Cochlea and Vestibular System


The ear has three parts: the outer ear, middle ear, and the inner ear. The first two are air filled; the latter is fluid filled. It contains the cochlea with the sensory cells that detect the sound and the vestibular system, which detects acceleration.

The outer ear consists of the pinna, which, in contrast to animals, does very little in humans, and the external auditory canal. At the end of the external auditory canal is the non-cellular tympanic membrane.

The tympanic membrane has air on both sides and thus readily captures the sound energy travelling down the external auditory canal. If it had fluid on the inner side, it would reflect about 99% of the sound energy. Thus, the air-filled middle ear is crucial.

The air-filled middle ear is connected to the pharynx via the Eustachian tube, which keeps the pressure on both sides of the tympanic membrane the same.





But how is the sound energy transfered into the cochlea, where the sensory cells are found? That is the function of the three ossicles, which form a lever system that transfers movements of the tympanic membrane to the oval window. This lies between the middle ear and the fluid-filled cochlea.

A crucial feature of the system is that the area of the tympanic membrane is about 25 times larger than that of the oval window. This allows all of the energy collected by the tympanic membrane to be applied to a much smaller area. This magnifies the force per unit area by 25 times, allowing the sound energy to enter the fluid of the cochlea. In this way, the sound energy is efficiently transfered with little reflection from the air to the fluid-filled cochlea.

The cochlea is essentially a long tube that is divided down the middle by the basilar membrane. The sound energy entering via the oval window is all applied on one side of the basilar membrane. Thus, sound entering the ear starts the basilar membrane vibrating.

But the basilar membrane is not uniform along its length, and different parts of the basilar membrane move the most in response to different frequencies of sound.

The portion of the basilar membrane near the oval window is more narrow and stiff. It thus moves preferentially with high frequencies. At the other end, the basilar membrane is wider and more flexible. This end moves most in response to low frequencies. Thus, the basilar membrane is laid out like a piano keyboard, with different frequencies (pitches) moving different regions preferentially.

The primary sensory cells in the cochlea are the hair cells. They lie along the basilar membrane. Thus, when the basilar membrane moves up and down, the hair cells move up and down.

The hair cells have projections at their top called stereocilia. When the stereocilia are moved, mechanically gated ion channels open, the hair cells are depolarized and as a result glutamate is released as a neurotransmitter. The glutamate then causes depolarization of the next neurons, which have axons that form the vestibulocochlear nerve.

But why are the stereocilia moved when the hair cells move up and down? This is because of the tectorial membrane, which lies over the stereocilia. The stereocilia are tweaked because the tectorial membrane does not move up and down when the basilar membrane moves up and down.


Otosclerosis

In otosclerosis, there is excess formation of bone about the oval window, leading to reduced mobility of the stapes. The progressive hearing loss tends to become apparent either in teenage years or early adulthood. There is a strong genetic component. The treatment is either a hearing aid or surgery, which is usually successful.

Otitis Media

See your lab notes for discussion of otitis media. Inflammation of the middle ear is usually the result of an upper respiratory tract infection, which moves up the Eustachian tubes. This is more likely in children with their short Eustachian tubes.

Deafness

See your lab notes for distinguishing conduction deafness and sensory deafness.



What membrane lies between the middle ear and the cochlea?

Answer

What membrane in the cochlea moves up and down in response to sound?

Answer

In what specific part of a hair cell are mechanically gated ion channels found?

Answer



Developmentally, the inner ear begins as a simple fluid-filled chamber, which gradually changes into the labyrinthine shape shown to the right. The coiled cochlea is discussed above. The remaining parts comprise the vestibular system, which sense acceleration. Acceleration refer to a change in velocity, and there are two types. When an object is rotating, it is constantly changing its direction and thus has rotational acceleration. On the other hand, an object with increasing speed in one direction has linear acceleration


Semicircular Canals

Rotational acceleration is sensed by the three semicircular canals, which each lie in a plane perpendicular to the other two. As shown to the right, each semicircular canal has an expanded region with a paddle-like structure, called the cupula (pink in figure), which extends across the canal. When the head is rotating in the plane of a canal, the inertia of the fluid causes the fluid in the canal to lag behind the wall of the canal. This puts pressure on the cupula and causes it to bend.


The bending of the cupula is sensed by a set of hair cells (red in figure) located on a structure called the crista. This is because the stereocilia of the hair cells extend into the cupula.

Utricle and Saccule

The utricle and saccule have a similar structure, which is shown to the right. They differ in their exact orientation. A mass of small calcium carbonate stones, called otoconia, lies on top of the otolithic membrane (shown in pink). A "carpet" of hair cells lies below this structure, with the stereocilia of the hair cells extending into the otolithic membrane.

During linear acceleration, the inertia of the mass of otoconia causes it to lag behind the movement of the head, bending the stereocilia of the hair cells.


Meniere's Disease

Meniere's disease is a disorder of the inner ear characterized by incapacitating episodes of vertigo leading to nausea and vomiting. Tinnitus (ringing sound) and hearing loss may also be present. The patients are often in their 30s or 40s. A typical attack lasts roughly an hour. The cause is unknown but involves an excessive accumulation of the endolymph fluid, which is the fluid in the vestibular system and cochlea.

Benign Paroxysmal Positional Vertigo

Benign Paroxysmal Positional Vertigo occurs when one of the otoconia breaks free and moves into usually the posterior semicircular canal. When the head position changes, the small stone moves through the canal, creating fluid movement. This deflects the cupula and causes vertigo. See your lab notes for further details about this disorder and its treatment.