The physiology of the eye. Human Eye Anatomy and Physiology: Understanding the Complexity of Vision

26.07.201805.04.2021 by alexxlab

How does the human eye work. What are the main components of the eye. How does light travel through the eye to create vision. What role do extraocular muscles play in eye movement. How do tears protect the eye.

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The Structure and Function of the Human Eye

The human eye is a marvel of biological engineering, serving as the primary organ for our sense of sight. Its complex structure allows us to perceive the world around us with remarkable detail and precision. Understanding the anatomy and physiology of the eye is crucial for appreciating how vision works and how various eye conditions can affect our sight.

The Eye’s Position and Protection

Our eyes are situated within bony cavities called orbits, which provide crucial protection and support. The orbits are surrounded by cranial bones, creating a safe housing for these delicate organs. But how else is the eye protected?

Eyelids with lashes act as a barrier against foreign particles
The palpebral conjunctiva lines the inner surface of the eyelids
Tears produced by the lacrimal gland wash away irritants

The conjunctiva extends over the white part of the eye (sclera), connecting the eyelids to the eyeball. This membrane plays a vital role in maintaining eye health and comfort.

The Journey of Light: From Cornea to Retina

Vision begins when light enters the eye. But what exactly happens to light as it travels through the eye’s structures? Let’s follow its path:

Light first passes through the cornea, the clear front “window” of the eye
The cornea bends light rays, accounting for about 60% of the eye’s refractive power
Light then travels through the pupil, an adjustable opening in the iris
The crystalline lens further focuses the light
Light passes through the vitreous humor, a clear gel filling the eyeball
Finally, light reaches the retina, where it’s converted into electrical signals

The Cornea: The Eye’s First Lens

The cornea is more than just a protective covering; it’s a crucial component in the visual process. How does the cornea contribute to vision? It acts as the eye’s first and most powerful lens, bending light rays so they can pass freely through the pupil. This refraction accounts for a significant portion of the eye’s focusing power.

The Iris and Pupil: Nature’s Camera Aperture

The iris, which gives our eyes their distinctive color, functions similarly to a camera’s aperture. How does the iris control light entry? It can contract or dilate, changing the size of the pupil in response to ambient light conditions. This adjustment helps regulate the amount of light entering the eye, ensuring optimal visual clarity in various lighting environments.

The Crystalline Lens: Fine-Tuning Focus

After passing through the pupil, light reaches the crystalline lens. This flexible structure plays a crucial role in focusing light precisely onto the retina. How does the lens adjust its focus? Through a process called accommodation, the lens can change its shape, becoming more or less curved to focus on objects at different distances.

Accommodation: The Eye’s Autofocus Mechanism

Accommodation is the eye’s ability to change its focal length to maintain a clear image of objects at varying distances. How does this process work?

Ciliary muscles surrounding the lens contract or relax
This changes the tension on the lens capsule
The lens becomes more spherical for near objects or flatter for distant objects
This shape change alters the lens’s refractive power, ensuring clear focus

The Retina: Where Light Becomes Signal

The retina is often compared to the film in a camera, but it’s far more complex. This light-sensitive layer at the back of the eye is where the transformation of light into neural signals begins. How does the retina process visual information?

Photoreceptor cells (rods and cones) capture light
These cells convert light into electrical impulses
The impulses are processed by various retinal neurons
Ganglion cells collect this information and send it to the brain

Rods and Cones: Specialized Light Detectors

The retina contains two types of photoreceptors: rods and cones. How do these cells differ in function?

Rods are more numerous and sensitive to low light, enabling night vision
Cones are less sensitive but allow for color vision and fine detail perception
There are three types of cones, each sensitive to different wavelengths of light

The Optic Nerve: The Visual Information Highway

Once the retina processes visual information, it needs to be sent to the brain for interpretation. This is where the optic nerve comes into play. How does the optic nerve function in the visual process?

It consists of over a million nerve fibers
It carries visual information from the retina to the brain
It acts as an extension of the brain, connecting the eye to visual processing centers

The optic nerve exits the back of the eye at a point called the optic disc. This area lacks photoreceptors, creating a natural blind spot in our visual field. However, our brain typically fills in this gap, making it unnoticeable in everyday vision.

Extraocular Muscles: Controlling Eye Movement

The ability to move our eyes is crucial for effective vision. This movement is controlled by six extraocular muscles attached to each eye. How do these muscles work together to control eye movement?

Four rectus muscles (superior, inferior, medial, lateral) control up/down and side-to-side movement
Two oblique muscles (superior and inferior) control rotational movements
These muscles work in coordinated pairs to produce smooth, precise eye movements

The Role of Oblique Muscles

The oblique muscles play a unique role in eye movement. Why are these muscles necessary? They compensate for the fact that the eye is not perfectly aligned on the sagittal plane. When we look up or down, the eye must also rotate slightly to compensate for the angle at which the rectus muscles pull.

Tear Production and the Lacrimal System

Tears are more than just a sign of emotion; they play a crucial role in maintaining eye health. How do tears protect and nourish the eye?

They lubricate the eye surface, reducing friction during blinking and eye movement
They wash away foreign particles and debris
They contain antibacterial compounds that help prevent infections
They provide oxygen and nutrients to the cornea

The Lacrimal Gland and Tear Drainage

The lacrimal gland, located beneath the lateral edge of the eye, produces tears. But where do these tears go? They flow across the eye surface and are drained through small openings called puncta, located in the inner corners of the eyelids. From there, tears travel through the lacrimal sac and nasolacrimal duct, eventually draining into the nasal cavity.

Visual Processing: From Eye to Brain

While the eye captures visual information, it’s the brain that interprets this data and creates our perception of the world. How does the brain process visual information?

Visual signals travel from the retina through the optic nerve
The optic chiasm allows information from both eyes to be combined
Signals reach the lateral geniculate nucleus in the thalamus
Information is then sent to the primary visual cortex in the occipital lobe
Higher visual areas process specific aspects like color, motion, and form

This complex process occurs rapidly, allowing us to perceive our visual world in real-time. The integration of information from both eyes also enables depth perception and stereoscopic vision.

Visual Pathways and Brain Specialization

The visual system in the brain is highly specialized. How is visual information distributed and processed?

The ventral stream or “what pathway” processes object recognition and form representation
The dorsal stream or “where pathway” handles spatial relationships and motion
Different areas of the visual cortex specialize in processing specific visual features like color, orientation, and motion

This specialization allows for efficient and detailed processing of the vast amount of visual information we receive every moment.

Common Eye Conditions and Their Impact on Vision

Understanding eye anatomy and physiology also helps in comprehending various eye conditions. What are some common eye problems and how do they affect vision?

Myopia (nearsightedness): Difficulty seeing distant objects clearly
Hyperopia (farsightedness): Difficulty seeing near objects clearly
Astigmatism: Blurred vision due to irregular cornea shape
Cataracts: Clouding of the eye’s natural lens
Glaucoma: Damage to the optic nerve, often due to increased eye pressure
Macular degeneration: Deterioration of the central part of the retina

Each of these conditions affects different parts of the eye, highlighting the importance of regular eye examinations and understanding the complex nature of our visual system.

Refractive Errors and Corrective Measures

Refractive errors like myopia, hyperopia, and astigmatism are common vision problems. How are these conditions corrected?

Eyeglasses: Lenses that compensate for the eye’s focusing deficiencies
Contact lenses: Similar to glasses but worn directly on the eye surface
Refractive surgery: Procedures like LASIK that reshape the cornea
Intraocular lenses: Artificial lenses implanted during cataract surgery

These corrective measures work by altering how light is focused onto the retina, compensating for the eye’s natural refractive errors.

The Evolution of the Human Eye

The human eye is a product of millions of years of evolution. How has the eye evolved over time? The eye’s development can be traced through various stages:

Light-sensitive proteins in single-celled organisms
Simple eyespots in multicellular organisms
Pin-hole camera eyes in early animals
Lens-bearing eyes in fish and other vertebrates
Highly specialized eyes in mammals and primates

Each stage brought improvements in visual acuity, color perception, and the ability to process complex visual information. The human eye represents one of the most advanced forms of this evolutionary journey.

Comparative Eye Anatomy Across Species

While the basic structure of the eye is similar across many species, there are fascinating variations. How do eyes differ among various animals?

Insects have compound eyes with multiple lenses
Many birds have tetrachromatic vision, allowing them to see ultraviolet light
Some deep-sea creatures have eyes adapted for low-light environments
Chameleons can move each eye independently

These variations showcase the diverse ways in which eyes have adapted to different environmental needs and lifestyles.

The Future of Eye Care and Vision Technology

As our understanding of eye anatomy and physiology grows, so does our ability to treat eye conditions and enhance vision. What advancements are on the horizon for eye care?

Gene therapy for inherited retinal diseases
Stem cell treatments for retinal regeneration
Advanced artificial retinas and neural implants
Non-invasive treatments for presbyopia
AI-assisted diagnosis and treatment planning

These emerging technologies promise to revolutionize how we prevent, diagnose, and treat eye conditions, potentially restoring sight to those with previously untreatable conditions.

The Integration of AI in Ophthalmology

Artificial intelligence is increasingly playing a role in eye care. How is AI being used in ophthalmology?

