D’YOUVILLE COLLEGE
BIOLOGY 108 -HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 2
SENSORY ORGANS II
Chapter 15
3.
The Ear (hearing and equilibrium):
a. External Ear (fig. 15 – 25): pinna and external acoustic meatus
• collection of sound waves, obstruction of foreign matter (coarse hairs),
waterproofing (ceruminous glands)
• tympanic membrane (internal boundary)
b. Middle Ear: also known as tympanic cavity
• three ossicles (malleus, incus, stapes – (figs. 15 – 25b & 15 – 26)) amplify
and transmit sound to oval window (attached to stapes) from tympanic membrane
(attached to malleus)
• tympanic reflex: two tiny muscles (stapedius and tensor tympani) damp
out loud sounds (fig. 15 – 26)
• round window, auditory tube: roles in pressure equalization
c. Internal Ear:
• osseous and membranous labyrinths (figs. 15 – 27 & 15 – 28)) containing
perilymph and endolymph:
• osseous labyrinth includes scala vestibuli, scala tympani, vestibule
and semicircular canals
• membranous labyrinth includes cochlear duct, utricle, saccule and
semicircular canals with ampullae
d. Physiology of hearing:
• cochlea (with its spiral organ of Corti) converts sound waves to nerve
impulses (fig. 15 – 28)
• sound waves are transmitted & amplified by tympanic membrane and
ossicles
• stapes sets oval window in vibration
• oval window vibrations generate pressure waves in perilymph
(equalized at round window); pressure waves set vestibular membrane vibrating,
producing pressure waves in endolymph of cochlear duct
• pressure waves in endolymph set basilar membrane vibrating
according to frequency of sound (deeper into cochlea for deeper pitches or lower
frequencies) (fig. 15 – 31)
• vibration of basilar membrane sets spiral organ of Corti vibrating (hairs
of hair cells distorted) against tectorial membrane causing generation of impulses
(fig. 15 – 28c)
• cochlear branch of vestibulocochlear nerve (cranial nerve VIII) carries
impulses to thalamus and onward to auditory cortex of temporal lobe (fig. 15 – 33)
e. Physiology of equilibrium:
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• utricle and saccule sense static equilibrium and semicircular canals sense
dynamic equilibrium
• sense organ of static equilibrium (monitoring position of head at rest) is
the macula (hair cells with hairs projecting into gelatinous covering containing
otoliths – (fig. 15 – 34); maculas are found in utricle & saccule); position of head
relative to gravity exerts pull on gelatinous membrane distorting hair processes &
generating impulses (fig. 15 – 35)
• sense organ of dynamic equilibrium (monitoring motion of head) is the
crista ampullaris (hair cells with hairs projecting into a flame-shaped cupula),
located within the ampulla of each semicircular canal (figs. 15 – 27 & 15 – 36); motion
of the head and fluid inertia distorts hairs producing impulses
• vestibular branch of vestibulocochlear nerve (cranial nerve VIII)
carries impulses to brainstem nuclei and cerebellum (fig. 15 – 37)