Special Senses Lecture

Hearing

Our ears actually serve two functions:1) Allow us to hear2) Maintain balance and equilibrium

Hearing and balance work with the help of mechanoreceptors in the ear.These receptors are activated by movement caused by sound vibrationswe receive or by movement of the head and body.

Anatomically, the ear is divided into (pg. 285):

1) Outer ear (hearing)2) Middle ear (hearing)3) Inner ear (hearing and balance)

outer ear middle ear

inner ear

Outer Ear:1) Pinna (or auricle): outer, visible part of

ear.

2) External auditory canal: canal that focuses sound waves into middle ear. Ceruminous glands lie under skin and secrete earwax.

3) Tympanic membrane (ear drum): receives sound waves and transmits them to bones of middle ear. Only vibrates freely when pressure on both sides of eardrum are equal.

pinna

External auditory canal

eardrum

Homeostatic imbalance:Otitis externa (“Swimmer’s Ear”):Infection of the external auditory canal caused by bacteria or fungus. Too much moisture in ear can irritate and break down skin in canal, allowing bacteria or fungus to penetrate, leading to infection.

bacteria or fungus in canal

Middle ear(like a small box filled with air within the temporal bone of skull)

Three ossicles of the middle ear:

1) Hammer (or malleus): receives sound vibrations from eardrum and transmits to anvil.

2) Anvil (or incus): receives vibrations from hammer and transmits to stirrup.

3) Stirrup (or stapes): receives vibrations from anvil and transmits to oval window.

hammer

anvil stirrup middle ear

The middle ear is connected to the throat (nasopharynx) via theEustachian tube. This tube serves to equalize air pressure on bothsides of the eardrum. Normally, it is closed or flattened but can open through swallowing or chewing. (Think of how chewinggum or swallowing can help relieve pressure in your ears whenflying on an airplane).

eustacian tube

Because of this ear/throat connection, the ear becomes morevulnerable to infections as bacteria and viruses can enter the ear viathe throat.

For example, Otis media (inflammation of the middle ear) is often dueto a bacterial infection. As a result, fluid and pus fill the middle earcavity. If severe, a myringotomy (puncture in eardrum) may benecessary.

Ouch!

On medial side of middle ear “box”, there are two membranous“windows”:1) Oval window: larger, superior membrane. Bones of stirrup presson oval window, which sets fluids of inner ear into motion.

2) Round window: smaller, inferior membrane. Serves as a pressurevalve for the inner ear. It dissipates pressure generated by inner ear fluid vibrations.

Oval window

Round window

Inner ear:•Consists of a maze of bony chambers called the bony labyrinth ( or osseous labyrinth). This is located deep in the temporal bone,behind the eye socket.

•The bony labyrinth is filled with perilymph, a plasma-like fluid that helps with sound transmission.

inner ear

Three subdivisions of the bony labyrinth:

1) Cochlea: helps with hearing. Within the cochlea is the Organ of Corti which contains hearing receptors (hair cells).

hair cells

cochlea

2) Semicircular canals: helps with balance and helps us respond to angular or rotation movements, such as twirling motions.

Within canal are the crista ampullaris, a cluster of hair cells that detect changes in our position & send impulses through the vestibulocochlear nerve, to brain stem, cerebellum, and spinal cord. This results in our ability to quickly correct our body’s position/equilibrium with reflexes and quick movements.

Semicircular canals

3) Vestibule: helps with balance & helps us determine “up”from “down” with the help of otoliths (tiny stones made of calcium salts embedded in a gel-like membrane).

vestibule

How sound is transmitted:

Sound waves reach the cochlea (after traveling through outer ear and middle ear). These vibrations set the cochlear fluids in motion. The cochlear fluids then move the tectorial membrane which then bends the hairs of the Organ of Corti. tectorial membrane bends

hair cells

This stimulates the hair cells, which then transmit an electrical impulse through the cochlear nerve (part of the 8th cranial nerve, the vestibulocochlear nerve) which travels to the auditory cortex in the temporal lobe of the brain. This is where the interpretation of sound occurs.

vestibulocochlear nervesends nerve impulse to brain

And, voila, sound!!