How does light transform into visual perception. What are the key components of the eye’s anatomy. How do physical properties of light interact with eye structures. What cellular mechanisms enable vision. How does the brain process visual information.

The Evolution and Importance of Human Vision

Human vision has come a long way from our primitive ancestors’ dichromatic sight, which only allowed for the interpretation of UV and red light. Approximately 30 million years ago, a significant evolutionary leap occurred with the development of trichromatic vision, thanks to the evolution of opsin genes. This advancement enabled humans to perceive a broader spectrum of colors, including black, white, red, green, blue, and the various shades in between.

The journey of visual perception, from the moment light reflects off an object to the creation of a visual image in the brain, is a complex and fascinating process. Understanding this intricate system requires delving into the anatomy and physiology of the eye, as well as the physical properties of light and vision.

The Physical Properties of Vision: How Light Interacts with the Eye

Vision is fundamentally a process of light interpretation. But how exactly does light enter the eye and get transformed into neural signals that the brain can understand?

The eye functions similarly to a camera, with a series of lenses and spaces that focus images. These components include:

• Vitreous humor
• Aqueous humor
• Crystalline lens
• Cornea

Each of these structures has its own refractive index, with an average of 1.34 due to their tissue composition. The refractive index is crucial in understanding how light behaves as it passes through different mediums.

Understanding Refraction in Vision

Refraction refers to the change in direction of a wave as it passes from one medium to another. In the context of vision, it’s the relationship between the velocity of light in air compared to its velocity when traveling through an object. While light travels at an astounding 300,000 km/s in air (with a refractive index of 1), its speed and trajectory change as it passes through the various structures of the eye.

This change in light behavior is essential for proper focusing of images on the retina. Any disease or condition that affects the refractive properties of the eye can significantly alter vision quality.

The Dynamic Role of the Crystalline Lens

The crystalline lens plays a crucial role in focusing light onto the retina. It’s a dynamic structure, capable of changing shape to accommodate for viewing objects at different distances. How does this work?

• For distant objects: The lens remains in a stretched, flatter configuration, allowing light to travel further into the eye due to a lower refraction index.
• For close objects: The lens becomes more spherical, causing light waves to converge at a closer point.

This shape-changing ability, known as accommodation, is controlled by the parasympathetic nervous system. The ciliary muscles contract, altering the lens shape and enabling clear focus on objects at varying distances.

The Cellular Basis of Vision: Rhodopsin and Light Absorption

At the cellular level, vision relies on a remarkable molecule called rhodopsin. This visual pigment, a modified form of vitamin A, is the key player in converting light into neural signals.

The Structure and Function of Rhodopsin

Rhodopsin consists of two main components:

1. Scotopsin: A protein component
2. Retinal: A photoreactive chromophore derived from vitamin A

The retinal component is covalently bound to one of the protein’s lysine residues in a protonated Schiff base (-N+=CH-). This structure is critical for rhodopsin’s light-absorbing properties.

How Rhodopsin Absorbs Light

The process of light absorption by rhodopsin involves several steps:

1. Photons elevate electrons within the conjugated pi system to higher energy orbitals.
2. This energy causes the retinal molecule to isomerize, changing from a cis to trans configuration (a 180-degree rotation).
3. This structural change triggers a cascade of events that ultimately leads to the generation of an electrical signal.

The Retina: Where Light Becomes Electrical Signals

The retina is a complex neural structure lining the back of the eye. It’s here that the conversion of light into electrical signals occurs, setting the stage for visual perception.

Photoreceptor Cells: Rods and Cones

The retina contains two types of photoreceptor cells:

• Rods: Responsible for vision in low light conditions (scotopic vision)
• Cones: Enable color vision and high visual acuity in bright light (photopic vision)

These cells contain the rhodopsin molecules that absorb light and initiate the visual transduction cascade.

The Visual Transduction Cascade

When light activates rhodopsin, it triggers a series of biochemical reactions known as the visual transduction cascade. This process involves several key steps:

1. Activation of the G-protein transducin
2. Stimulation of cGMP phosphodiesterase
3. Reduction of intracellular cGMP levels
4. Closure of cGMP-gated ion channels
5. Hyperpolarization of the photoreceptor cell

This hyperpolarization leads to a decrease in neurotransmitter release, which is interpreted by the brain as a light signal.

From Retina to Brain: The Visual Pathway

Once the electrical signals are generated in the retina, they must be transmitted to the brain for processing and interpretation. This journey involves a complex network of neurons and specialized brain regions.

The Optic Nerve and Optic Chiasm

The axons of retinal ganglion cells bundle together to form the optic nerve. These nerves from both eyes meet at the optic chiasm, where some fibers cross over to the opposite side of the brain. This partial crossing allows for binocular vision and depth perception.

The Lateral Geniculate Nucleus and Visual Cortex

From the optic chiasm, visual information travels to the lateral geniculate nucleus (LGN) of the thalamus. The LGN acts as a relay station, processing and sorting visual information before sending it to the primary visual cortex in the occipital lobe.

In the visual cortex, different aspects of the visual scene are processed in specialized areas:

• V1 (primary visual cortex): Basic feature detection
• V2: Simple shape recognition
• V4: Color processing
• V5/MT: Motion detection

Color Vision: The Trichromatic Theory

Human color vision is based on the trichromatic theory, which posits that we have three types of cone cells, each sensitive to different wavelengths of light:

• S-cones: Short wavelengths (blue)
• M-cones: Medium wavelengths (green)
• L-cones: Long wavelengths (red)

The combination of signals from these three types of cones allows us to perceive the full spectrum of colors. How does this work in practice?

When light of a particular wavelength enters the eye, it stimulates the three types of cones to different degrees. The brain interprets the relative activation of these cones to determine the perceived color. For example, yellow light stimulates both red and green cones, leading to the perception of yellow.

