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SENSORY ORGANS (VISION,
HEARING, AND EQUILIBRIUM)
Ears
HEARING
How old are your ears? ASAP Science
 Hearing Test
 Discovery: How Hearing Works
 Process of Hearing: Animation **
 Organ of Corti
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2.) Ears
 a. External ear
 (1) Auricle (a.k.a. pinna) collects sound waves
 (2) External auditory meatus – S shaped tube in
temporal bone, approx 2.5 cm
 b. Middle ear
 (1) Tympanic cavity – air filled space in temporal
bone
 (2) Tympanic membrane (eardrum)
 (a) Outer surface: thin layer of skin; inner:
mucous membrane
 (b) Vibrates from sound waves
THE EAR
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(3) Auditory ossicles (3)
 (a) Hammer (malleus), anvil (incus), stirrup
(stapes)
 (b) Bridge from the eardrum to inner ear –
transmit vibrations from eardrum to fluid of
inner ear
 (c) Stapes – transmits vibrations to fluid in
oval window of inner ear, causing fluid to
vibrate
 (d) Also amplify vibrations (large SA of
eardrum to smaller SA of oval window leads to
increased vibration force & pressure)
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(4) Eustachian tubes-connect middle ear to
nasopharynx aka pharyngotympanic (auditory)
tubes
 (a) Equalize air pressure on inside of eardrum
 (b) Allow bacteria & infections to travel to ear
from nose are
 c. Inner ear-complex system of intercommunicating
chambers & tubes-a.k.a. labyrinths (2)
 (1) Osseous labyrinth-bony canal in temporal
bone
 (2) Membranous labyrinth-tube inside osseous
labyrinth (has same shape)
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LABYRINTHS
(a) Endolymph-fluid inside membranous labyrinth
 (b) Perilymph-fluid between the two labyrinths
 (3) Parts of labyrinths
 (a) Cochlea-bony core & thin bony self (threads on a
screw)
 i. Shelf divides bony labyrinth into 2
compartments:
• Upper: scala vestibuli (oval window leads to
scala vestibuli)
• Lower: scala tympani (leads to round window)
 ii. Cochlear duct: membranous labyrinth between
the scala vestibuli & scala tympani; houses the
organ of Corti
 iii. Vestibular membrane-separates the cochlear
duct from the scala vestibuli
 iv. Basilar membrane-separates the cochlear
duct from the scala tympani
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v. Organ of Corti
• [a] contains hearing receptors called hair cells
• [b] on upper surface of basilar membrane
• [c] covered by tectorial membrane
• [d] best frequency perception: 2-3 thousand cycles
per second (cps)
(b) semicircular canals-equilibrium
 (c) vestibule-bony chamber between cochlea &
semicircular canals; for hearing & equilibrium
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d. Auditory nerve pathway
(1) auditory nerve to temporal lobes (cochlear nerve to
medulla to midbrain to thalamus to auditory cortex in
temporal lobes)
 (2) some fibers cross over to other side (not all); so
impulses are interpreted by both sides of brain
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e. sense of equilibrium-two parts (sense of
static & dynamic equilibrium)
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(1) static equilibrium-maintains stability of head &
body when they’re motionless
 (a) organs of static equilibrium located in vestibule
(between cochlea & semicircular canals)
 (b) membranous labyrinth inside vestibule expands
into 2 chambers: the utricle & the saccule
 (c) macula-small patch of hair cells in saccule and
anterior wall of utricle
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i. hair cells and supporting cells
ii. Hair cells project into a gelatinous mass studded
with otoliths (CaCO3 crystals)
iii. Otoliths in mass of material increase inertia
(resistance to change in motion)
iv. Bending hair cells leads to sending of impulses
informing brain of head’s position
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(2) dynamic equilibrium-balance during
movement
 (a) three semicircular canals (at right angles &
in different planes) detect motion
 (b) Ampullae-swelling on ends of semicircular
canals where they join the vestibule
 (c) crista ampularris-sensory organs in
ampullae
 i. sensory cells & supportive cells
 ii. Hairs extend upward into gelatinous mass
called the cupula
 iii. Respond to changes in acceleration &
deceleration
 iv. Impulses sent to cerebellum for analysisactivate muscles to maintain balance
LABYRINTHS