Ear

The ear functions for both hearing and balance. It is connected to the brain by the vestibulocochlear (auditory) nerve (CN VIII).

The ear has three regions:

1. Outer ear (auricle or pinna) - Is a large cartilagenous structure. This structure is subdivided into several parts such as the helix (rim), antihelix, tragus, antitragus, and concha. At the bottom of the auricle there is usually a fleshy lobe.

The external auditory meatus is the opening that leads from the auricle to the tympanic membrane. It contains many ceruminous glands for the production of cerumen (ear wax). Cerumen functions to keep the tympanic membrane soft and waterproof. The external auditory meatus also contains many small hairs. Together the hairs and the cerumen help prevent foreign objects from reaching the tympanic membrane.

The tympanic membrane (ear drum) is the innermost portion of the outer ear. It is a thin double layered membrane that is very sensitive to pain via CN V and CN X. The tympanic membrane transmits vibrations to the malleus in the middle ear.

2. Middle ear (tympanic cavity) - is located in the petrous portion of the temporal bone. It is defined as the area between the tympanic membrane and the oval and round windows. There are 2 openings (entrances/exits) from this cavity:

a. epitympanic recess - connects this cavity to the mastoid air cells.

b. auditory (eustachian) tube - connects this cavity to the nasopharynx for equilibration of air pressure.

The middle ear contains three ossicles (bones). These form a tandem connection from the tympanic membrane to the oval window. In order these are the malleus, incus, and stapes. These three bones act as levers to intensify the vibrations being passed from the tympanic membrane to the oval window. This force is also magnified by the step down effect from the large tympanic membrane to the small oval window. Together these two factors increase the vibratory force about 20 fold. To regulate the amount of force transmitted to the oval window there are two regulatory muscles:

a. tensor tympany m. - which functions to pull the malleus inward.

b. stapedius m. - which functions to pull the stapes outward.

3. Inner ear (labyrinth) - contains the functional organs for balance and hearing. It is divided into two portions, the bony labyrinth and the membranous labyrinth.

The bony labyrinth is divided into three areas:

1. vestibule - contains the oval and round windows. It also contains the membranous labyrinth in two connected sacs:

a. utricle larger sac

b. saccule

Both of these sacs are sensitive to gravity and linear movement.

2. semicircular canals - There are three of these canals in each ear. one to represent each plane. These are sensitive to angular acceleration/deceleration and rotational movement. They contain semicircular ducts with a membranous ampulla. These connect to the upper part of the utricle.

3. Cochlea - is coiled 2 times to resemble a snail shape. The cochlea is divided into 3 chambers:

a. scala vestibuli - starts at the oval window. It is the uppermost chamber.

b. scala tympani - is the lower chamber. It goes to the round window.

c. cochlear duct - is the middle chamber. Its roof is the vestibular chamber. Its floor is the basilar membrane.

The scala vestibuli and tympani contain perilymph and are connected at the helicotrema.

The cochlear duct contains endolymph and ends at the helicotrema.

The Organ of Corti is inside the cochlear duct. It contains the receptors that transform mechanical vibrations into nerve impulses. These are located in the basilar membrane of the organ of corti thus making it the functional unit of hearing.

High intensity waves are transmitted through the vestibular membrane and the basilar membrane into the perilymph. This causes displacement of the perilymph which displaces the hair cells of the basilar membrane. The displacement of the hair cells begins an impulse to the brain.

Equilibrium - already talked about semicircular canals and saccule and utricle.

Due to microvilli and a kinocilium

20 - 50 microvilli and 1 kinocilium

If microvilli deflect toward the kinocilium we will get a depolarization.

If microvilli deflect away from the kinocilium we will get a hyperpolarization.

Macula of the saccule, not in the utricle. The macula has otoliths, which are small crystals of calcium carbonate to give the mass inertia.

The semicircular canals have an ampulla at their base. Here we find an elevated area known as the crista ampullaris. It has many hair cells. These hair cells reach into a dome shaped gelatinous mass called the

cupula. When the hair cells are bent by a spinning motion impulses are sent to the brain via the vestibular portion of CN VIII.