Color Blindness and Its Causes

Color blindness occurs when one or more types of cone cells are absent or functioning abnormally. The most common form is red-green color blindness, which affects about 8% of males and 0.5% of females. What causes this condition?

Color blindness is typically genetic, caused by mutations in the genes that code for cone photopigments. These mutations are usually X-linked recessive, which explains the higher prevalence in males. In rare cases, color blindness can also be acquired due to damage to the retina, optic nerve, or brain areas involved in color processing.

The Importance of Eye Health and Regular Check-ups

Given the complexity of the visual system, maintaining eye health is crucial for optimal vision. Regular eye check-ups can help detect and address potential issues early on. What should these check-ups include?

• Visual acuity tests
• Refraction assessment
• Eye pressure measurement
• Examination of the retina and optic nerve
• Color vision testing

Early detection of eye conditions such as glaucoma, macular degeneration, or diabetic retinopathy can significantly improve treatment outcomes and prevent vision loss.

Protecting Your Vision in the Digital Age

In our increasingly digital world, eye strain and computer vision syndrome have become common issues. How can we protect our eyes while using digital devices?

1. Follow the 20-20-20 rule: Every 20 minutes, look at something 20 feet away for 20 seconds
2. Adjust screen brightness and contrast for comfort
3. Use anti-glare screens or glasses
4. Ensure proper lighting in your workspace
5. Take regular breaks from screen time

By implementing these strategies and maintaining regular eye check-ups, we can help preserve our vision and ensure optimal eye health throughout our lives.

Future Directions in Vision Research and Treatment

The field of vision science continues to advance, offering new insights and potential treatments for various eye conditions. What are some of the exciting developments on the horizon?

Gene Therapy for Inherited Eye Diseases

Researchers are making significant progress in developing gene therapies for inherited retinal diseases. These treatments aim to replace faulty genes or introduce new genes to correct genetic abnormalities that cause vision loss. For example, Luxturna, a gene therapy for a rare form of inherited blindness, was approved by the FDA in 2017, marking a milestone in the treatment of genetic eye disorders.

Artificial Retinas and Visual Prostheses

For individuals with severe retinal degeneration, artificial retinas and visual prostheses offer hope for restoring some degree of vision. These devices typically involve an external camera that captures visual information and transmits it to an implant in the retina or visual cortex. While current devices provide limited visual acuity, ongoing research aims to improve their resolution and functionality.

Stem Cell Therapies for Retinal Regeneration

Stem cell research holds promise for treating conditions like age-related macular degeneration and retinitis pigmentosa. Scientists are exploring ways to use stem cells to regenerate damaged retinal cells or create new photoreceptors. Early clinical trials have shown encouraging results, though more research is needed to ensure the safety and efficacy of these treatments.

Advanced Imaging Technologies

Cutting-edge imaging technologies are revolutionizing how we diagnose and monitor eye diseases. Optical coherence tomography angiography (OCTA), for instance, allows for non-invasive visualization of retinal blood vessels, aiding in the early detection of conditions like diabetic retinopathy and glaucoma. What other imaging advancements are on the horizon?

• Adaptive optics scanning laser ophthalmoscopy: Provides cellular-level imaging of the retina
• Hyperspectral imaging: Enables detailed analysis of retinal metabolism
• Artificial intelligence-assisted image analysis: Improves accuracy and efficiency in diagnosing eye conditions

These technologies not only enhance our understanding of eye diseases but also pave the way for more personalized and effective treatments.

Neurofeedback and Vision Training

Emerging research suggests that neurofeedback and vision training techniques may help improve visual function in certain conditions. These approaches aim to enhance neural plasticity and optimize visual processing in the brain. While still in early stages, these techniques show promise for conditions like amblyopia (lazy eye) and certain forms of visual impairment.

As our understanding of the visual system continues to grow, so too does our ability to develop innovative treatments and preventive strategies. The future of vision science holds exciting possibilities for preserving and enhancing this crucial sense that connects us to the world around us.

Physiology, Vision – StatPearls – NCBI Bookshelf

Introduction

Vision is one of the five senses the body uses to interpret its surroundings. In the past, our primitive ancestors had what is called “dichromatic vision,” allowing for interpretation of only UV light and red light. About 30 million years ago, the trichromatic part of vision came to existence due to the evolution of opsin genes.[1] Humans can now see black, white, red, green, and blue, as well as the colors in between this spectrum since the retina and the brain are equipped to differentiate them. What happens between an object and a synapse in the most posterior part of the brain is the fascinating journey that we will cover.

Issues of Concern

Physical Properties of Vision

Vision cannot be discussed without knowing the physical properties of optics. The eye receives light that then is traduced into energy. That energy goes into the optic nerve as an action potential and travels to specific nuclei in the brain, where it is processed. But, how does that light get into the eye to be processed into an action potential to be sent to the brain?

The eye is composed of a series of lenses and spaces that give focus to images, just as a camera does. It is composed of the vitreous humor, aqueous humor, the crystalline lens, and the cornea, and each of these has its own refraction index (the average being 1.34, because of the content of these tissues). Light travels in the air in the form of waves. The term “refraction index” refers to the relation between the velocity of light in the air compared to its velocity when it travels across an object. Light travels at a velocity of 300,000 km/s in the air.

