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Human Anatomy & Physiology
6/25/2017
07.5 Mechanisms of Hearing & Equilibrium
Organs of Hearing
Organ of Corti
 Located within the cochlea
 Receptors = hair cells on the basilar membrane
 Gel-like tectorial membrane is capable of bending hair
cells
 Cochlear nerve (a division of cranial nerve VIII) attached to
hair cells transmits nerve impulses to temporal lobe
Perilymph vibrations from sound waves move the tectorial membrane
Hair cells are bent by the membrane
Length of the fibers in the basilar membrane “tunes” specific regions
to vibrate at specific frequencies
An action potential starts in the cochlear nerve.
Continued stimulation can lead to adaptation:
 When the same sounds keep reaching the ears, auditory
receptors tend to adapt or stop responding to those sounds.
 We are no longer aware of those sounds.
 This can be good. We are no longer aware of the continual
noise of a fish tank or the drone of a running motor after the
first few seconds.
Homeostatic Imbalances (p 289-291)
Deafness
 Defined as hearing loss of any degree
Two kinds of Deafness
 Conduction
 Sensorineural
Conduction deafness
 Occurs when something interferes with the conduction of sound
vibrations
o Ie. build up of earwax
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o Ie. Fusion of the ossicles (called otosclerosis
o Ie. Ruptured eardrum
o Ie. Otitis media
Sensorineural deafness
 Occurs when there is degeneration or damage to the receptor
cells in the organ of Corti, cochlear nerve, or to neurons going
to the brain.
 Often results from extended listening to excessively loud
sounds.
 A nervous system structure problem
Ménière’s disease
 Exact cause unknown, possible causes:
o Arteriosclerosis
o Degeneration of cranial nerve VIII
o Increased pressure of the inner ear fluids
 Progressive deafness
 Individuals become nauseated, often have vertigo so severe
they cannot stand without extreme discomfort
 Anti-motion sickness drugs often prescribed.
Organs of Equilibrium
Receptor cells are in two structures within the Vestibular apparatus
 Vestibule
 Semicircular canals
Equilibrium has two functional parts
 Static equilibrium
 Dynamic equilibrium
Static Equilibrium
Maculae
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Receptors in the vestibule
Report on the position of the head
Respond to straight line movements
Sends information via the vestibular nerve
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Anatomy of the maculae
 Hair cells are embedded in the otolithic membrane
 Otoliths (tiny stones) float in a gel around the hair cells
 Movements cause otoliths to bend the hair cells
Dynamic Equilibrium
Crista ampullaris
 Receptors in the semicircular canals
 Respond to angular or rotator movements
 Tuft of hair cells covered by the:
o Cupula (a gelatinous cap)
 Also sends information via the vestibular nerve
Inertia causes the endolymph in semicircular duct to lag behind.
Cupula moves in opposite direction of the body, bending hair cells
with it.
Keep in mind receptors in the semicircular canals, vestibule, sight,
proprioceptors of muscles & tendons all are important in providing
information used to control balance to the cerebellum.
Remember to:
Reduce, Recite, Reflect, & Review!
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