How does the inner ear contribute to hearing and balance. What are the main components of the inner ear. How do the bony and membranous labyrinths interact. What is the clinical relevance of inner ear disorders.

The Fascinating World of Inner Ear Anatomy

The inner ear, a marvel of biological engineering, is a complex structure hidden within the temporal bone of the skull. It plays a crucial role in two of our most important senses: hearing and balance. Understanding its intricate anatomy is essential for medical professionals, researchers, and anyone interested in the wonders of human physiology.

Anatomical Position and Structure: Unraveling the Inner Ear’s Location

Where exactly is the inner ear located? The inner ear is nestled deep within the petrous part of the temporal bone, strategically positioned between the middle ear laterally and the internal acoustic meatus medially. This protected location ensures the delicate structures of the inner ear are well-shielded from external influences.

The Two Main Components: Bony and Membranous Labyrinths

The inner ear consists of two primary components:

• Bony labyrinth: A series of intricate cavities within the temporal bone
• Membranous labyrinth: A system of ducts and sacs contained within the bony labyrinth

These two components work in harmony to facilitate our hearing and balance functions.

The Bony Labyrinth: A Protective Housing for Delicate Structures

The bony labyrinth serves as a protective shell for the more delicate membranous structures within. It consists of three main parts:

1. Vestibule: The central chamber
2. Cochlea: A spiral-shaped structure responsible for hearing
3. Semi-circular canals: Three loop-shaped structures involved in balance

These bony cavities are lined with periosteum and filled with a fluid called perilymph, which plays a crucial role in the transmission of sound vibrations.

The Vestibule: The Inner Ear’s Central Hub

Why is the vestibule considered the central part of the bony labyrinth? The vestibule acts as a connecting point between different parts of the inner ear. It communicates anteriorly with the cochlea and posteriorly with the semi-circular canals. Additionally, it houses two important components of the membranous labyrinth: the saccule and utricle, which are vital for our sense of balance.

The Cochlea: A Spiral-Shaped Wonder of Sound Perception

The cochlea is perhaps the most recognizable part of the inner ear, with its distinctive spiral shape. It houses the cochlear duct, which is the primary organ of hearing. The cochlea’s structure is a marvel of efficiency:

• It twists around a central bony core called the modiolus
• The modiolus contains branches of the cochlear nerve
• A bony ledge called the spiral lamina extends from the modiolus, supporting the cochlear duct

The cochlear duct divides the cochlea into two main chambers:

• Scala vestibuli: Located above the cochlear duct
• Scala tympani: Situated below the cochlear duct

These chambers are filled with perilymph and play a crucial role in the mechanics of hearing.

Semi-circular Canals: The Guardians of Balance

How do the semi-circular canals contribute to our sense of balance? The three semi-circular canals – anterior, lateral, and posterior – are oriented at right angles to each other. This arrangement allows them to detect rotational movements of the head in all three spatial planes. Each canal has a swollen end called the ampulla, which contains sensory hair cells that respond to fluid movement within the canals.

The Membranous Labyrinth: A Delicate Network of Fluid-Filled Structures

Within the protective bony labyrinth lies the membranous labyrinth, a continuous system of ducts and sacs filled with endolymph. This intricate network includes:

• Cochlear duct: The organ of hearing
• Three semi-circular ducts: Corresponding to the bony semi-circular canals
• Saccule and utricle: Otolith organs involved in detecting linear acceleration and head position

The membranous labyrinth is surrounded by perilymph, creating a fluid-within-fluid system that is crucial for the proper functioning of the inner ear.

The Cochlear Duct: Home of the Organ of Corti

The cochlear duct, also known as the scala media, is a critical component of the hearing mechanism. It has a triangular cross-section and is bounded by:

• Lateral wall: Formed by the spiral ligament
• Roof: Reissner’s membrane, separating it from the scala vestibuli
• Floor: Basilar membrane, separating it from the scala tympani

The basilar membrane supports the Organ of Corti, which contains the hair cells responsible for converting mechanical vibrations into electrical signals that can be interpreted by the brain as sound.

Vasculature: Nourishing the Inner Ear

How is the inner ear supplied with blood? The inner ear receives its blood supply primarily from the labyrinthine artery, which is a branch of the anterior inferior cerebellar artery. This artery divides into:

• Anterior vestibular artery: Supplying the utricle and parts of the semi-circular ducts
• Common cochlear artery: Further dividing to supply the cochlea and the remaining vestibular structures

Proper blood supply is crucial for the functioning of the inner ear, as any disruption can lead to hearing and balance disorders.