The index of refraction of air is 1, the same value as in a vacuum. This refraction index changes when light travels through objects, as it gets slower going through glass, for example. With all of the above then we can infer that light gets slower and its trajectory gets modified slightly as it goes through the eye, and it can be also inferred that every disease that affects the refractive properties of the eye will significantly alter vision. [2][3]

When light waves come across a spherical lens, these waves converge into a focal point, and in the eye, this focal point is projected towards a single area, which is the retina. For this to be accomplished, the crystalline lens must be a dynamic structure. The lens is a capsule filled with water and filamentous proteins; it has a stretched configuration in resting. So, if we imagine this light wave coming across this stretched, very flat lens, then we can assume that light will go farther into the eye because the refraction index is lower. With this crystalline configuration, we can see things clearly even though they are far away because they get projected further into the eye. But when we focus a closer object, then the lens has to change its shape into a more spherical one, in order for the light waves to converge into a closer point, as discussed earlier. This is accomplished by the parasympathetic system. So the nervous system has a role here! Yes, because the parasympathetic system is in charge of constricting fibers in the ciliary muscle. Contraction of these muscle fibers makes the crystalline lens become rounder, as will be discussed later. All of these processes have to be intact for the accommodation reflex to be accomplished.[4]

Cellular Level

At the heart of these organic devices is the visual pigment rhodopsin, a modified molecule of vitamin A. This molecule, which consists of allylic carbons, contains a great deal of conjugated, pi electrons. Recall in organic chemistry an allylic group is a carbon atom singly bonded to another carbon atom, which is in turn double bonded to a carbon atom. The electrons within these alternating pi bonds of the rhodopsin molecule are not as well-defined as the electrons in a saturated carbon chain (no double bonds) or in a singly double-bonded (think simple structure) molecule of ethylene (aka ethene, C2h5).[5]

Rhodopsin consists of the protein scotopsin and the photoreactive chromophore retinal, which is derived from vitamin A. Retinal is covalently bound to one of the protein’s lysine residues in a protonated Schiff base (-N+=CH-). The chromophore is the light-absorbing center of the molecule. It functions by facilitating the absorption of photons to potential energy, which supplies the energy needed to allow isomerization of the chromophore molecule from cis to trans (180-degree rotation). Although the reaction mechanism is involved, it can be summarized as:

• Photons elevate electrons within the conjugated pi system to higher energy orbitals (dictated by the level of resonance within the chromophore).

• The molecule rotates around the double-bond, changing from cis to trans configuration.

• By the end of the mechanism, the excited electrons’ energy levels have fallen back to the ground state.[6]

At the reaction completion, the chromophore has changed to the more stable trans configuration. Thus, within rhodopsin, light absorption leads to a chemical reaction that forces part of the rhodopsin molecule to translocate, changing protein conformation, and exposing active sites. This activated form of rhodopsin is known as metarhodopsin II. Before it can get to the metarhodopsin phase, rhodopsin decays through a series of intermediates, and these changes occur in the matter of milliseconds. Metarhodopsin II activates many copies of the G protein transducin (by replacing transducin’s GDP with GTP). Many activated transducin complexes activate cyclic nucleotide phosphodiesterase (PDE), which can itself hydrolyze 1000 molecules of cGMP to 5′-GMP per second. cGMP-gated channels in the plasma membrane of these rods (or cones) allow sodium ion influx at high cGMP concentrations; this is balanced by cation exchanger-mediated glutamate efflux, maintaining cell depolarization in dark conditions. At low cGMP concentrations, these channels close, stopping sodium ion influx and reducing glutamate efflux, all leading to cell hyperpolarization in light conditions. Thus, light-induced rod/cone state changes lead to hyperpolarization of the photoreceptor cells. Conversely, photoreceptor cells without the presence of light exist in the depolarized state. [7][8][9]

After this cascade of events, the enzyme rhodopsin kinase quickly binds metarhodopsin II, phosphorylating and halting its activity. The protein arrestin binds phosphorylated metarhodopsin II. Metarhodopsin II is unstable and will split within minutes, leading to opsin and free trans-retinal. Trans-retinal is transported to pigment epithelial cells that convert trans-retinal back to 11-cis-retinal, which eventually is recombined with opsin within cones/rods to reform rhodopsin. Guanylate cyclase restores cGMP concentration, and the cone/rod is ready to respond to another light exposure event.[9]

Additionally, phototransduction is subject to regulation by a calcium-mediated pathway to quickly diffuse a large gradient response, which is important in events such as sudden flashes of light in the dark. In dark conditions, the intracellular calcium level is high due to calcium diffusion through cGMP-gated channels. Lack of frequent light response leads to higher intracellular cGMP concentrations and allows more calcium to enter the cell per second. Calcium ion binding to rhodopsin kinase increases the rate of rhodopsin phosphorylation, reducing transducin activation. Calcium ion binding to guanylate cyclase accelerates the restoration of cGMP concentration. And calcium ion binding to calmodulin increases cGMP affinity to its gated channel.

Development

The surface of the human retina contains about three million cones and one hundred million rods, but there are just 1.5 million ganglion cells; meaning that for every ganglion cell, there are sixty rods and two cones. Cones transmit color information, whereas rods have greater sensitivity to low-light conditions. However, the distribution of rods and cones tends to be different depending on the part of the retina. In the central retina, for example, there are almost only cones and a lot of ganglion cells making synapses, which explains why the central retina confers the highest visual acuity. In contrast, in the peripheral retina, there are more rods than cones, and the visual acuity in these peripheral regions is decreased. There are several different types of ganglion cells: W, X, and Y ganglion cells. Some of these are responsible for detecting changes in color intensity (cones), and some are more specialized in detecting changes in contrast (rods). These differences depend on the part of the retina in which the ganglion cells are receiving the stimuli.[10]

The interneural connections of ganglia (bipolar cells) allow for low-level visual processing, adjusting the gain of the signal to transmit light gradients rather than absolute light intensity. Thus, relative differences within the light field and the object’s visual patterns are emphasized, as opposed to binary hit/miss signal information. This process is crucial because rods and cones can distinguish light intensity varying by ten orders of magnitude; however, the ganglia of the optic nerve can only transmit about 1% of this range.[11]

Color vision results from the combination of signals from three pigment types within cones: red, green, and blue pigments that correspond to cone types L, M, and S (RGB-LMS), respectively. Those colors correspond to the wavelengths of peak light absorption intensities of the modified chromophores. Remember, excited electrons are vital to producing Schiff-base modifications, which can be further classified as red shift or blue shift modifications.