Automated analysis of retinal images for disease detection
Predictive modeling for progression of eye diseases
Personalized treatment plans based on vast datasets
AI-assisted surgical planning and guidance

These AI applications have the potential to improve diagnostic accuracy, streamline treatment processes, and make advanced eye care more accessible globally.

Eye anatomy and physiology – how the eye and vision work

The human eye, the organ responsible for the sense of sight, is a very complex structure. We use our vision in almost every activity, so the eye is a most important organ.

How vision works

Sight begins when light rays from an object enter the eye through the cornea, the clear front “window” of the eyeball. The cornea is actually responsible for about sixty percent of the eyeball’s light-ray-bending capability. The cornea’s refractive power bends the light rays in such a way that they pass freely through the pupil, the size-changing hole in the iris.

Watch an animation on “How vision works”

The iris, the structure that gives the eyes color, works like a shutter in a camera. It has the ability to enlarge and shrink, depending on how much light the environment is sending into the pupil.

After passing through the iris, the light rays strike the eye’s crystalline lens. This clear, flexible structure works much like the lens in a camera – shortening and lengthening its width in order to focus light rays properly.

In a normal eyeball, after exiting the back of the lens, the light rays pass through the vitreous — a clear, jelly-like substance that fills the globe of the eyeball. The vitreous humor helps the eye hold its spherical shape. Finally, the light rays land and come to a sharp focusing point on the retina.

Continuing with our “camera” analogy, the retina’s function is much like the film in a camera. It is responsible for capturing all of the light rays, processing them into light impulses through millions of tiny nerve endings, then sending these light impulses through over a million nerve fibers to the optic nerve.

The optic nerve is sort of like an extension of the brain. It is a bundled cord of more than a million nerve fibers. The light impulses travel through this nerve fiber to the brain, where they are interpreted as an image.

Vision | Anatomy and Physiology I

Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 1). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

Figure 1. The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

Figure 2. Extraocular Muscles The extraocular muscles move the eye within the orbit.

The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially.

The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane.

When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 1). The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.

The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light.

The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception. The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor. The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see Figure 3). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.

Figure 3. Structure of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.

As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1.

The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea. Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs.

Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 4). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.

Figure 4. Photoreceptor (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Longer wavelengths of less than 380 nm fall into the infrared range, whereas shorter wavelengths of more than 720 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.

Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. Before interacting with a photon, retinal’s flexible double-bonded carbons are in the cis conformation. This molecule is referred to as 11-cis-retinal. A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans– conformation, forming all-trans-retinal, which has a straight hydrocarbon chain (Figure 5).

Figure 5. Retinal Isomers The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.

The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina. Until the retinal molecule is changed back to the 11-cis-retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.

Figure 6. Comparison of Color Sensitivity of Photopigments Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.

In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.

The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.

Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex.

The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals. ” What aspect of retinal processing is simplified by that statement? Explain your answer.

Sensory Nerves

Once any sensory cell transduces a stimulus into a nerve impulse, that impulse has to travel along axons to reach the CNS. In many of the special senses, the axons leaving the sensory receptors have a topographical arrangement, meaning that the location of the sensory receptor relates to the location of the axon in the nerve. For example, in the retina, axons from RGCs in the fovea are located at the center of the optic nerve, where they are surrounded by axons from the more peripheral RGCs.

Spinal Nerves

Generally, spinal nerves contain afferent axons from sensory receptors in the periphery, such as from the skin, mixed with efferent axons travelling to the muscles or other effector organs. As the spinal nerve nears the spinal cord, it splits into dorsal and ventral roots. The dorsal root contains only the axons of sensory neurons, whereas the ventral roots contain only the axons of the motor neurons. Some of the branches will synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, or even the anterior (ventral) horn, at the level of the spinal cord where they enter. Other branches will travel a short distance up or down the spine to interact with neurons at other levels of the spinal cord. A branch may also turn into the posterior (dorsal) column of the white matter to connect with the brain. For the sake of convenience, we will use the terms ventral and dorsal in reference to structures within the spinal cord that are part of these pathways. This will help to underscore the relationships between the different components. Typically, spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain.

Cranial Nerves

Cranial nerves convey specific sensory information from the head and neck directly to the brain. For sensations below the neck, the right side of the body is connected to the left side of the brain and the left side of the body to the right side of the brain. Whereas spinal information is contralateral, cranial nerve systems are mostly ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons, such as the olfactory, optic, and vestibulocochlear nerves. Other cranial nerves contain both sensory and motor axons, including the trigeminal, facial, glossopharyngeal, and vagus nerves (however, the vagus nerve is not associated with the somatic nervous system). The general senses of somatosensation for the face travel through the trigeminal system.

Physiology of the Eye – 2nd Edition

Preface to the Second Edition

Acknowledgments

1 Review of Ocular Anatomy

Gross Anatomy

Internal Anatomy

Sciera and Cornea

Corneoscleral Junction

Uvea

Retina

Crystalline Lens

Vitreous Body

Chambers of the Eye

Blood Vessels in the Eye

Nonvisual Nerves of the Eyeball

Eyelids and Conjunctiva

2 The Aqueous Humor

Source

Chemistry

Rate of Production

Outflow Path

3 The Intraocular Pressure

Methods of Measurement

Schiotz Tonometry

Applanation Tonometry

Errors in Tonometry

MacKay-Marg Tonometer, Durham-Langham Pneumatonometers, and Noncontact Tonometers

Normal Intraocular Pressure

Factors Affecting Intraocular Pressure

Estimating Changes in Intraocular Pressure

Facility of Outflow and Tonography

Other Factors Influencing Intraocular Pressure

4 The Vitreous Body

Anatomy

Chemical Properties

Physical Properties

Osmosis

Flow Conductivity

5 The Lens

Structure

Accommodation and Elasticity

Transparency

Metabolism and Nutrition

Cataract

6 The Cornea I: Form, Swelling Pressure, Transport Processes and Optics

Structure

Swelling Pressure

The Thickness—Hydration Relationship

In Vivo Measurement of Swelling—Imbibition Pressure

Transparent Process

Theory of Diffusion

Theory of Bulk Flow

Stromal Diffusion

Bulk Flow in the Stromal

The Limiting Layers

Diffusion

Bulk Flow

Corneal Bulk Water Flow

Physical Optics

Transmission and Absorption

Scattering

Refractive Index

Birefringence

7 Cornea II: Metabolism, Oxygen, Carbon Dioxide, and Contact Lens Wear

Glucose

Amino Acids

Oxygen

Corneal Oxygen Requirements

The Critical Oxygen Tension

Oxygen Flux and Distribution of Oxygen

The Open Eye

The Closed Eye

The Cornea in an Oxygen-Free Environment

The Tight Oxygen Impermeable Contact Lens

The Case of the Tight Gas-Permeable Contact Lens

Oxygen Tension at the Corneal Surface

The Case of the Gas-Impermeable Contact Lens with Tear Pumping

The Case of the Oxygen-Permeable Contact Lens with Tear Pumping

Multilayered Cornea

Carbon Dioxide

Corneal Thickness Control

8 The Sciera

Swelling Pressure, Diffusion, and Bulk Flow

Tissue Mechanics of Sciera (and Cornea)

9 Retina

Structure

Metabolism

Photochemistry

Flow Conductivity and Retinal Adhesion

Density Measurement

Flow Conductivity Measurement

10 The Tears and the Lids

The Tears

Function

Film Structure

Sources

Composition

Stability

Production Rate

Evaporation and Temperature

The Lids

Lid Activity

References

Index

15.

5 Vision – Anatomy & Physiology

Vision

Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 15.5.1). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

Figure 15.5.1 – The Eye in the Orbit: The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 15.5.2). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye to moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 15.5.1).

The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.

Figure 15.5.2 – Extraocular Muscles: The extraocular muscles move the eye within the orbit.

The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible (Figure 15. 5.3). The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris—the colored part of the eye. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception.

The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor.

The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve (see Figure 15.5.3). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins. This creates a “blind spot” in the retina, and a corresponding blind spot in our visual field.

Figure 15.5.3 – Structure of the Eye: The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.

Note that the photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity, or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 15.5.3). As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea.

Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 15.5.4). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.

Figure 15. 5.4 – Photoreceptor: (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nm. Wavelengths of electromagnetic radiation longer than 720 nm fall into the infrared range, whereas wavelengths shorter than 380 nm fall into the ultraviolet range. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.

Figure 15.5.5 – Retinal Isomers: The retinal molecule has two isomers, (a) one before a photon interacts with it and (b) one that is altered through photoisomerization.

The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue (Figure 15.5.6). The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from a rod’s corresponding RGC.

Figure 15.5.6 – Comparison of Color Sensitivity of Photopigments: Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.

External Website

Watch this video to learn more about a transverse section through the brain that depicts the visual pathway from the eye to the occipital cortex. The first half of the pathway is the projection from the RGCs through the optic nerve to the lateral geniculate nucleus in the thalamus on either side. This first fiber in the pathway synapses on a thalamic cell that then projects to the visual cortex in the occipital lobe where “seeing,” or visual perception, takes place. This video gives an abbreviated overview of the visual system by concentrating on the pathway from the eyes to the occipital lobe. The video makes the statement (at 0:45) that “specialized cells in the retina called ganglion cells convert the light rays into electrical signals. ” What aspect of retinal processing is simplified by that statement? Explain your answer.

