457
CHAPTER TWELVE
Nervous System III
The ear of a young person with normal hearing can
detect sound waves with frequencies varying from about
20 to 20,000 or more vibrations per second. The range of
greatest sensitivity is between 2,000 and 3,000 vibrations
per second
(f
g. 12.15)
.
and release neurotransmitter to the outside. The neu-
rotransmitter stimulates the ends of nearby sensory nerve
F
bers, and in response, they transmit nerve impulses along
the cochlear branch of the vestibulocochlear nerve (cranial
nerve VIII) to the brain.
Cochlear duct
Tectorial membrane
Scala tympani
Hair cells
Basilar
membrane
(a)
(b)
FIGURE 12.14
Spiral organ. (
a
) A micrograph of the spiral
organ and the tectorial membrane (300×). (
b
) A scanning electron
micrograph of hair cells in the spiral organ, looking down on the “hairs”
(bright yellow) (6,700×).
Scala vestibuli (perilymph)
Scala tympani (perilymph)
Cochlear duct
(endolymph)
Facial nerve
Oval window
Vestibular
membrane
Tectorial
membrane
Receptors
(hair cells)
20,000 cps
1,500 cps
500 cps
20 cps
Round
window
Tympanic
cavity
Tympanic
membrane
Malleus
Incus
Stapes
FIGURE 12.15
Receptors in regions of the cochlear duct sense
diF
erent frequencies of vibration, expressed in cycles per second (cps).
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