Red shift or blue shift modifications denote whether the shift is toward peak absorptions at longer or shorter wavelengths, respectively. The average absorption maxima for 11-cis-retinal occurs at a wavelength of 380nm. If an experimenter were to expose 11-cis-retinal to EM radiation at this wavelength, the 11-cis-retinal would most readily absorb energy, as opposed to with an EM radiation at a wavelength of 280nm. Studies have demonstrated that when retinal is chemically modified to exhibit a more conjugated, distributed pi-electron system, redshift Schiff-base modification is observed. This means the visual pigment exhibits more significant resonance than before, and light is maximally absorbed in a longer wavelength. In contrast, when retinal is chemically modified to exhibit a less conjugated, less distributed pi-electron system, blue shift Schiff-base modification is observed. Here, the visual pigment exhibits less significant resonance than before, and light is maximally absorbed in a shorter wavelength. L cones have peak absorptions at 555-565 nm, M cones at 530-537 nm and S cones at 415-430 nm.[12]

Thus, color vision arises from the shifted cones’ peak absorption levels and ultimately the brain’s interpretation of the composition of these points of wavelength absorption. The entire pathway is sometimes referred to as the retinoid cycle.

Organ Systems Involved

The sense of vision involves the eye and the series of lenses of which it is composed, the retina, the optic nerve, optic chiasm, the optic tract, the lateral geniculate nuclei in the thalamus and the geniculocalcarine tract that projects to the occipital cortex.

Mechanism

The information coming from the ganglion cells of the retina reaches the optic nerves, and then the action potentials travel to a region called the optic chiasm (where the optic nerve fibers of both eyes cross in the midline and then form the optic tract). The direction of the visual information here is slightly different, as the ipsilateral temporal side of it passes directly into the ipsilateral part of the cortex, whereas the nasal part of vision gets crossed to the contralateral part of the brain, traveling to the opposite occipital cortex. Therefore, downstream to the optic chiasm, every optic tract has information from both eyes, from the temporal ipsilateral part of the visual field and the nasal contralateral part of it. This visual information then gets integrated into the lateral geniculate nuclei of the thalamus and then projects to the visual cortex. Before visual information reaches the thalamus, it can also travel to other structures such as the pretectal nuclei and the superior colliculus in the brainstem (to generate visual reflexes to focus on certain objects) or to the suprachiasmatic nuclei of the hypothalamus (to regulate the circadian rhythms), etc.[13][14]

When the information reaches the thalamus, it has to be ordered like paperwork in an office. So, to accomplish this task, the lateral geniculate nucleus has six layers of neural networks so that information can be integrated and put in order. Layers II, III, and V receive information from the ipsilateral temporal visual field, and layers I, IV and VI receive information from the contralateral nasal visual fields. To make it more interesting, layers I and II are made of magnocellular neurons, and layers III, IV, V, and VI are made of parvocellular neurons. The retina also contains magnocellular and parvocellular neurons, which are subtypes of the ganglion cells (the cells that receive information at the end of the retinal visual pathway). In the retina, the “magnocellular” type of ganglion cells receive information about black and white contrast and rapid changes in object positions, and the “parvocellular” type of neurons receive information about color. So the lateral geniculate nucleus has two layers of neurons dedicated exclusively to the integration of information about black and white contrast and visual field changes, and it has four layers assigned to the combination of color. From here, all of these color and contrast cues go to the visual cortex, where the information is then processed and interpreted.[15]

Related Testing

When evaluating an ophthalmologic complaint, it is very important to delineate the timing of the onset of symptoms. Ocular symptoms may be sudden or progressive, unilateral or bilateral, be associated with pain, photophobia, or discharge. To gain insight from the patient´s medical history, previous ocular conditions, and medications, history of recent ocular surgery or general surgery, the attending clinician must ask intentionally, otherwise, there could be missing information. 

A complete physical examination is necessary, inquire about changes in vision with each eye, make a pupillary exam, evaluate extraocular muscle movements and if you are suspecting a CNS lesion or in the optic pathway, test the confrontation visual fields. 

Pupil Examination

Pupil abnormalities are one of the most common challenges faced by clinicians. In order to evaluate the pupillary function correctly, one must understand the principles of the physiology of the pupil. The pupil either dilates or constricts. The pupillary dilation is mediated by the sympathetic nervous system, the constricting function is mediated by the parasympathetic nervous system.

• The constricting pupillary pathway starts in the midbrain, there we can find the Edinger – Westphal nucleus. When the eye is exposed to a very near object or to light, this information travels through the optic nerve first, in order for the brain to integrate and process it. Just after the optic chiasm and before information processing at the level of the thalamus (lateral geniculate nuclei), visual information of space and light goes to the EW nucleus after making synapsis in the olivary nuclei. Then, the third cranial nerve acquires parasympathetic fibers originating in these nuclei and travel together. In the subarachnoid space, they travel in the dorsal part of the nerve, very near to the posterior communicating artery. When the oculomotor nerve enters the cavernous sinus, these fibers are configurated more peripherally. Then, the oculomotor nerve reaches the anterior part of the cavernous sinus and soon will reach the superior orbital fissure. Once it reaches the SOF, the oculomotor nerve divides into two divisions: a superior division that innervates the superior palpebrae levator muscle and the superior rectus. The inferior division of the third cranial nerve innervates the rest of the extraocular muscles, except for the lateral rectus (innervated by the abducens) and the superior oblique (innervated by the trochlear nerve). In the posterior aspect of the globe, there is the ciliary ganglion. Here, fibers from the third cranial pair synapse to give origin to the short ciliary nerves that will innervate the pupillary muscles and that will configure the lens alongside the pupil. The near triad is composed of miosis, convergence, and accommodation, this triad is under the influence of more than one brain area, namely the mesencephalic reticular formation, the raphe interpositus, and the superior colliculus.  