Central Pathway of Visual Information

The connections of the optic nerve are more complicated than those of other cranial nerves. Instead of the connections being between each eye and the brain, visual information is segregated between the left and right sides of the visual field. In addition, some of the information from one side of the visual field projects to the opposite side of the brain. Within each eye, the axons projecting from the medial side of the retina decussate at the optic chiasm. For example, the axons from the medial retina of the left eye cross over to the right side of the brain at the optic chiasm. However, within each eye, the axons projecting from the lateral side of the retina do not decussate. For example, the axons from the lateral retina of the right eye project back to the right side of the brain. Therefore the left field of view of each eye is processed on the right side of the brain, whereas the right field of view of each eye is processed on the left side of the brain (Figure 15.5.7).

Figure 15.5.7 – Segregation of Visual Field Information at the Optic Chiasm: Contralateral visual field information from the lateral retina projects to the ipsilateral brain, whereas ipsilateral visual field information has to decussate at the optic chiasm to reach the opposite side of the brain.

A unique clinical presentation that relates to this anatomic arrangement is the loss of lateral peripheral vision, known as bilateral hemianopia. This is different from “tunnel vision” because the superior and inferior peripheral fields are not lost. Visual field deficits can be disturbing for a patient, but in this case, the cause is not within the visual system itself. A growth of the pituitary gland presses against the optic chiasm and interferes with signal transmission. However, the axons projecting to the same side of the brain are unaffected. Therefore, the patient loses the outermost areas of their field of vision and cannot see objects to their right and left.

Extending from the optic chiasm, the axons of the visual system are referred to as the optic tract instead of the optic nerve. The optic tract has three major targets, two in the diencephalon and one in the midbrain. The connection between the eyes and diencephalon is demonstrated during development, in which the neural tissue of the retina differentiates from that of the diencephalon by the growth of the secondary vesicles. The connections of the retina into the CNS are a holdover from this developmental association. The majority of the connections of the optic tract are to the thalamus—specifically, the lateral geniculate nucleus. Axons from this nucleus then project to the visual cortex of the cerebrum, located in the occipital lobe. Another target of the optic tract is the superior colliculus.

In addition, a very small number of RGC axons project from the optic chiasm to the suprachiasmatic nucleus of the hypothalamus. These RGCs are photosensitive, in that they respond to the presence or absence of light. Unlike the photoreceptors, however, these photosensitive RGCs cannot be used to perceive images. By simply responding to the absence or presence of light, these RGCs can send information about day length. The perceived proportion of sunlight to darkness establishes the circadian rhythm of our bodies, allowing certain physiological events to occur at approximately the same time every day.

Cortical Processing of Visual Information

Likewise, the topographic relationship between the retina and the visual cortex is maintained throughout the visual pathway. The visual field is projected onto the two retinae, as described above, with sorting at the optic chiasm. The right peripheral visual field falls on the medial portion of the right retina and the lateral portion of the left retina. The right medial retina then projects across the midline through the optic chiasm. This results in the right visual field being processed in the left visual cortex. Likewise, the left visual field is processed in the right visual cortex (see Figure 15.5.7). Though the chiasm is helping to sort right and left visual information, superior and inferior visual information is maintained topographically in the visual pathway. Light from the superior visual field falls on the inferior retina, and light from the inferior visual field falls on the superior retina. This topography is maintained such that the superior region of the visual cortex processes the inferior visual field and vice versa. Therefore, the visual field information is inverted and reversed as it enters the visual cortex—up is down, and left is right. However, the cortex processes the visual information such that the final conscious perception of the visual field is correct. The topographic relationship is evident in that information from the foveal region of the retina is processed in the center of the primary visual cortex. Information from the peripheral regions of the retina are correspondingly processed toward the edges of the visual cortex. Similar to the exaggerations in the sensory homunculus of the somatosensory cortex, the foveal-processing area of the visual cortex is disproportionately larger than the areas processing peripheral vision.

In an experiment performed in the 1960s, subjects wore prism glasses so that the visual field was inverted before reaching the eye. On the first day of the experiment, subjects would duck when walking up to a table, thinking it was suspended from the ceiling. However, after a few days of acclimation, the subjects behaved as if everything were represented correctly. Therefore, the visual cortex is somewhat flexible in adapting to the information it receives from our eyes (Figure 15.5.8).

Figure 15.5.8 – Topographic Mapping of the Retina onto the Visual Cortex: The visual field projects onto the retina through the lenses and falls on the retinae as an inverted, reversed image. The topography of this image is maintained as the visual information travels through the visual pathway to the cortex.

The cortex has been described as having specific regions that are responsible for processing specific information; there is the visual cortex, somatosensory cortex, gustatory cortex, etc. However, our experience of these senses is not divided. Instead, we experience what can be referred to as a seamless percept. Our perceptions of the various sensory modalities—though distinct in their content—are integrated by the brain so that we experience the world as a continuous whole.

In the cerebral cortex, sensory processing begins at the primary sensory cortex, then proceeds to an association area, and finally, into a multimodal integration area. For example, the visual pathway projects from the retinae through the thalamus to the primary visual cortex in the occipital lobe. This area is primarily in the medial wall within the longitudinal fissure. Here, visual stimuli begin to be recognized as basic shapes. Edges of objects are recognized and built into more complex shapes. Also, inputs from both eyes are compared to extract depth information. Because of the overlapping field of view between the two eyes, the brain can begin to estimate the distance of stimuli based on binocular depth cues.

External Website

Watch this video to learn more about how the brain perceives 3-D motion. Similar to how retinal disparity offers 3-D moviegoers a way to extract 3-D information from the two-dimensional visual field projected onto the retina, the brain can extract information about movement in space by comparing what the two eyes see. If movement of a visual stimulus is leftward in one eye and rightward in the opposite eye, the brain interprets this as movement toward (or away) from the face along the midline. If both eyes see an object moving in the same direction, but at different rates, what would that mean for spatial movement?

Everyday Connections –

Depth Perception, 3-D Movies, and Optical Illusions

The visual field is projected onto the retinal surface, where photoreceptors transduce light energy into neural signals for the brain to interpret. The retina is a two-dimensional surface, so it does not encode three-dimensional information. However, we can perceive depth. How is that accomplished?

Two ways in which we can extract depth information from the two-dimensional retinal signal are based on monocular cues and binocular cues, respectively. Monocular depth cues are those that are the result of information within the two-dimensional visual field. One object that overlaps another object has to be in front. Relative size differences are also a cue. For example, if a basketball appears larger than the basket, then the basket must be further away. On the basis of experience, we can estimate how far away the basket is. Binocular depth cues compare information represented in the two retinae because they do not see the visual field exactly the same.

The centers of the two eyes are separated by a small distance, which is approximately 6 to 6.5 cm in most people. Because of this offset, visual stimuli do not fall on exactly the same spot on both retinae unless we are fixated directly on them and they fall on the fovea of each retina. All other objects in the visual field, either closer or farther away than the fixated object, will fall on different spots on the retina. When vision is fixed on an object in space, closer objects will fall on the lateral retina of each eye, and more distant objects will fall on the medial retina of either eye (Figure 15.5.9). This is easily observed by holding a finger up in front of your face as you look at a more distant object. You will see two images of your finger that represent the two disparate images that are falling on either retina.

These depth cues, both monocular and binocular, can be exploited to make the brain think there are three dimensions in two-dimensional information. This is the basis of 3-D movies. The projected image on the screen is two dimensional, but it has disparate information embedded in it. The 3-D glasses that are available at the theater filter the information so that only one eye sees one version of what is on the screen, and the other eye sees the other version. If you take the glasses off, the image on the screen will have varying amounts of blur because both eyes are seeing both layers of information, and the third dimension will not be evident. Some optical illusions can take advantage of depth cues as well, though those are more often using monocular cues to fool the brain into seeing different parts of the scene as being at different depths.

Figure 15.5.9 – Retinal Disparity: Because of the interocular distance, which results in objects of different distances falling on different spots of the two retinae, the brain can extract depth perception from the two-dimensional information of the visual field.

There are two main regions that surround the primary cortex that are usually referred to as areas V2 and V3 (the primary visual cortex is area V1). These surrounding areas are the visual association cortex. The visual association regions develop more complex visual perceptions by adding color and motion information. The information processed in these areas is then sent to regions of the temporal and parietal lobes. Visual processing has two separate streams of processing: one into the temporal lobe and one into the parietal lobe. These are the ventral and dorsal streams, respectively (Figure 15.5.10). The ventral stream identifies visual stimuli and their significance. Because the ventral stream uses temporal lobe structures, it begins to interact with the non-visual cortex and may be important in visual stimuli becoming part of memories. The dorsal stream locates objects in space and helps in guiding movements of the body in response to visual inputs. The dorsal stream enters the parietal lobe, where it interacts with somatosensory cortical areas that are important for our perception of the body and its movements. The dorsal stream can then influence frontal lobe activity where motor functions originate.