Innervation: The Neural Network of the Inner Ear

The inner ear is innervated by the vestibulocochlear nerve (cranial nerve VIII), which has two distinct components:

1. Cochlear nerve: Responsible for transmitting auditory information
2. Vestibular nerve: Carrying balance and spatial orientation information

These nerves connect the sensory cells of the inner ear to the brain, allowing us to perceive sound and maintain balance.

The Cochlear Nerve: Transmitting Sound Information

The cochlear nerve originates from the spiral ganglion, located in the modiolus of the cochlea. It receives input from the hair cells of the Organ of Corti and transmits this information to the cochlear nuclei in the brainstem.

The Vestibular Nerve: Maintaining Balance and Spatial Awareness

The vestibular nerve consists of superior and inferior divisions. It receives input from the hair cells in the semi-circular ducts, utricle, and saccule. This information is then relayed to the vestibular nuclei in the brainstem, contributing to our sense of balance and spatial orientation.

Clinical Relevance: Understanding Inner Ear Disorders

The complex structure of the inner ear makes it susceptible to various disorders that can significantly impact a person’s quality of life. One such condition is Meniere’s disease.

Meniere’s Disease: A Complex Inner Ear Disorder

What is Meniere’s disease and how does it affect the inner ear? Meniere’s disease is a chronic condition characterized by episodes of vertigo, fluctuating hearing loss, tinnitus, and a feeling of fullness in the affected ear. It is believed to be caused by an abnormal build-up of endolymph in the membranous labyrinth, although the exact etiology remains unclear.

Symptoms of Meniere’s disease include:

• Sudden episodes of vertigo lasting from 20 minutes to several hours
• Fluctuating sensorineural hearing loss, often affecting low frequencies
• Tinnitus or ringing in the ears
• Aural fullness or pressure in the affected ear

Treatment for Meniere’s disease typically involves a combination of medications to manage symptoms, dietary changes to reduce fluid retention, and in some cases, surgical interventions.

Beyond Meniere’s: Other Inner Ear Disorders

The inner ear’s complex anatomy makes it susceptible to a variety of other disorders, each with its own set of challenges and treatment approaches. Some of these conditions include:

Benign Paroxysmal Positional Vertigo (BPPV)

What causes the sudden, brief episodes of vertigo associated with BPPV? This common inner ear problem occurs when tiny calcium carbonate crystals (otoconia) become dislodged from the utricle and migrate into one of the semi-circular canals. As a result, the brain receives false signals about head movement, leading to brief but intense episodes of vertigo triggered by certain head positions.

Treatment for BPPV often involves specific head maneuvers designed to guide the displaced crystals back to their proper location. These maneuvers, such as the Epley maneuver, can be highly effective in resolving symptoms.

Labyrinthitis and Vestibular Neuritis

How do infections affect the inner ear? Labyrinthitis and vestibular neuritis are inflammatory conditions that can affect different parts of the inner ear:

• Labyrinthitis: Inflammation of both the cochlear and vestibular portions of the inner ear, often caused by viral infections
• Vestibular neuritis: Inflammation primarily affecting the vestibular nerve, usually due to viral infections

Both conditions can cause severe vertigo, nausea, and balance problems. Treatment typically involves medications to manage symptoms and vestibular rehabilitation exercises to help the brain compensate for the altered balance signals.

Acoustic Neuroma

What is an acoustic neuroma and how does it impact inner ear function? An acoustic neuroma, also known as a vestibular schwannoma, is a benign tumor that develops on the vestibular portion of the vestibulocochlear nerve. While not strictly an inner ear disorder, it can significantly impact inner ear function as it grows.

Symptoms of acoustic neuroma may include:

• Gradual hearing loss on the affected side
• Tinnitus
• Balance problems
• Facial numbness or weakness (in advanced cases)

Treatment options for acoustic neuroma depend on the size and growth rate of the tumor, and may include observation, radiation therapy, or surgical removal.

The Future of Inner Ear Research and Treatment

As our understanding of inner ear anatomy and physiology continues to grow, so do the possibilities for new treatments and interventions. What are some of the exciting developments on the horizon for inner ear medicine?