• The dilating pupillary pathway consists of a set of three neurons. The first-order neuron is located in the paraventricular nucleus of the hypothalamus and it travels along the lateral aspect of the brainstem to make synapsis with the second-order neurons located in the ciliospinal nuclei (of Budge), this nucleus extends from C8 to T12 and its fibers run along the lung apex and the subclavian artery and they ascend through the common carotid artery to the third-order neurons that constitute the stellate cervical ganglion. The neurons that originate from this ganglion travel along the internal carotid artery and external carotid artery. The internal carotid artery fibers innervate the Müller muscles and the pupillary dilator muscle; they run along the abducens nerve and the ophthalmic artery. The external carotid fibers innervate the sweat glands of the face. 

Anisocoria refers to the discrepancy between the size of both pupils. Physiologic anisocoria is a dilation difference of 1 mm or less between the two pupils. Anisocoria may be monocular or binocular, in miosis or mydriasis. 

– Monocular mydriasis: This results from damage to the parasympathetic fibers innervating the pupil. It can be localized to the oculomotor nerve fibers before they reach the ciliary ganglion or after they synapse with it. Always examine the complete function of the third cranial nerve, including the function of the superior and inferior division. Ask yourself if it is a complete or a partial third nerve palsy, is associated with pain, is there any pupil involvement, and if there are signs of abnormal regeneration. The two more common causes of a third nerve palsy are an intracranial aneurysm or an ischemic lesion. Other slow-growing lesions may cause an eye to go mydriatic, like a meningioma. When the ciliary fibers are damaged, they cause a tonic pupil. This means a pupil that stays in mydriasis but typically resolves a little when stimulated by the accommodation reflex. The third cranial nerve is resilient to traumatic injury. When there is a traumatic injury of the oculomotor nerve, there surely is damage to the abducens nerve and trochlear nerve as well. The Adie pupil, as its called, is more common in females, tends to be idiopathic, and may be associated with non-responsive deep tendon reflexes. This is called Holmes-Adie syndrome.

– Binocular mydriasis: These may result from excessive sympathetic stimulation. They may be physiological in the case of an anxiety attack. The concern is the overstimulation of the SNS by drugs, such as cocaine, tricyclic antidepressants, or sympathomimetics. Some cases might be secondary to iris injury or a systemic disease acting as a parasympatholytic such as botulism (never Myasthenia gravis). The bilateral optic neuropathy may present with bilateral mydriasis, as well as diabetic neuropathy.

– Monocular miosis: This presentation arises from the disruption of the sympathetic innervation to the eye. Patients often present with Horner syndrome: is comprised of unilateral miosis, ptosis, and hemifacial ipsilateral anhidrosis. The clinical presentation may vary depending on the order neuron injured. First-order neuron injuries are very uncommon, unless the lesion is at the level of the medulla, being part of a Wallenberg syndrome. Lesion at the level of the ciliospinal nucleus comes from a Brown-Séquard syndrome, commonly. A Pancoast tumor, which is a tumor located a the lung apex, may cause Horner syndrome. Carotid dissection is a rather common cause of Horner syndrome and one should be very aware of this pathology. Note that the syndrome might not be complete in presentation, depending on the level of injury, one might find a partial Horner syndrome. Not every miotic eye is a Horner syndrome. It may be a chronic Adie pupil becoming miotic out of fatigue of the sympathetic nervous system.

– Bilateral miosis: Bilaterally small pupils are not uncommon. This results from a predominance in parasympathetic action over sympathetic action. It may be caused by sedating medications, lesions in the pons, lesions in the diencephalon or chronic reinnervation from a ganglionopathy. One pathology of concern is the Argyll-Robertson pupil. in which both pupils are miotic but are irregular. It is being said that the lesion responsible for this chronic syphilis manifestation is at the level of the midbrain, there’s no evidence supporting that theory.[16]

Clinical Significance

Improper Color Vision Recognition/Color Blindness

Many forms of color vision recognition abnormalities are present in the population, with most having a genetic origin. Very few individuals are truly color blind, but instead, individuals who are considered color-blind see a disrupted range of colors. The most common forms are protanopia and deuteranopia, conditions arising from loss of function of one of the cones, leading to dichromic vision. Protanopia is the loss of L cones (red) resulting in green-blue vision only. Deuteranopia is the loss of M cones (green) resulting in red-blue vision only. Both are X-linked alleles, therefore almost exclusively occurring in males, occurring with a prevalence of 1%. Loss of S cones does rarely occur in 0.01% of males and females. In these cases, one of the cones does not function, and one of the others is expressed instead in its place.

Similar to above, but not as severe in its symptoms, is the condition called “anomalous trichromatic vision” (tritanomaly), in which all three cones are present but the color vision is aberrant. The two common forms of color blindness, protanomaly and deuteranomaly, result in loss of L or M cones, respectively, and the lost cones are replaced with cones of intermediate spectral tuning. Both are X-linked and occur in 7% of males.[17]

Diseases affecting color vision but not affecting cones  

In addition to disorders of proper color recognition, many diseases in vision display phototransduction defects that affect many portions of the signal pathway and its regulation. Here, not only is color vision function decreased, but the monochromatic vision is worsened as well.