Figure 15.5.10 – Ventral and Dorsal Visual Streams: From the primary visual cortex in the occipital lobe, visual processing continues in two streams—one into the temporal lobe and one into the parietal lobe.

Disorders of the…Brain: Prosopagnosia

The failures of sensory perception can be unusual and debilitating. A particular sensory deficit that inhibits an important social function of humans is prosopagnosia, or face blindness. The word comes from the Greek words prosopa, that means “faces,” and agnosia, that means “not knowing.” Some people may feel that they cannot recognize people easily by their faces. However, a person with prosopagnosia cannot recognize the most recognizable people in their respective cultures. They would not recognize the face of a celebrity, an important historical figure, or even a family member like their mother. They may not even recognize their own face.

Prosopagnosia can be caused by trauma to the brain, or it can be present from birth. The exact cause of proposagnosia and the reason that it happens to some people is unclear. A study of the brains of people born with the deficit found that a specific region of the brain, the anterior fusiform gyrus of the temporal lobe, is often underdeveloped. This region of the brain is concerned with the recognition of visual stimuli and its possible association with memories. Though the evidence is not yet definitive, this region is likely to be where facial recognition occurs.

Though this can be a devastating condition, people who suffer from it can get by—often by using other cues to recognize the people they see. Often, the sound of a person’s voice, or the presence of unique cues such as distinct facial features (a mole, for example) or hair color can help the sufferer recognize a familiar person. In the video on prosopagnosia provided in this section, a woman is shown having trouble recognizing celebrities, family members, and herself. In some situations, she can use other cues to help her recognize faces.

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The inability to recognize people by their faces is a troublesome problem. It can be caused by trauma, or it may be inborn. Watch this video to learn more about a person who lost the ability to recognize faces as the result of an injury. She cannot recognize the faces of close family members or herself. What other information can a person suffering from prosopagnosia use to figure out whom they are seeing?

The Physiology of the Eye on Steam

About This Game

“The Physiology of the Eye” is an interactive VR platform, that features two modes. It starts in a real-time automatic training mode that guides you through our visual content. Then, at any time in the application, the user can push the “Mode” button on the VR controller and switch to our interactive training mode. This allows the user to choose which areas they need to focus on and learn at their own pace. This includes a new labeling system which allows you to expand each label to find out more about the exact 3D piece you have selected and to get an in-depth understanding of each structure.

These new labels also have audio clips attached to them so you can hear exactly how to pronounce the various parts of the model. Even in Chinese. This application also includes an assessment system to judge your learning comprehension with our interactive testing system. It will also provide you with a final GPA at the end of the course.

Quotes:

“…facilitates learning through active engagement in the content and allows learners to apply content in ways that quickly and effectively solve challenges and create new opportunities.” – Charles H. Patti, Ph.D., University of Denver and Queensland University of Technology.
“…photorealism aesthetics and scientific accuracy, the perfect mix to take education to the next level.” – Alban Denoyel, CEO & Co-founder @ Sketchfab
“A beautiful example of how virtual reality is poised to transform life sciences and education.” – Sean Wagstaff, NVIDIA

Features:

Automatic mode: Like a traditional text book, with sections in the chapter and voice over talking to you about the model in front of you.
Interactive mode: Use this mode to pick the models up, scale, rotate, as well as pull on different parts of the model to see how it functions. You can also select the different parts of the model to find out what they are and learn at your own pace.
Scientifically accurate representations
Quizzes with final GPA

About the team:

Intervoke is a startup company with a dynamic group of people who are responsible for creating award winning 3D animations for over 20 years. This company was started to create world class scientifically accurate content and 3D animation to effectively communicate various complex biological interactions inside the human body. Our team consists of 3D animators, medical illustrators, Unity code engineers, virtual reality specialists, and custom tailored physicians. We have come together to custom engineer virtual reality training programs and 3D animations for scientific visualization. Using photo-realistic thought provoking visuals, we can achieve detailed scientific accuracy. All while presenting content using custom designed clean interactive interfaces. We are passionate about science and technology and maintain the highest possible quality.

Adler’s Physiology of the Eye: Clinical Application, Tenth E… : Journal of Neuro-Ophthalmology

Adler’s Physiology of the Eye: Clinical Application, Tenth Edition; Paul L. Kaufman and Albert Alm, Editors. Mosby, St. Louis, MO, 2003. ISBN: 0-323-01136-5. $102.00

Scope: This tenth edition of Adler’s Physiology of the Eye is a comprehensive, single-volume, multi-authored textbook of ocular physiology designed to be a reference text for preclinical scientists and clinicians. It emphasizes clinical applications of the exponentially expanding knowledge base of ocular physiology, morphology, and molecular biology.

Contents: The book is divided into 14 sections, each with a separate editor. Each section covers broad topics, including the ocular surface, the cornea and sclera, the lens, optics and refraction, accommodation and presbyopia, aqueous humor hydrodynamics, the vitreous, the retina, visual perception, the optic nerve, the central visual pathways, the pupil, ocular circulation, and extraocular muscles/eye movements. Topics covered include anatomy, development, physiology, and pathophysiology. Each chapter is a thoroughly referenced (more than 500 references in some chapters) up-to-date review with extensive figures, illustrations, diagrams, and clinical photographs.

Of particular interest to neuro-ophthalmologists are the sections about retina, optic nerve, visual perception, the central visual pathways, the pupil, extraocular muscles, and eye movements. The visual perception section provides a comprehensive overview of several areas rarely discussed, including entoptic phenomena, visual acuity, early visual processing of spatial form, binocular vision, temporal properties of vision, development of vision in infancy, perimetry and visual field testing, color vision, and visual adaptation. The central visual pathways section is organized anatomically, including chapters on the retino-geniculate projections, the lateral geniculate nucleus, the primary visual cortex, and the extrastriate cortex, as well as a chapter on visual deprivation. The final section on extraocular muscles and eye movements has three chapters, with the latter two including explanations of three-dimensional rotations of the eye and the neural control of eye movements. Interesting new additions to the text include updated descriptions of the neural circuitry of the retina, ophthalmic facial anatomy, and ocular circulation.

Strengths: This is an updated and revised edition of a classic. It is arguably the most important single-volume reference for scientists and clinicians seeking information or insight into any aspect of vision science. At the same time, it is enjoyable to read, chapter by chapter, for anyone who is fascinated by the workings of the eye and its brain connections. The chosen authors are all recognized experts in their fields, and the text covers all aspects of the visual system. Information gathered from basic science research is used to explain ocular physiology. Clinical correlations aid in the understanding and application of the basics of vision science to patient care. The figures and tables are clear and complement the text well. The chapters on the pupil, the optic nerve, ocular circulation, the extraocular muscles, three-dimensional rotations of the eye, and the neural control of eye movements are spectacular.

Weaknesses: A minor criticism is that many of the photographs of the eye and histologic sections would be better viewed as color reproductions. There is a small color plate section in the back of the book that includes 22 color figures that seem to be arbitrarily chosen. Like any other multi-authored text, there is some unevenness to the writing style.

Recommended audience: This book is intended for preclinical scientists and clinicians but is equally useful for ophthalmologists and students of vision science.

Critical appraisal: The editors have created a comprehensive, up-to-date text on ocular physiology. This book should be an invaluable addition to any neuro-ophthalmologist’s collection. Dr. Adler would be proud of this latest version of his classic text.

Nicholas J. Volpe, MD

Scheie Eye Institute; University of Pennsylvania; Philadelphia, Pennsylvania

Physiology of the Eye – MCAT Social and Behavioral Sciences

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Eye Physiology – Ophthalmic

There are people with very low vision in whom the cornea, aqueous humor, lens and vitreous body are transparent, there are no disturbances from the visual pathways and brain centers, as well as light-receiving elements of the retina. Poor vision of these people is due to the blurred image on the retina due to the refractive features of the eye. Distinguish between physical and clinical refraction.

Physical refraction is the refractive power of the optical system of the eye, expressed in diopters.Diopter (diopter) is a unit of measurement of the strength of an optical system. One diopter (1.0 diopters) equals the power of a biconvex lens with a focal length of 1 meter. The shorter the focal length, the stronger the refractive power of the lens and the weaker the refractive power of the lens, the longer its focal length. (A 2.0 D lens has a focal length of 50 cm, a 4.0 D lens has 25 cm, a 10.0 D lens has 10 cm, etc.)

In diopters, you can also measure the refractive power of concave lenses. The strength of concave lenses can be calculated by compensating for the refraction given by convex optical glasses.

Concave lens to compensate for a 1.0 D convex lens, i.e. restoring the parallel direction of a parallel beam of rays refracted by a convex lens with a force of 1.0 diopters, has the same optical power in, but with the opposite sign. This concave lens is called a 1.0 diopter lens. A -1.0 D lens scatters a beam of parallel light beams as much as a +1.0 D lens collects them.

The refractive apparatus of the eye is the cornea, aqueous humor, lens and vitreous body.

Any complex refractive system is characterized by its cardinal points, which determine the diopter effect of the system. It has six cardinal points – two focal points (back and front), two nodal and two main ones.

Focal points are the points at which parallel rays are collected, refracted in the system. Consequently, the posterior focus in the eye will be at the point at which, after refraction, parallel rays are collected, reaching the eye from the front. If a parallel beam falls on the eye system from behind, then after refraction it will be collected in the front focus.