Gene Therapy for Hearing Loss

Researchers are exploring the potential of gene therapy to treat certain forms of genetic hearing loss. By introducing functional copies of defective genes into the inner ear, it may be possible to restore or preserve hearing function in individuals with specific genetic mutations.

Stem Cell Therapies

The potential use of stem cells to regenerate damaged hair cells in the cochlea is an area of intense research. If successful, this approach could offer hope for individuals with sensorineural hearing loss, which is currently considered irreversible.

Improved Cochlear Implants

Ongoing research aims to enhance the performance of cochlear implants, which are electronic devices that can provide a sense of sound to individuals with severe to profound hearing loss. Future implants may offer more natural sound perception and improved speech understanding in noisy environments.

Vestibular Implants

Similar to cochlear implants, vestibular implants are being developed to restore balance function in individuals with bilateral vestibular loss. These devices aim to provide artificial sensory input to the vestibular system, potentially improving balance and reducing the risk of falls.

Targeted Drug Delivery Systems

Researchers are working on developing new methods to deliver medications directly to the inner ear. These targeted delivery systems could potentially improve the efficacy of treatments for various inner ear disorders while minimizing systemic side effects.

As we continue to unravel the mysteries of the inner ear, these advancements and others yet to be discovered hold the promise of improving the lives of millions of people affected by hearing and balance disorders. The intricate anatomy of the inner ear, once a barrier to treatment, may soon become the key to unlocking new therapeutic possibilities.

The Inner Ear – Bony Labyrinth – Membranous Labryinth

• 1 Anatomical Position and Structure
  • 1.1 Bony Labyrinth
  • 1.2 Membranous Labyrinth
• 2 Vasculature
• 3 Innervation
• 4 Clinical Relevance: Meniere’s Disease

The ear can be divided into three parts: the outer ear, middle ear and inner ear.

The inner ear is the innermost part of the ear, and houses the vestibulocochlear organs. It has two main functions:

• To convert mechanical signals from the middle ear into electrical signals, which can transfer information to the auditory pathway in the brain.

• To maintain balance by detecting position and motion.

In this article, we shall look at the anatomy of the inner ear – its position, structure, and neurovascular supply.

By TeachMeSeries Ltd (2023)

Fig 1 – Overview of the ear

Anatomical Position and Structure

The inner ear is located within the petrous part of the temporal bone. It lies between the middle ear and the internal acoustic meatus, which lie laterally and medially respectively. The inner ear has two main components – the bony labyrinth and membranous labyrinth.

• Bony labyrinth – consists of a series of bony cavities within the petrous part of the temporal bone. It is composed of the cochlea, vestibule and three semi-circular canals. All these structures are lined internally with periosteum and contain a fluid called perilymph.

• Membranous labyrinth – lies within the bony labyrinth. It consists of the cochlear duct, semi-circular ducts, utricle and the saccule. The membranous labyrinth is filled with fluid called endolymph.

The inner ear has two openings into the middle ear, both covered by membranes. The oval window lies between the middle ear and the vestibule, whilst the round window separates the middle ear from the scala tympani (part of the cochlear duct).

Bony Labyrinth

The bony labyrinth is a series of bony cavities within the petrous part of the temporal bone. It consists of three parts – the cochlea, vestibule and the three semi-circular canals.

Vestibule

The vestibule is the central part of the bony labyrinth. It is separated from the middle ear by the oval window, and communicates anteriorly with the cochlea and posteriorly with the semi-circular canals.

Two parts of the membranous labyrinth; the saccule and utricle, are located within the vestibule.

Cochlea

The cochlea houses the cochlea duct of the membranous labyrinth – the auditory part of the inner ear. It twists upon itself around a central portion of bone called the modiolus, producing a cone shape which points in an anterolateral direction. Branches from the cochlear portion of the vestibulocochlear (VIII) nerve are found at the base of the modiolus.

Extending outwards from the modiolus is a ledge of bone known as spiral lamina, which attaches to the cochlear duct, holding it in position. The presence of the cochlear duct creates two perilymph-filled chambers above and below:

• Scala vestibuli: Located superiorly to the cochlear duct. As its name suggests, it is continuous with the vestibule.

• Scala tympani: Located inferiorly to the cochlear duct. It terminates at the round window.