1. Congenital Stationary Night Blindness (CSNB)

One such disease is congenital stationary night blindness. It is a genetic defect resulting in functional cones but dysfunctional rods. In this disease, many potential culprits have been identified including abnormal rhodopsin, arrestin, rod transducin, rod phosphodiesterase, and rhodopsin kinase. Studies have demonstrated that in some populations of this disease, rods are permanently transmitting light signals. In CSNB, b-waves are reduced (in CSNB type 2) or absent (in CSNB type 1) during an electroretinogram (ERG). There are currently no treatments for this disorder. 

2. Retinitis Pigmentosa (RP)

Another disease affecting rod function is retinitis pigmentosa, which is a genetically inherited disease characterized by progressive degeneration of the retina, leading to blindness. Frequently, it begins in the early phase as night blindness. Vision loss first occurs in the periphery and progresses towards the center of vision, manifesting as tunnel vision. RP is associated with faulty rod functioning; if cones begin to be affected, then blindness eventually results. RP is characterized by reduced or absent a-waves and b-waves during an ERG. It has a prevalence of 1 in 3500 individuals.

3. Malnutrition-Associated

Deficiency in the essential nutrient vitamin A leads to night blindness, and this can eventually lead to permanent blindness through the deterioration of the receptor outer segments.

Experimental Therapies

Currently, there are no FDA-approved treatments for CSNB. However, the promise of gene-therapy interventions is on the horizon. Recently, FDA-approval was gained for retinal gene therapy (voretigene neparvovec) that utilizes the adeno-associated virus (AAV) and RPE65 gene, which can treat an uncommon form of RP called Leber congenital amaurosis (LCA). This was the first FDA-approved gene therapy for an inherited disorder.[18]

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Figure

Arteries of the Eye, Ophthalmic Artery; Internal Carotid Artery. Contributed by Gray’s Anatomy Plates

Figure

The Tunics of the Eye, Plan of retinal neurons. Contributed by Gray’s Anatomy Plates

Figure

Figure 1. (A) When light is incident on the skin (thick red arrow), most of it tends to get reflected back (thin red arrow), while the remaining gets refracted (oblique orange arrow), diffracted (yellow shooting arrows) or absorbed (crimson area). On (more…)

Figure

The retinoid cycle. Contributed by Arturo López de Nava, MS

Figure

Retina anatomy. Image courtesy Orawan

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Bowmaker JK. Evolution of colour vision in vertebrates. Eye (Lond). 1998;12 ( Pt 3b):541-7. [PubMed: 9775215]

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Human eye | Definition, Anatomy, Diagram, Function, & Facts

cross section of the human eye

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human eye, in humans, specialized sense organ capable of receiving visual images, which are then carried to the brain.

Anatomy of the visual apparatus

Structures auxiliary to the eye

The eye is protected from mechanical injury by being enclosed in a socket, or orbit, which is made up of portions of several of the bones of the skull to form a four-sided pyramid, the apex of which points back into the head. Thus, the floor of the orbit is made up of parts of the maxilla, zygomatic, and palatine bones, while the roof is made up of the orbital plate of the frontal bone and, behind this, by the lesser wing of the sphenoid. The optic foramen, the opening through which the optic nerve runs back into the brain and the large ophthalmic artery enters the orbit, is at the nasal side of the apex; the superior orbital fissure is a larger hole through which pass large veins and nerves. These nerves may carry nonvisual sensory messages—e.g., pain—or they may be motor nerves controlling the muscles of the eye. There are other fissures and canals transmitting nerves and blood vessels. The eyeball and its functional muscles are surrounded by a layer of orbital fat that acts much like a cushion, permitting a smooth rotation of the eyeball about a virtually fixed point, the centre of rotation. The protrusion of the eyeballs—proptosis—in exophthalmic goitre is caused by the collection of fluid in the orbital fatty tissue.

It is vitally important that the front surface of the eyeball, the cornea, remain moist. This is achieved by the eyelids, which during waking hours sweep the secretions of the lacrimal apparatus and other glands over the surface at regular intervals and which during sleep cover the eyes and prevent evaporation. The lids have the additional function of preventing injuries from foreign bodies, through the operation of the blink reflex. The lids are essentially folds of tissue covering the front of the orbit and, when the eye is open, leaving an almond-shaped aperture. The points of the almond are called canthi; that nearest the nose is the inner canthus, and the other is the outer canthus. The lid may be divided into four layers: (1) the skin, containing glands that open onto the surface of the lid margin, and the eyelashes; (2) a muscular layer containing principally the orbicularis oculi muscle, responsible for lid closure; (3) a fibrous layer that gives the lid its mechanical stability, its principal portions being the tarsal plates, which border directly upon the opening between the lids, called the palpebral aperture; and (4) the innermost layer of the lid, a portion of the conjunctiva. The conjunctiva is a mucous membrane that serves to attach the eyeball to the orbit and lids but permits a considerable degree of rotation of the eyeball in the orbit.

The conjunctiva

The conjunctiva lines the lids and then bends back over the surface of the eyeball, constituting an outer covering to the forward part of this and terminating at the transparent region of the eye, the cornea. The portion that lines the lids is called the palpebral portion of the conjunctiva; the portion covering the white of the eyeball is called the bulbar conjunctiva. Between the bulbar and the palpebral conjunctiva there are two loose, redundant portions forming recesses that project back toward the equator of the globe. These recesses are called the upper and lower fornices, or conjunctival sacs; it is the looseness of the conjunctiva at these points that makes movements of lids and eyeball possible.

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The fibrous layer

The fibrous layer, which gives the lid its mechanical stability, is made up of the thick, and relatively rigid, tarsal plates, bordering directly on the palpebral aperture, and the much thinner palpebral fascia, or sheet of connective tissue; the two together are called the septum orbitale. When the lids are closed, the whole opening of the orbit is covered by this septum. Two ligaments, the medial and lateral palpebral ligaments, attached to the orbit and to the septum orbitale, stabilize the position of the lids in relation to the globe. The medial ligament is by far the stronger.