Node points are the points through which the rays pass without refraction. Major points are the points where refraction begins.

In the refractive system of the eye, the posterior nodal point is close to the anterior nodal point, and the posterior major point is very close to the anterior principal point, therefore, simplifying the optical system of the eye, it can be assumed that there is one major point located in the anterior chamber 2 mm from the cornea, one nodular 7 mm behind the cornea (slightly in front of the posterior pole of the lens) and two focal – posterior (23-24 mm posterior to the anterior surface of the cornea) and anterior (15-17 mm in front of the eye).

To study the refractive system of the eye, we need to determine, first of all, the refractive indices of the aqueous humor and the lens, the radii of curvature of the anterior surface of the cornea, the anterior and posterior surfaces of the lens, the thickness of the lens and cornea, the depth of the anterior chamber and the length of the anatomical axis of the eye.

The radius of curvature of the cornea is on average 7.8 mm. The depth of the anterior chamber is 3.0 mm. The radius of the front surface of the lens is 10 mm, the back surface is 6 mm. The lens thickness is 3.6-5.0 mm.

The refractive index of aqueous humor is 1.33.

The refractive index of the lens is 1.43.

The average refractive power of the eye in newborns is 77.0-80.0 diopters (according to E.I.Kovalevsky), in older children and adults – 60. 0 diopters with variation within 52.0-68.0 diopters.

Clinical refraction is the ratio of the anteroposterior axis of the eye to the strength of the refractive apparatus.

If the focus of parallel rays refracted in the eye system turns out to be on the retina, this means that the length of the focal length of the given refractive system of the eye coincides with the length of the anteroposterior axis of the eye.This is the so-called proportional refraction – emmetropia (Emmetropia).

If parallel rays, refracted in the lens, are collected in front of the retina, this means that the focal length does not coincide with the length of the anteroposterior axis of the eye. In this case, the eye is longer than the strength of its refractive apparatus requires. This is a disproportionate refraction – myopia (Myopia) .

If the parallel rays are collected behind the retina, because the length of the focal length of the refractive apparatus of the eye is greater than the length of the anteroposterior axis of the eye, i.e.That is, the refractive apparatus is weak for the eye, which is shorter than necessary for this system – this is a disproportionate refraction – Hypermetropia .

According to the distance of the focus of the ametropic eye from the retina, weak, medium and strong degrees of refractive errors are distinguished. With a weak degree of ametropia, visual acuity is impaired slightly, although it cannot be complete due to a small circle of light scattering (each luminous point gives a circle of light scattering of the larger diameter, the further the focus is from the retina and, consequently, the lower visual acuity).Myopia and hyperopia are included in the concept of ametropia.

With moderate degrees of ametropia, there is a large loss of vision. With a high degree of ametropia, visual acuity is always very low, because the focus is very far from the retina.

Myopia:

weak degree – up to 3. 0 diopters;
medium degree – up to 6.0 diopters;
high degree – over 6.0 diopters.

Hyperopia:

weak – up to 2.0 diopters;
average – from 2.0 to 5.0 diopters;
high – above 5.0 diopters.

In the myopic eye, parallel rays collect in front of the retina. Such an eye needs rays that require more refraction than parallel ones. Then the refractive power will be insufficient to collect these rays into their main focus, i.e. in front of the retina, but will collect further, i.e. on the retina. Such rays are divergent rays located closer to infinity. When approaching a point, the rays emanating from it will hit the retina. This point will be the further point of clear view for the given eye.

In the hyperopic eye, parallel rays will gather behind the eye. This eye needs to send rays that require less refraction than parallel ones. These rays are converging rays before entering the eye. Such rays must converge even before the eye, so that after refraction in the eye, they are collected just on the retina. The converging rays are beyond infinity, i.e. in negative, non-existent space.

With a myopia of 1.0 diopters, the further point of clear vision is at a distance of one meter from the eye.With myopia more than 1.0 diopters, it is even closer. In the myopa, the further point of a clear view can be determined in the simplest way. The patient is offered to read a book in a well-lit room. The doctor gradually moves away from him with a book in his hands. The greatest distance at which the subject is still able to parse the type indicates the position of the further point of clear view.

Physiology of the organ of vision

LECTURE No. 2

THEME: PHYSIOLOGY OF THE VISION BODY.

The main function of the visual analyzer
a person is the perception of light, and
also the shapes of objects of the surrounding world
and their position in space, light
causes complex changes in the retina,
causing the so-called
visual act. So the light
is an adequate irritant for
organ of vision. Light – magnetic vibrations
with a certain frequency (369-760 mmk –
visible part of the spectrum).

It is believed that light irritation in
primarily perceives rhodopsin
(visual purple).

Transformation of light energy into
the retina is carried out as a result
vital processes of receptors
– rods and cones, including
photochemical destruction reactions and
restoration of rhodopsin in close connection
with metabolism. Chemical products
transformations in photoreceptors, as well as
the resulting electrical
potentials are annoying
for other layers of the retina, where
excitation impulses carrying visual
information to the central nervous system. Excitement from sticks
and cones are transmitted to bipolar and
retinal ganglion cells.Continuous
photochemical process (rhodopsin synthesis)
is impossible without the presence of vitamins A and
B ₂, ATP, nicotinamide, etc.
lack of these substances in the body
such visual functions are impaired,
how light perception, adaptation, develops
hemeralopia (night blindness). but
the process of perception, as a rule, is not
limited to sight, but involves
tactile, gustatory sensations. Processes
visual perception, flowing in
the eye are an integral part
brain activity.They are closely related
with thinking.

Due to the limited speed, light
(3 at 10 ¹⁰ m / s) and a certain delay
nerve impulses entering the brain,
a person sees the past (disappeared). Behind
one second the light beam has time
will fly around the Earth more than 7 times.

The retina perceiving light in
functionally can be
divided into a central (spot area
retina) and peripheral (all the rest
surface of the retina). Respectively
this distinguishes between central and
peripheral vision.Moreover,
also distinguish the nature of vision (monocular,
binocular).

The most perfect visual
perception is possible provided that
the image of the object falls on the area
retinal spots, especially its central
pits. Peripheral retina
this ability is significantly
to a lesser extent. The further from the center
to the periphery of the retina is projected
the picture of the object, the less it is
distinctly.

Max Schultz advanced the theory of duality
view on the distribution of responsibilities
between the sticks (there are about 13 million) and
cones (7 million).Central office
retinas (cones) provide daytime
vision and color perception, and peripheral
(sticks) – night (scotopic), or
twilight (mesoscopic) vision
(light perception, dark adaptation).

In the retina, there are 3 types
processes:

retinomotor reaction – consists
is that depending on the degree and
cone luminous flux intensity
come to the fore in bright light
and vice versa, and the light hits everything
elements.
photochemical reaction – associated with
decomposition of rhodopsin and iodopsin. For
so that they constantly recover
need constant supply
nutrients and the presence of magic,
to have time to rest.
electrical reaction. When decomposing
rhodopsin and iodopsin arise
positive and negative ions,
which form the fields, the result
what is the occurrence of the difference
potentials, which, according to Lazarev’s theory,
is a trigger for
the appearance of visual images in
bark.

Visual organ functions:

visual acuity (central vision)
field of view (peripheral vision)
color perception
dark adaptation

Visual acuity – ability
distinguish the human eye separately
two luminous points located on
maximum distance from the eye and
minimum distance between each other.

Visual acuity allows detailed
study subjects. Visual acuity
carried out by the macular region
(yellow spot), which always coincides
visual axis of the eye. Next to yellow
spot visual acuity decreases (if
yellow spot 1, then near 0.01).

Anatomical features of the macular
area:

the visual axis is projected into the macula
in the macular region there are only
one cones
each cone from the macula corresponds
one “own” individual bipolar
a cell, and on the periphery of such a picture
not observed
in the macular region, the retina
thinned, which is necessary for improvement
her trophics

The angle of view is formed by extreme points
subject and the nodal point of the eye.

It was found that the smallest angle of view,
under which the eye can distinguish 2 points
is equal to 1 degree. This value of the angle of view
taken as an international unit
visual acuity and on average is
1 unit (1.0).

At an angle of view of 1 degree, the value
retinal image is 4 in 10 ^-3,
that is, 4 μm, and the cone diameter is also
is equal to 0.002 – 0.0045 mm. This correspondence
confirms the view that for
separate perception of two points
it is necessary that two such elements
(cones) were separated by at least one
non-ray element
shine.However, visual acuity equal to 1,
is not limiting. Exist
nationalities and tribes with a sharpness
vision reaches 6 or more units.

To determine visual acuity
used tables that are built
decimally. They are the smallest
signs are visible at an angle of 5 degrees
from a distance of 5 m.If these signs
differ between the subjects, then according to the formula
Snellen visus = d / D, in which
d is the distance from which
the patient actually sees the line, D
– the distance from which the patient should
would see a line with visual acuity
1, visual acuity is 5/5, that is, 1.0.
This is the 10th row in the table. Above her is the 9th
the character string is constructed this way
that from 5 meters they can be read at
visual acuity less by 0.1, that is
0.9, etc.