Semi-circular Canals

There are three semi-circular canals: anterior, lateral and posterior. They contain the semi-circular ducts, which are responsible for balance (along with the utricle and saccule).

The canals are situated superoposterior to the vestibule, at right angles to each other. They have a swelling at one end, known as the ampulla.

By TeachMeSeries Ltd (2023)

Fig 2 – The three parts of the bony labyrinth.

Membranous Labyrinth

The membranous labyrinth is a continuous system of ducts filled with endolymph. It lies within the bony labyrinth, surrounded by perilymph. It is composed of the cochlear duct, three semi-circular ducts, saccule and the utricle.

The cochlear duct is situated within the cochlea and is the organ of hearing. The semi-circular ducts, saccule and utricle are the organs of balance (also known as the vestibular apparatus).

Cochlear Duct

The cochlear duct is located within the bony scaffolding of the cochlea. It is held in place by the spiral lamina. The presence of the duct creates two canals above and below it – the scala vestibuli and scala tympani respectively. The cochlear duct can be described as having a triangular shape:

• Lateral wall – Formed by thickened periosteum, known as the spiral ligament.

• Roof – Formed by a membrane which separates the cochlear duct from the scala vestibuli, known as the Reissner’s membrane.

• Floor – Formed by a membrane which separates the cochlear duct from the scala tympani, known as the basilar membrane.

The basilar membrane houses the epithelial cells of hearing – the Organ of Corti. A more detailed description of the Organ of Corti is beyond the scope of this article.

By TeachMeSeries Ltd (2023)

Fig 3 – Structure of the cochlea, and borders of the cochlear duct.

Saccule and Utricle

The saccule and utricle are two membranous sacs located in the vestibule. They are organs of balance which detect movement or acceleration of the head in the vertical and horizontal planes, respectively.

The utricle is the larger of the two, receiving the three semi-circular ducts. The saccule is globular in shape and receives the cochlear duct.

Endolymph drains from the saccule and utricle into the endolymphatic duct. The duct travels through the vestibular aqueduct to the posterior aspect of the petrous part of the temporal bone. Here, the duct expands to a sac where endolymph can be secreted and absorbed.

Semi-circular Ducts

The semi-circular ducts are located within the semi-circular canals, and share their orientation. Upon movement of the head, the flow of endolymph within the ducts changes speed and/or direction. Sensory receptors in the ampullae of the semi-circular canals detect this change, and send signals to the brain, allowing for the processing of balance.

By TeachMeSeries Ltd (2023)

Fig 4 – The components of the membranous labyrinth.

Vasculature

The bony labyrinth and membranous labyrinth have different arterial supplies. The bony labyrinth receives its blood supply from three arteries, which also supply the surrounding temporal bone:

• Anterior tympanic branch (from maxillary artery).
• Petrosal branch (from middle meningeal artery).
• Stylomastoid branch (from posterior auricular artery).

The membranous labyrinth is supplied by the labyrinthine artery, a branch of the inferior cerebellar artery (or, occasionally, the basilar artery). It divides into three branches:

• Cochlear branch – supplies the cochlear duct.
• Vestibular branches (x2) – supply the vestibular apparatus.

Venous drainage of the inner ear is through the labyrinthine vein, which empties into the sigmoid sinus or inferior petrosal sinus.

By TeachMeSeries Ltd (2023)

Fig 5 – The labyrinthine artery arising from the basilary artery

Innervation

The inner ear is innervated by the vestibulocochlear nerve (CN VIII). It enters the inner ear via the internal acoustic meatus, where it divides into the vestibular nerve (responsible for balance) and the cochlear nerve (responsible for hearing):

• Vestibular nerve – enlarges to form the vestibular ganglion, which then splits into superior and inferior parts to supply the utricle, saccule and three semi-circular ducts.

• Cochlear nerve – enters at the base of the modiolus and its branches pass through the lamina to supply the receptors of the Organ of Corti.

The facial nerve, CN VII, also passes through the inner ear, but does not innervate any of the structures present.

Clinical Relevance: Meniere’s Disease

Meniere’s disease is a disorder of the inner ear, characterised by episodes of vertigo, low-pitched tinnitus, and hearing loss.

The symptoms are thought to be caused by an excess accumulation of endolymph within the membranous labyrinth, causing progressive distension of the ducts. The resulting pressure fluctuations damage the thin membranes of the ear that detect balance and sound.