The muscles of the lids

Closure of the lids is achieved by contraction of the orbicularis muscle, a single oval sheet of muscle extending from the regions of the forehead and face and surrounding the orbit into the lids. It is divided into orbital and palpebral portions, and it is essentially the palpebral portion, within the lid, that causes lid closure. The palpebral portion passes across the lids from a ligament called the medial palpebral ligament and from the neighbouring bone of the orbit in a series of half ellipses that meet outside the outer corner of the eye, the lateral canthus, to form a band of fibres called the lateral palpebral raphe. Additional parts of the orbicularis have been given separate names—namely, Horner’s muscle and the muscle of Riolan; they come into close relation with the lacrimal apparatus and assist in drainage of the tears. The muscle of Riolan, lying close to the lid margins, contributes to keeping the lids in close apposition. The orbital portion of the orbicularis is not normally concerned with blinking, which may be carried out entirely by the palpebral portion; however, it is concerned with closing the eyes tightly. The skin of the forehead, temple, and cheek is then drawn toward the medial (nose) side of the orbit, and the radiating furrows, formed by this action of the orbital portion, eventually lead to the so-called crow’s feet of elderly persons. It must be appreciated that the two portions can be activated independently; thus, the orbital portion may contract, causing a furrowing of the brows that reduces the amount of light entering from above, while the palpebral portion remains relaxed and allows the eyes to remain open.

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Opening of the eye is not just the result of passive relaxation of the orbicularis muscle but also is the effect of the contraction of the levator palpebrae superioris muscle of the upper lid. This muscle takes origin with the extraocular muscles at the apex of the orbit as a narrow tendon and runs forward into the upper lid as a broad tendon, the levator aponeurosis, which is attached to the forward surface of the tarsus and the skin covering the upper lid. Contraction of the muscle causes elevation of the upper eyelid. The nervous connections of this muscle are closely related to those of the extraocular muscle required to elevate the eye, so that when the eye looks upward the upper eyelid tends to move up in unison.

The orbicularis and levator are striated muscles under voluntary control. The lids also contain smooth (involuntary) muscle fibres that are activated by the sympathetic division of the autonomic system and tend to widen the palpebral fissure (the eye opening) by elevation of the upper, and depression of the lower, lid.

In addition to the muscles already described, other facial muscles often cooperate in the act of lid closure or opening. Thus, the corrugator supercilii muscles pull the eyebrows toward the bridge of the nose, making a projecting “roof” over the medial angle of the eye and producing characteristic furrows in the forehead; the roof is used primarily to protect the eye from the glare of the sun. The pyramidalis, or procerus, muscles occupy the bridge of the nose; they arise from the lower portion of the nasal bones and are attached to the skin of the lower part of the forehead on either side of the midline; they pull the skin into transverse furrows. In lid opening, the frontalis muscle, arising high on the forehead, midway between the coronal suture, a seam across the top of the skull, and the orbital margin, is attached to the skin of the eyebrows. Contraction therefore causes the eyebrows to rise and opposes the action of the orbital portion of the orbicularis; the muscle is especially used when one gazes upward. It is also brought into action when vision is rendered difficult either by distance or the absence of sufficient light.

The outermost layer of the lid is the skin, with features not greatly different from skin on the rest of the body, with the possible exception of large pigment cells, which, although found elsewhere, are much more numerous in the skin of the lids. The cells may wander, and it is these movements of the pigment cells that determine the changes in coloration seen in some people with alterations in health. The skin has sweat glands and hairs. As the junction between skin and conjunctiva is approached, the hairs change their character to become eyelashes.

The glandular apparatus

The eye is kept moist by secretions of the lacrimal glands (tear glands). These almond-shaped glands under the upper lids extend inward from the outer corner of each eye. Each gland has two portions. One portion is in a shallow depression in the part of the eye socket formed by the frontal bone. The other portion projects into the back part of the upper lid. The ducts from each gland, three to 12 in number, open into the superior conjunctival fornix, or sac. From the fornix, the tears flow down across the eye and into the puncta lacrimalia, small openings at the margin of each eyelid near its inner corner. The puncta are openings into the lacrimal ducts; these carry the tears into the lacrimal sacs, the dilated upper ends of the nasolacrimal ducts, which carry the tears into the nose.

The evaporation of the tears as they flow across the eye is largely prevented by the secretion of oily and mucous material by other glands. Thus, the meibomian, or tarsal glands, consist of a row of elongated glands extending through the tarsal plates; they secrete an oil that emerges onto the surface of the lid margin and acts as a barrier for the tear fluid, which accumulates in the grooves between the eyeball and the lid barriers.

The structure and functions of the eye, the anatomy of the eye

A person sees not through the eyes, but through the eyes, from where information is transmitted through the optic nerve, chiasm, visual tracts to certain areas of the occipital lobes of the cerebral cortex, where the picture of the external world that we see is formed. All these organs make up our visual analyzer or visual system.

The presence of two eyes allows us to make our vision stereoscopic (that is, to form a three-dimensional image). The right side of the retina of each eye transmits the “right side” of the image through the optic nerve to the right side of the brain, the left side of the retina does the same. Then the two parts of the image – the right and the left – the brain connects together.

Since each eye perceives “its own” picture, if the joint movement of the right and left eyes is disturbed, binocular vision may be disturbed. Simply put, you will start seeing double or you will see two completely different pictures at the same time.

Basic functions of the eye

• optical system that projects an image;
• system that perceives and “codes” the received information for the brain;
• “service” life support system.

Structure of the eye

The eye can be called a complex optical instrument. Its main task is to “transmit” the correct image to the optic nerve.