Visus is measured in abstract
units. Visual acuity depends on
the diameter of the cones in the fundus, then
there is the smaller it is, the visual acuity is
better.

If the subject does not see
top line from 5 m (he has
visus <0.1), then the finger count is checked from a distance of up to 0.5 m.If the patient is not sees this too, then light perception is checked (visus = 1 /), which can be both with the correct one, and incorrect light projection.

Three main reasons leading to
decrease in visual acuity:

Clinical refraction (myopia,
hyperopia, astigmatism).
Clouding of the optical media of the eye
(cornea, lens, vitreous
body).
Retinal diseases and n.
Opticus.

Line of sight.

The field of view is that volume of space
which the human eye sees when
motionless field of gaze and motionless
position of the head (given that the field
gaze is useful to the vision of both eyes).The visual field is a function of the peripheral
department of the retina, namely the rod
apparatus.

Physiological boundaries of the visual field
depend on the state of the visual
apparatus of the eye and visual centers.

Scotoma – loss of part of the visual field.
Distinguish:

Physiological (blind spot, scotomas
due to the passage of blood vessels),
pathological.
Positive (human perceived)
and negative (imperceptible).
By location – central,
paracentral and peripheral.
Absolute – that is, in this area
the patient does not see anything at all and
relative – the patient continues
see, but the objects blur.

Color perception – function of the cone
apparatus, is determined using tables
Rabkin.

M.V. Lomonosov in 1975 showed for the first time
what if 3 lights are counted in the color wheel
basic, then mixing them in pairs
(3 pairs) you can create any others
(intermediate in these pairs in color
circle).This was confirmed by Thomas Jung in
England (1802), later Helmholtz in Germany.
Thus, the basic
three-component color theory
vision. There are 3 primary colors:
red, green, purple, with them
mixing you can get any colors,
except for black.

Dark adaptation – adaptation
organ of vision to conditions of reduced
illumination. Breaking the dark
adaptations are called hemeralopia (chicken
blindness). Her types:

symptomatic – occurs when
various diseases of the organ of vision
(retinal pigmentary degeneration)
essential – associated with a deficiency
vitamin A, liver disease
(xerophthalmia).

90,000 Health and stereo 3D. Part one, physiological / Smart things

Advances and advances in technology are somehow linked to concerns about potential health problems. Sometimes these fears are completely justified: a lot of unsuspecting people have time to die before it turns out that radiation can be fatal. Sometimes these fears are mostly far-fetched: today, few people remember about the passions that seethed at the time of the appearance of color televisions, but it came to recommendation, ridiculous by today’s standards, not to sit in front of the screen for more than half an hour.

But more often than not, fears about the potential harm of a particular new technology to health hang in uncertainty, dividing even scientists into two almost equal camps of skeptics and optimists. Until now, no one can put an end to the protracted dispute about the potential dangers of mobile phones, although both sides managed to refute each other’s arguments a thousand times over. And the controversy about the health effects of genetically modified foods!

However, it is unlikely that the inhabitants of large cities, who daily and no longer in the first generation inhale a hellish mixture instead of air, may not understand the nature of the occurrence of numerous allergies.And if after a couple of generations everyone’s tails grow at once, what difference does it make if this happens because of the frequent attachment of the cell phone to the brain or from the regular eating of artificially modified oatmeal?

Stereoscopic 3D vision, which has become fashionable in the wake of the latest advances in electronics, also has not been without accusations. More often they talk about the potential to influence the well-being of the viewer, but there are also direct accusations of the possibility of causing harm to vision and health in general.As in any other, not fully studied industry, in the wake of increased interest, many scientific, semi-scientific and even charlatan (or paid, but no less charlatan) statements about the harmful effects of stereoscopy regularly appear. Or, conversely, about the benefits.

In this publication, we are not going to either defend or refute anyone’s calculations or conclusions on the topic of stereo 3D. Today we will try to “dissect” this question, to decompose the general problem into its constituent components.And only after that, having analyzed the details, we will try to draw cautious conclusions – what exactly is worth being afraid of, and where they are simply trying to confuse and intimidate us with pseudoscientific rubbish.

If you really want to understand the nature of the formation of a stereoscopic image, so as not to be like those who write in the comments “3D sucks!” losers, we will have to delve into some of the details of human physiology, somewhat beyond the scope of a school textbook of anatomy.Believe me, the author does not want to burden the reader with unnecessary or common truths at all, but nevertheless I have to remind you of something, otherwise we will not succeed in a substantive conversation.

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Physiology of vision: stereoscopy and color

So, stereoscopic (binocular) vision is the greatest gift at the disposal of a person. The eyes in the stereoscopic visual system appear as two sources of visual information, the pupils of which are spaced about 65 mm apart in space.Do not forget that a three-dimensional picture is, first of all, the result of the work of our brain, which builds a three-dimensional image as a result of synchronous processing of two streams of information.

Due to the perception of the surrounding reality by each eye at a certain angle relative to the other eye, the visual section of our brain has the ability not only to restore volume, but also to estimate the sizes of various objects and the distance to them with a greater or lesser degree of reliability.It is on this that the simplest methods of “deception” of sight in cinema are based, when on the screen we see midgets or gullivers, depending on the director’s idea and the skill of the operator. At the same time, additional information about the size of visible objects and about the distance to them can be obtained even with one eye, but in motion, and this technique is also perfectly used in cinematography and games, even without any 3D.

The next important point is the internal structure of the eye. Within the framework of today’s article, we are interested in such specific characteristics as “autofocus”, aperture and angle of view.

The eye lens – a kind of “lens” of the visual system – has a wonderful property to automatically refocus the incoming information on the retina (again, the word “automatically” is used here to simplify the complex feedback processes between the eye muscles, optic nerves and the visual brain). This happens due to a change in the shape of the lens itself under the influence of certain eye muscles (according to one version, this is the annular ciliary muscle, according to the other, these are oblique and longitudinal muscles, but we do not need these subtleties at all to establish the truth about 3D).

A hole in the iris, known as the pupil, plays the role of a variable aperture, limiting excess light from entering the retina. This system also operates almost instantaneously, ranging from about 2 mm (f / 8.3) at significant brightness to 8 mm (f / 2.1) in the dark (normally, the pupil diameter is about 4 mm), and with a significant excess of the light flux, additional mechanisms also come into play – eyelashes when we squint, or even eyelids, when the brightness is completely unbearable.Such facts as the perception of the retina of contrast at a level of about 1: 100 and the ability to distinguish even 1: 1,000,000 after a very long adaptation in the dark, as well as “cones” and “rods” for the perception of color and monochrome information, we shall consider within the framework of this article we will not. They are, of course, important in the formation of a stereoscopic picture, but their role is secondary.

The last important point for us is the angle of view. On average, the field of view of each eye includes a view of 95 ° external field horizontally, 60 ° internal field horizontally, 60 ° up and 75 ° down (in fact, the circular view diagram for different eyes has a very specific shape and is individual for each).Without being distracted by the peculiarities of the optic nerve (like a “blind spot”), which are secondary for us today, let us focus on the fact that the zone of high optical sharpness, in which our eye sees the clearest image, is … only 2 degrees.

Yes, that’s right. It may seem to you that you are seeing at the same time with great sharpness in a much larger angle of view, but this is not so, for the perception of a high-definition image over a large area, the signal system from the retina, optic nerve and brain simply needs constant eye movement.

Now – attention! A very important point for understanding further: the movement of each of the eyes is provided by six muscles: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique and superior oblique muscles of the eyeball.

However, this is not all: there are many ways to move the eyeball . For example, eyes resting in a dream move under the eyelids in a completely different way than when tracking a moving object. And even if you look at a stationary object or, as they say, look at one point, the eyeballs still make small micro-movements, usually at an almost imperceptible angle of about 0.2 degrees.

But that’s not all! Imagine that control over the muscles that determine the displacement of the eyeballs is in different cases different areas of the brain! For example, if we talk about the vestibulo-ocular reflex – when, for example, you follow a moving object with your eyes and at the same time turn your head yourself, in this case the frontal lobes of the brain control the movement of the eye muscles.

Now imagine how difficult it is to control the movement of each eye, and even more so with two eyes simultaneously.

But for the final point in the definitions of today’s article, it is necessary to describe another function of the eye muscles – the so-called “convergence of the optical axes of the eyes”, or convergence. The essence of this function is very simple: if the object under consideration is far enough from the viewer, then the conditional optical axes “lens – retina” of two eyes can be represented as practically parallel lines. But the closer the object under consideration is, the greater the angle of convergence of these optical convergent axes.Up to examining the tip of your own nose, when the eyes are already incredibly squinting, and the optical axes intersect here, on the nose.

Of course, the closer the object under consideration is, the more detailed, if you will, “versatile” information about the volume of this object is received by the brain, and vice versa, the further away, the more similar the “picture” received by two eyes will be and the less information about the volume will be received by the brain.

However, this medal also has a downside: even on the tip of the nose at the same time, both eyes can no longer really focus.If you try to transfer information with “transcendental” convergence to the brain through the eyes with the help of technical means, you can imagine how your brains will “boil” in such a situation.

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Subtotals: the surprising is near, but is it forbidden?