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Anatomy of the inner ear

Anatomy of the inner ear

Timothy C. Hain, MD •
Page last modified:
March 31, 2023

While there are several other surveys of inner ear anatomy, we have diffidently
here set out to put the content in our lecture on this subject on the web.

Overview of peripheral anatomy

Mechanical sensors (canals and otoliths) respond to angular and linear movement. This is an artist’s drawing, based on the classic drawing by Max Brodel, and some liberties were taken for clarity. For example, the length of the internal auditory canal (IAC) is very short here. See the commentary about accuracy below. We have attempted to provide some “real” images from MRI and CT scans below.

The semicircular canals are very small rings — about 1 cm in diameter. The image above is an MRI of a normal set of inner ears, taken in the “axial” view. The snail shaped objects at the top (front of head) are the cochlea. The rings are the semicircular canals. The IAC here is, of course, anatomically accurate, and is much longer than what is shown on the artist’s illustration above.

 

The image above is taken from a CT scan shows that the anterior canal diameter is less than 1 cm – -smaller than a dime !

There are three semicircular canals – -the anterior, superior and horizontal. Between the three canals the brain can determine rotational velocity in three dimensions.

There are also two otolith organs (click here for more). The otoliths respond to linear acceleration such as gravity or changing velocity of movement in a straight line.

• Cochleo-Vestibular(8th) cranial nerve (click here
  for an animation)
• Spiral (cochlear) and Scarpa’s (vestibular) ganglion
• Brainstem nuclei

On the accuracy of the above illustration (the “master ear” adapted from Brodel).

The illustration of the inner ear at the top of this page, the “Master ear”, is an artists redrawing of an illustration originally due to Max Brodel (1946). This small book was published posthumously, based on drawings made originally in 1939.

Jackler et al (2014) pointed out that this illustration (which has been used by many) has numerous errors — they state “Numerous anatomic errors exist such as a 180-degree reversal of the incus and a markedly foreshortened internal auditory canal. ” Some of these errors were presumably made to avoid a “cluttered image”. Nevertheless, others are just anatomic errors such as the “180 degree reversal of the incus”. Jackler et al (2014) provide a link to a more correct illustration here. The more accurate illustration does not reveal the structures within the ear as clearly as the Brodel drawing, but the accuracy is of course very important, especially to surgeons.

Also see:Illustration from Jackler et al (2014)•Educational Models of the labyrinthine anatomy involved with BPPV.

References:

• Brodel, M., et al. (1946). Three unpublished drawings of the anatomy of the human ear. Philadelphia, WB Saunders.
• Jackler, R. K., et al. (2014). “Revisiting Max Brodel’s 1939 classic coronal illustration of the ear.” Otol Neurotol 35(3): 555-560.

outer, middle and inner ear

The human ear is one of the most important organs, which not only allows us to hear the sounds that surround us, but also helps to maintain balance.

What parts does the human ear consist of

• Outer ear
• Middle ear
• Inner ear.

Outer ear

The outer ear is the only externally visible part of the hearing organ. It consists of:

• The auricle, which collects sounds and directs them to the external auditory meatus.
• External auditory meatus, which is designed to conduct sound vibrations from the auricle to the tympanic cavity of the middle ear. Its length in adults is approximately 2.6 cm. The surface of the external auditory canal also contains sebaceous glands that secrete earwax that protects the ear from germs and bacteria.
• The tympanic membrane that separates the outer ear from the middle ear.

Middle ear

The middle ear is the air-filled cavity behind the eardrum. It is connected to the nasopharynx by the Eustachian tube, which equalizes pressure on both sides of the eardrum. That is why, if a person’s ears are blocked, he reflexively begins to yawn or swallow. Also in the middle ear are the smallest bones of the human skeleton: the hammer, anvil and stirrup. They are not only responsible for the transmission of sound vibrations from the outer ear to the inner ear, but also amplify them.

Inner ear

The inner ear is the most complex part of hearing, which, due to its intricate shape, is also called a labyrinth. It consists of:

• The vestibule and semicircular canals, which are responsible for the sense of balance and position of the body in space.
• Liquid-filled snails. It is here that sound vibrations enter in the form of vibration. Inside the cochlea is the organ of Corti, which is directly responsible for hearing. It contains about 30,000 hair cells that pick up sound vibrations and transmit the signal to the auditory cortex. It is interesting that each of the hair cells reacts to a certain sound purity, which is why, when they die, hearing loss occurs and the person stops hearing the sounds of the frequency for which the dead cell was responsible.