Cornea is the transparent membrane covering the front of the eye. There are no blood vessels in it, it has a large refractive power. Enters the optical system of the eye. The cornea borders on the opaque outer shell of the eye – the sclera. See the structure of the cornea.

Anterior chamber is the space between the cornea and the iris. It is filled with intraocular fluid.

Iris – similar in shape to a circle with a hole inside (pupil). The iris consists of muscles, with the contraction and relaxation of which the size of the pupil changes. It enters the choroid of the eye. The iris is responsible for the color of the eyes (if it is blue, it means that there are few pigment cells in it, if it is brown, there are many). Performs the same function as the aperture in a camera, adjusting the light output.

Pupil – hole in the iris. Its dimensions usually depend on the level of illumination. The more light, the smaller the pupil.

Lens is the “natural lens” of the eye. It is transparent, elastic – it can change its shape, “focusing” almost instantly, due to which a person sees well both near and far. Encapsulated, held in place by eyelash band . The lens, like the cornea, is part of the optical system of the eye.

Vitreous body is a gel-like transparent substance located in the back of the eye. The vitreous body maintains the shape of the eyeball and is involved in intraocular metabolism. Enters the optical system of the eye.

Retina – consists of photoreceptors (they are sensitive to light) and nerve cells. Receptor cells located in the retina are divided into two types: cones and rods. In these cells, which produce the enzyme rhodopsin, the energy of light (photons) is converted into electrical energy of the nervous tissue, i.e., a photochemical reaction.

The sticks are highly light sensitive and allow you to see in poor light, they are also responsible for peripheral vision. Cones, on the contrary, require more light for their work, but it is they that allow you to see fine details (responsible for central vision), make it possible to distinguish colors. The largest accumulation of cones is in the fovea centralis (macula), which is responsible for the highest visual acuity. The retina is adjacent to the choroid, but loosely in many areas. It is here that it tends to flake off in various diseases of the retina.

Sclera is the opaque outer shell of the eyeball, which passes into the transparent cornea in the anterior part of the eyeball. 6 oculomotor muscles are attached to the sclera. It contains a small number of nerve endings and blood vessels.

Choroid – lines the posterior sclera, adjacent to the retina, with which it is closely connected. The choroid is responsible for the blood supply to the intraocular structures. In diseases of the retina, it is very often involved in the pathological process. There are no nerve endings in the choroid, therefore, when it is ill, pain does not occur, usually signaling some kind of malfunction.

Optic nerve – with the help of the optic nerve, signals from the nerve endings are transmitted to the brain.

Useful reading

General questions about treatment in the clinic

Structure of the human eye: anatomy, description

Structure and function of the human eye

Our eyes are a complex apparatus that allows a person to receive information and see the world in all its glory. The eyes have a very complex structure – they combine various tissue structures and vascular systems.

Understanding what the eye consists of, how its components work and what functions they perform, helps to carefully and competently treat one’s own vision, recognize “signals” of violations and, accordingly, make decisions to eliminate them.

Eyeball

Due to its spherical or spherical shape, the human eye has received the name “eyeball”. It is located in the eye socket – a special bone structure of the skull that protects the eyeball from damage. The anterior surface of the eyeball is protected by the eyelids.

The six external muscles provide movement of the eyeball and create binocular vision – that is, seeing with two eyes, which creates a three-dimensional image (stereoscopic vision).

Through the lacrimal ducts, the outflow of tear fluid occurs. The lacrimal glands produce tears that moisturize the surface of the eyeball and create a protective film on its surface.

Sheaths of the eye

There are several layers of the eye.

Conjunctiva is the outer transparent membrane that covers the surface of the eye and the inside of the eyelids. The conjunctiva aids in the necessary gliding during eyeball movement.

Fibrous membrane is a gas sheath that consists of the sclera, limbus, and cornea.

Sclera is a white shell that performs protective and supporting functions. This is the densest part of the eyeball.

Limbo is the transitional membrane of the eye between the sclera and the cornea. The limbus contains stem cells that regenerate the outer layers of the cornea.

Cornea is the transparent part of the fibrous membrane. It is abundantly supplied with nerves, therefore it is characterized by high sensitivity. The transparency of the cornea allows light rays to penetrate into the eye.

Choroid – part of the eye that provides blood supply to the eye and trophism (nutrition) of its cells. The choroid is composed of:

1. Choroid – a structure that is in close contact with the retina and sclera, performing the functions of nutrition and shock absorption
2. Ciliary body – (providing vision at different distances) and producing (producing) aqueous humor (intraocular fluid)
3. The iris, which determines the color of our eyes, and the pupil, the opening for light rays to enter
4. The retina is the inner lining of the eye. The retina is a very thin, less than one-third of a millimeter (300 microns) thick, light-sensitive tissue. It provides the main functions of the organs of vision – central and peripheral (lateral) vision. This is where the optic nerve fibers begin, through which impulses pass to the brain, providing our visual perception.

The thin sensitive part of the retina – macula – is responsible for the central vision of a person.

Eye cameras

The chambers of the eye are closed and interconnected spaces. They contain intraocular fluid that nourishes the cornea and lens. There are two chambers in the eyeball – anterior and posterior.

Anterior is located between the cornea and the iris. And between the peripheral part of the cornea and the iris, the angle of the anterior chamber is isolated. In this place, the outflow of intraocular fluid is carried out.

Posterior chamber is located between the iris and the posterior surface of the ciliary body.

Behind the iris is the lens . It has the shape of a biconvex lens and is attached to the ciliary (ciliary) body with the help of a large number of thin ligaments.

Behind the lens, the cavity of the eyeball is filled with the vitreous body , which is necessary to maintain the tension of the cell membranes of the eye (turgor).

Optical system of the eye

The optical system of the eye is our vision. The optical system allows a person to visually perceive all kinds of objects of the external world.