I would like to congratulate those who have honestly read up to this moment and got imbued with the details about the principle of our vision. Now it will be much easier for you to deal with the reasons for indifference to stereo 3D or rejection of it by you or your friends, as well as with the reasons for the occurrence of certain unpleasant symptoms when watching stereoscopic content.

Now you and I are sufficiently prepared to examine under a magnifying glass every technology popular today for forming a stereoscopic image – anaglyph, passive-polarization, active-shutter, autostereoscopy, and also quite reasonably determine the flaws of each of them from the point of view comfortable (or, conversely, uncomfortable) perception of a volumetric image.

In addition, we are now fully prepared to substantively discuss all the “shoals” of game developers and Hollywood goofs, resulting in the emergence of “suck” samples of stereo 3D-culture.

Thus, we have arrived at the following classification of the types of 3D stereo perception problems:

Physiological problems – deviations and health disorders, including vision problems, excluding or limiting a person’s ability to perceive the surrounding reality in the volume of
Technological problems – imperfection of technical means for reproducing stereoscopic content, leading to poor-quality and / or insufficiently comfortable perception of a virtual 3D picture
Human Factors Issues – stereoscopic 3D content, technically and artistically unsuccessful, produced by inexperienced, insufficiently trained and / or untalented professionals.

Today, having meticulously understood the structure of the human eye, its interaction with the brain, plus armed with some statistics, we will draw several conclusions regarding the problems of perception of a stereo 3D physiological plan. We will look at other aspects in the second part.

First conclusion: d Even if you take a perfectly shot 3D movie or a perfectly made stereo 3D game, even if you assemble an excellent installation for viewing stereo 3D, stereoscopic content is initially available, alas, not to every .According to statistics, about 8% of viewers are completely unable to perceive stereo 3D in any form. The reasons are different, but the result, alas, is the same. The author of these lines, who twice lost the lens of his left eye and restored it twice, sincerely sympathizes with those who cannot see the volume with two eyes at all.

In fact, the lack of volumetric vision is the fate of not only those who look at the world with one eye. The fact is that, according to numerous scientific studies, volumetric perception of the surrounding reality is formed in the human brain only in the first years of his life .Moreover, the formation process actually ends at preschool age, at about 5-6 years. For everyone, it happens in different ways, and the depth of stereoscopic perception reaches a purely individual level: for some it is more, for others it is less, and for some it remains almost without development at all.

So two sighted eyes are not yet a guarantee of normal perception of stereoscopic 3D content.

In addition, there are many additional physiological factors that limit the perception of a three-dimensional picture and even volume, as such, in dynamics.The entire spectrum of ophthalmic problems – from poor development of one (or both) optic nerves to retinal detachment, from different diopters for each eye to uneven development of the muscles of each eye, and so on, and so on.

In the end, the developmental features of the vestibular apparatus can make it difficult to watch stereofilms and lead to dizziness or even nausea. What is there to go far: with all his fanaticism in relation to stereo 3D and a great love for volumetric cinema, the author of these lines experienced quite a nasty feeling on himself when watching the first minutes of the film “Smurfs 3D”.Despite the cleverly selected background and excellent reproduction of perspective, the “falling” angle of the camera spinning in a corkscrew, with all due respect, cannot be called successful. If not to say even more rudely (but I would like to). And this despite the fact that further, throughout the film (and in other 3D-tapes), such a nasty feeling never had to be experienced.

Second conclusion: p Imagine the entire range of the possible state of vision and the vestibular apparatus of a person – from completely healthy to complete blindness.Without even coming close to the problems of technology, creativity in the creation of films and games in today’s publication, we can already state the fact: out of the average 15% of viewers leaving 3D cinemas with the experience of unpleasant sensations, not everyone should blame bad 3D technology or bad 3D content , the problems most likely lie in their state of health.

We will talk about the rest in the second part of our material.

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Anatomy of the eye and adnexa Ophthalmic

The organ of vision has a complex structure and is represented by the optical system, neural pathways and neurons of the brain, which analyze the information received. How a person perceives the world around depends on the functioning of each link.

Eye structure

The human eye is a paired sensory organ that perceives electromagnetic waves and provides the function of vision.Three shells are distinguished in the structure of the eye:

Outer shell. Represented by fibrous tissue, which has a high density and strength. Due to these properties, it maintains the shape of the eyeball, gives it elasticity and is the place of fixation of the oculomotor muscles. In the front part, the outer shell is represented by a transparent cornea, which has a refractive power and is one of the components of the optical system. The back of it is called the sclera.
Medium casing. Consists of a dense network of blood vessels and is responsible for eye trophism and excretion of metabolic products. It contains the choroid, the iris and the ciliary body. The choroid provides nutrients to the retina and rebuilds the permanently degraded visual pigments. Each person’s iris has its own color, it is represented by muscles that change the size of the pupil, thereby regulating the amount of incoming light. The ciliary body fixes the lens, participates in the process of accommodation and releases the aqueous humor necessary for the normal functioning of the eye.
Inner sheath or retina. Represented by receptors that convert electromagnetic waves in the form of light into information, which is then transmitted to the brain.

Another important component of the organ of vision is the lens. It is a biconvex lens that participates in the processes of light transmission and light refraction. Depending on the conditions, it can change its shape, which allows you to focus your gaze on the desired object.The lens is part of the optical system of the eye and provides clear vision both near and far.

On its way, light passes not only the lens, but also the fluid of the anterior and posterior chambers of the eye, which is called aqueous humor. Normally, it is transparent. The secretion of aqueous humor occurs in the ciliary body. Then it enters the posterior chamber of the eye, and from there through the pupil into the anterior chamber. From there, aqueous humor is absorbed into the bloodstream through the Schlemm canal. Up to 9 ml of aqueous humor is produced per day, which constantly circulates and supplies the lens, cornea and vitreous with nutrients.In addition, it plays an important role in light refraction, participates in the creation of normal intraocular pressure and protects the internal structures of the eye from adverse factors.

The cavity of the eyeball, located between the lens and the retina, is filled with a substance resembling a transparent gel – this is a stele body. The vitreous body has fixation points in which the intraocular structures are held, ensuring their normal position. The vitreous body provides the eye turgor and its correct anatomical shape.

With various types of visual impairment, one or another anatomical structure is involved in the pathological process. The task of the ophthalmologist at the diagnostic stage is to find the cause that led to the disease. For this, the specialist is assisted by various research methods that allow obtaining objective information about the state of different membranes of the eye.

Somov V.V. Clinical ophthalmology. – M .: MEDpress-inform, 2005.

Avetisov E.S. et al. Pediatric Ophthalmology Guide.- M .: Medicine, 1987.

Eye diseases: textbook / ed. A.A. Bochkareva – M .: Medicine, 1989.

Anatomy and physiology of the eye | Likon – contact lenses and care products Kiev.

23 Apr 2010

The eye socket is a bony cavity or cavity in which the eyeball is located. It consists of seven small bones that protect the eye from physical damage.

Adjacent Organs – The tissues surrounding the eye, including the eyelashes and eyelids, are called adjacent organs.

The eyelashes consist of several rows of short vellus hairs located along the edge of the eyelids and perform the following functions:

trapping air dust particles, eyelashes prevent clogging of the eyes;

contribute to the rapid closure of the eyelids with the threat of damage.

Eyelashes are characterized by natural growth and shedding. New lashes grow quickly.

Each eyelid is made up of a movable plate of fibrous tissue that covers and opens the front of the eyeball. Outside, the eyelids are covered with skin, and from the inside – with a mucous membrane. By distributing the tear fluid secreted by the lacrimal glands, the eyelids protect the eye from any foreign matter that could damage the cornea. The lacrimal openings are also located on the eyelids, through which the tear is diverted.

Both eyelids are covered with a sensitive membrane called the conjunctiva. The palpebral conjunctiva lines the inner surface of the eyelids and becomes the bulbar conjunctiva. The bulbar conjunctiva is located on the front of the eyeball (the visible white part of the eye) and is in contact with the surface of the palpebral conjunctiva.

Since the conjunctiva is a continuous membrane, contact lenses cannot be behind it, that is, behind the eye.The conjunctiva is in constant direct contact with the lenses, so a healthy conjunctiva is needed to successfully use contact lenses. When the eye is irritated, the blood vessels of the conjunctiva dilate and the eye turns red. The conjunctival membrane contains glands that promote the formation of a tear film to moisturize the surface of the eye.

The layer of water and nutrients on the cornea is called the tear film. This film is constantly being produced and removed from the surface of the eye.The lacrimal glands of the eyelids are involved in its formation, the outflow occurs through the lacrimal openings into the nasolacrimal duct and further into the nose. The tear film provides nourishment to the cornea in the form of oxygen, glucose, salts and minerals. It moisturizes the cornea and eyelids, preventing them from drying out, and also acts as a lubricant, making it easier for the eyelids to slide over the cornea. Foreign particles dissolve in the tear film and are removed from the surface of the eye. Dangerous microorganisms are exposed to the antibacterial enzymes (lysozymes) of the tear.The tear film consists of three layers: the outer lipid (fatty) layer, the central aqueous layer and the inner mucin layer, which facilitates the connection of the film with the cornea.