Auditory pathways

The auditory pathways are a collection of nerve fibers responsible for the transmission of nerve impulses from the cochlea to the auditory centers located in the temporal lobes of the brain. It is there that the processing and analysis of complex sounds, for example, speech, takes place. The speed of transmission of the auditory signal from the outer ear to the centers of the brain is approximately 10 milliseconds.

Sound perception

The ear sequentially converts sounds into mechanical vibrations of the tympanic membrane and auditory ossicles, then into vibrations of the fluid in the cochlea, and finally into electrical impulses, which are transmitted along the pathways of the central auditory system to the temporal lobes of the brain for recognition and processing.

Receiving nerve impulses, the brain not only converts them into sound, but also receives additional, important information for us. This is how we distinguish between the pitch and loudness of a sound and the time interval between the moment the sound is picked up by the right and left ears, which allows us to determine the direction in which the sound comes. At the same time, the brain analyzes not only the information received from each ear separately, but also combines it into a single sensation. In addition, the so-called “templates” of familiar sounds are stored in our brain, which helps the brain to quickly distinguish them from unfamiliar ones. With hearing loss, the brain receives distorted information, sounds become quieter and this leads to errors in their interpretation. The same problems can arise as a result of aging, head injuries and neurological diseases. This proves only one thing: for good hearing, the work of not only the hearing organ, but also the brain is important!

Marina Alexandrovna Palaznik

Otorhinolaryngologist-audiologist of the first category. Work experience: since 2010. Conducts reception of adults and children.

Reception of adults and children:

Minsk, st. Plekhanova, 27

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Center for good hearing

structure, functions and age features.

Structure
inner ear.

Internal
The ear is one of the three parts of the hearing organ.
and balance. Is the most difficult
department of the hearing organs, because of its
intricate shape is called a labyrinth.

Bone
labyrinth consists of vestibule, cochlea,
semicircular canals.

U
standing human snail is
in front, and semicircular canals behind,
there is a cavity between them.
irregular shape – vestibule. Inside
bony labyrinth is membranous
maze that has exactly the same
three parts, but smaller, and between
the walls of both labyrinths are
a small gap filled with transparent
liquid – perilymph, and the cavity
membranous labyrinth – endolymph.

Snail

Each
part of the inner ear
a specific function. For example, a snail
is an organ of hearing: sound
fluctuations that are from the external auditory
passage through the middle ear into
internal auditory canal
vibrations are transmitted to the fluid filling
snail. Inside the snail is the main
membrane (lower membranous wall),
on which the organ of Corti is located
accumulation of various supporting cells
and special sensory epithelial
hair cells, which
perilymph vibrations perceive
auditory stimuli in the range
16-20000 vibrations per second, convert
they are transmitted to the nerve endings VIII
pairs of cranial nerves
vestibulocochlear nerve; further
nerve impulse enters the cortex
auditory center of the brain.

vestibule
and semicircular canals

threshold
and semicircular canals – sense organs
balance and body position in space.
The semicircular canals are located in three
mutually perpendicular planes
and filled with translucent gelatinous
liquid; inside the channels are
sensitive hairs immersed in
liquid, and at the slightest movement
body or head in fluid space
in these channels is displaced, pressing
on the hairs and generating impulses in
endings of the vestibular nerve
the brain receives information instantly
about changing the position of the body. Job
vestibular apparatus allows
a person to navigate accurately
space during the most complex movements
– for example, jumping into the water from a springboard
and turning around a few times
in the air, in the water the diver instantly
knows where is up and where is down.

Functions
inner ear.

Internal
ear, how the auditory organ performs
the following functions: watch
in the description of the snail!

Internal
ear, as an organ of balance, will perform
the following functions: watch
in the description of the vestibule and semicircular
channels!

features of the inner ear.

internal
the ear of the newborn is well developed,
its size is close to that of an adult
person. The bony walls of the semicircular
canals are thin, gradually thicken
in the pyramid of the temporal bone.

IN
postnatal ontogenesis continue
myelination (myelin sheath)
electrical insulating sheath
axons of many neurons) and
synaptogenesis (synapses are specialized
functional contacts between excitable
cells that serve to transmit and
signal conversion) central
auditory pathways and centers.