Page 95 - 84 human physiolofy part-1

Senses

the chain to transfer vibrations from the eardrum to the inner ear. The arrangement of these 3
bones is a sort of Rube Goldberg device: movement of the tympanic membrane causes movement
of the first bone, which causes movement of the second, which causes movement of the third.
When this third bone pushes down, it causes movement of fluid within the cochlea (a portion of
the inner ear). This particular fluid only moves when the stapes footplate is depressed into the
inner ear. Unlike the open ear canal, however, the air of the middle ear is not in direct contact
with the atmosphere outside the body. The Eustachian tube connects from the chamber of the
middle ear to the back of the pharynx. The middle ear in humans is very much like a specialized
paranasal sinus, called the tympanic cavity, it, like the paranasal sinuses, is a hollow mucosa
lined cavity in the skull that is ventilated through the nose. The mastoid portion of the temporal
bone, which can be felt as a bump in the skull behind the pinna, also contains air, which
ventilates through the middle ear.

Inner Ear (Cochlea, Vestibule, and Semi-Circular Canals)
The inner ear includes both the organ of hearing (the cochlea) and a sense organ that is attuned to
the effects of both gravity and motion labyrinth or vestibular apparatus. The balance portion of
the inner ear consists of three semi-circular canals and the vestibule. The inner ear is encased in
the hardest bone of the body. Within this ivory hard bone, there are fluid-filled hollows. Within
the cochlea are three fluid filled spaces: the tympanic canal, the vestibular canal, and the middle
canal. The eighth cranial nerve comes from the brain stem to enter the inner ear. When sound
strikes the ear drum, the movement is transferred to the footplate of the stapes, which presses into
one of the fluid-filled ducts of the cochlea. The hair cells in the organ of Corti are tuned to certain
sound frequencies, being responsive to high frequencies near the oval window and to low
frequencies near the apex of the cochlea.
The fluid inside this duct is moved, flowing against the receptor cells of the organ of Corti, which
fire. These stimulate the Spiral Ganglion, which sends information through the auditory portion
of the eighth cranial nerve to the brain.

Hair Cell
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at the top, for which
they are named. These cilia are the mechanosensors for hearing. Lightly resting atop the longest
cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the
cilia and allowing electric current into the hair cell. Hair cells, like the photoreceptors of the eye,
show a graded response, instead of the spikes typical of other neurons. One may ask how such a
wiggle of a hair bundle triggers a difference in membrane potential. Special hair cells are the
actual sensory receptors which will fire off action potentials when they are disturbed.
Immediately over the hair cells of the organ of Corti is an overhanging “tectorial membrane.”
When the Bones of the Middle Ear vibrate the oval window, these vibrations are transmitted to
the fluid within the cochlea and eventually cause the round window on the cochlea to bulge
outward. These vibrations disturb the membrane on which the Organ of Corti is located, causing
the hair cells to “rub” against the overhanging tectorial membrane. The disturbed hair cells will
then fire action potentials. The current model is that cilia are attached to one another by “tip
links”, structures which link the tips of one cilium to another. Stretching and compressing the tip
links may open an ion channel and produce the receptor potential in the hair cell. These graded
potentials are not bound by the “all or none” properties of an action potential. There are far fewer
hair cells than afferent nerve fibers in the cochlea. The nerve that innervates the cochlea is the
vestibulocochlear nerve, or cranial nerve number VIII. Neuronal dendrites innervate cochlear hair
cells. The neurotransmitter itself is thought to be glutamate. At the presynaptic juncture, there is a
distinct “presynaptic dense body” or ribbon. This dense body is surrounded by synaptic vesicles

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