The lipid layer is a fatty film that prevents the tear film from drying out. The aqueous layer is a weakly alkaline solution (pH 7.35), consisting of water (98%), mineral and nutrients, enzymes, (ions), dissolved salts and proteins. This layer, which makes up 90% of the thickness of the tear film, provides the cornea with oxygen and nutrients. The mucinous (mucous) layer connects the tear film and the corneal epithelium.

An important factor in adapting to contact lenses is the chemical structure of the tear film, since the lens rests on it. Typical film thickness is 7 µm.The average volume of the eye lacrimal fluid is 6 μl. 10–20 seconds would be enough for all the tear film to evaporate, but we involuntarily blink every 5–10 seconds and restore the film.

A patient with a violation of any layer of the tear film is not very suitable for wearing contact lenses. The condition of the tear film can be assessed in two ways: in the Schirmer test, strips of paper are used to indirectly determine the amount of the water layer of the film, the time to disintegration test (TBP) shows the time it takes for the tear film to evaporate from the surface of the cornea (normally 10–15 seconds).

The cornea is a transparent, avascular, highly sensitive, multi-layered, dome-shaped membrane that limits the anterior chamber of the eye. Convex at the center, the cornea flattens at the junction with the sclera, which is called the limbus. The changing curvature of the cornea makes it difficult to measure and fit contact lenses.Therefore, contact lenses usually have a different curvature: central peripheral.

90,000 The structure of the eye in animals, the perception of the surrounding world. | “Companion”

The eye is a spherical organ, states of three main membranes (outer, middle and inner). It has internal structures, due to the structure of which there is a reception and transmission of impulses for processing in the brain. Each structure of the eye has its own purpose.Briefly, but in detail, let us dwell on each of them.

The lens (5) is a transparent, biconvex structure located inside the eyeball and is located immediately behind the iris (2) of the eye. The function of the lens is to focus the light source on the retina of the eye (6). The iris is the colored part of the eye, located behind the cornea (4) in front of the lens, contains melanin pigment. Depending on the amount of pigment, the iris has a peculiar color and is individual for each species of animals and humans.Thanks to the iris, the biometrics of the human eye have been created. The iris helps control the amount of light entering the eye through the pupil (1).

The cornea (4) of the eye in animals occupies almost the entire visible part of the eye, is transparent, transmits light to the retina of the eye.
The ciliary (ciliary body) is located behind the iris near the lens. The main functions of the ciliary body: the production of intraocular fluid (aqueous humor), which fills the anterior sphere of the eye.In addition, the ciliary body consists of muscles, thanks to which the eye can focus on objects at different distances.
The retina, or retina (6), is made up of nerves that line the back of the eye. The retina perceives light and creates impulses that are sent through the optic nerve (7) to the brain. The optic nerve is located at the back of the eye. This structure is responsible for the visible image. The retina is riddled with millions of nerve fibers that converge to form the optic nerve.When light hits the retina, it turns into a nerve impulse and is transmitted along these fibers along the optic nerve to the brain. This is how animals perceive the world around them.

Features of the anatomy and physiology of the child’s eye

A child’s visual system is different from that of an adult and is constantly evolving.

How do newborn children see when they recognize their mother and why is the color of the eyes in the first days of life far from always final?

Anatomy and physiology of the organ of vision in children

The child’s eyeball is still too small, which is why the focusing of light rays does not take place on the retina, but behind it.So farsightedness at an early age is a natural physiological phenomenon!

Babies see objects blurry, they do not distinguish small details, they focus better at a distance of 20-25 cm. Curiously, this is the approximate distance to the mother’s face when she holds the baby to her breast.

With growth and development, hyperopia decreases and completely disappears by the age of 6-7 years. The eyeball grows significantly in the first year of life, at the age of 5 it is almost the same as that of an adult, but it will finally form only by the age of 17-18.

Also, the retina, optic nerve, oculomotor muscles continue to actively form, the visual cortex of the brain develops – everything you need not only for vision, but also for perception and understanding of what you see.

Children’s vision at different periods

Even in the womb (at about 5 months), the baby reacts to light or lack of it. In the first 2-3 weeks, the child focuses his gaze only on large objects.By the end of 1 month, he can already distinguish silhouettes.

From 2 months, the baby learns to focus his gaze, to follow moving objects. It is especially good to catch bright objects with a glance, because around the same period, the child begins to perceive colors. Best of all distinguishes between red and yellow, later begins to see green and blue, and then other colors and shades.

At the age of three months, objects become visible up close. And the baby also recognizes the faces of the closest ones – those who are constantly in contact with him, repeats their facial expressions.From the 4th month of life, the baby can distinguish shapes and basic colors. From the 7th – he already sees different objects well: shape, size and color play a role. And from the 8th he recognizes familiar people, distinguishes their faces.

At 1–1.5 years old, children actively comprehend the world around them, but they also see a two-dimensional picture. And only when they start to crawl and walk – to move independently in space – binocular vision begins to form. Binocular, or stereoscopic, spatial vision is the ability to see with both eyes, when the brain “adds up” a single volumetric image from what it sees.The process of development of binocular vision at the age of 7–15 years comes to an end in full.

It is important to remember that each child is unique and the described developmental periods are individual, may differ by 1–2 months. But if something bothers you in the development of your baby, be sure to seek help from a specialist, sign up for a diagnosis. After all, the earlier a problem is discovered, the easier it is to solve it.

How vision is tested in children

The ophthalmologist performs the first examination immediately after birth.If there are no problems, eyesight is checked in the first month of life, 1 year, at the age of 3 and 5-6 years. At school, children undergo annual medical examinations.

Depending on the age, during the examination, the ophthalmologist notes whether the baby’s gaze is fixed on objects and whether he reacts to light, checks eye movement, face recognition, and reactions to stimuli. For example, although immediately after birth, the child still does not distinguish between objects and objects, but in the first 4–6 weeks of life, the reaction to light is checked – the pupils constrict as a defense mechanism.

In preschoolers, vision is assessed using special tables with images of objects: a mushroom, an asterisk, a horse, etc. Before that, you need to make sure that the pictures are well known to the baby. They are asked to draw a well-known figure, for example, a circle (with one eye closed in turn). When they learn the alphabet using the traditional letter table.

Children sometimes do not complain to their parents about visual defects, simply because they do not know how things really should look.Poor vision delays development, which is why regular check-ups with an ophthalmologist are so important. Adults also need to track alarming symptoms: a child squints, literally buries his nose at a TV or tablet monitor, reads or examines pictures at close range, complains of a headache, rubs his eyes, often stumbles, and loses things.

Interesting and important to know!

In the first months of life, the sensitivity of the cornea is reduced.If something gets into the eye, then the newborns do not feel pain and do not react, which means it may go unnoticed by the parents. Foreign bodies – cilia, dust particles, villi – threaten with inflammation.
Light is necessary for proper development of eye muscles and pigmentation, but not direct sunlight. The child’s lens is not yet fully formed. While it is developing, it is not protected from the harmful effects of ultraviolet radiation, so it is advisable to go for walks with a child in a panama hat, a cap or in a stroller with a visor.
Babies cry without tears at first – this is normal. But if in the first couple of months of life the mucous plugs that close the tear ducts do not dissolve, it is worth contacting an ophthalmologist.
It often happens that immediately after birth, the iris in newborns is very light or even blue. This is due to the lack of pigmentation. The final eye color is formed most often by the age of two.
Children’s vision needs to be developed: hang different bright toys and moving mobiles at a distance of 20–30 cm, periodically replace them with new ones; take the baby in your arms and show him things around; actively use different facial expressions in communication with your child; put it in the crib in different ways so that the eyes do not squint to one side.
Considering the fact that the visual system continues to form until the age of 18, children with vision problems are not recommended to carry out laser correction.Instead, it is more advisable to choose non-surgical methods for improving vision, adapted to age-related changes. These are accommodation training, physiotherapy procedures, hardware treatment, special night lenses and other methods that help correct vision and prevent eye diseases.

The physiology of the eye. Human Eye Anatomy and Physiology: Understanding the Complexity of Vision

The Structure and Function of the Human Eye

The Eye’s Position and Protection

The Journey of Light: From Cornea to Retina

The Cornea: The Eye’s First Lens

The Iris and Pupil: Nature’s Camera Aperture

The Crystalline Lens: Fine-Tuning Focus

Accommodation: The Eye’s Autofocus Mechanism

The Retina: Where Light Becomes Signal

Rods and Cones: Specialized Light Detectors

The Optic Nerve: The Visual Information Highway

Extraocular Muscles: Controlling Eye Movement

The Role of Oblique Muscles

Tear Production and the Lacrimal System

The Lacrimal Gland and Tear Drainage

Visual Processing: From Eye to Brain

Visual Pathways and Brain Specialization

Common Eye Conditions and Their Impact on Vision

Refractive Errors and Corrective Measures

The Evolution of the Human Eye

Comparative Eye Anatomy Across Species

The Future of Eye Care and Vision Technology

The Integration of AI in Ophthalmology

Eye anatomy and physiology – how the eye and vision work

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Anatomy and physiology of the eye | Likon – contact lenses and care products Kiev.

Features of the anatomy and physiology of the child’s eye

Anatomy and physiology of the organ of vision in children

Children’s vision at different periods

How vision is tested in children

Interesting and important to know!

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