Download Three major areas of ear 1. External (outer) ear – hearing only 2

Three major areas of ear
1.
External (outer) ear – hearing only
2.
Middle ear (tympanic cavity) – hearing only
3.
Internal (inner) ear – hearing and equilibrium
Receptors for hearing and balance respond to separate stimuli
Are activated independently
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External Ear
Auricle (pinna)Composed of
Helix (rim); Lobule (earlobe)
Funnels sound waves into auditory canal
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External acoustic meatus (auditory canal)
Short, curved tube lined with skin bearing hairs, sebaceous glands, and
ceruminous glands
Transmits sound waves to eardrum
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Tympanic membrane (eardrum)
Boundary between external and middle ears
Connective tissue membrane that vibrates in response to sound
Transfers sound energy to bones of middle ear
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A small, air-filled, mucosa-lined cavity in temporal bone
Flanked laterally by eardrum
Flanked medially by bony wall containing oval (vestibular) and round
(cochlear) windows
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Epitympanic recess—superior portion of middle ear
Mastoid antrum
Canal for communication with mastoid air cells
Pharyngotympanic (auditory) tube—connects middle ear to nasopharynx
Equalizes pressure in middle ear cavity with external air pressure
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Three small bones in tympanic cavity: the malleus, incus, and stapes
Suspended by ligaments and joined by synovial joints
Transmit vibratory motion of eardrum to oval window
Tensor tympani and stapedius muscles contract reflexively in response
to loud sounds to prevent damage to hearing receptors
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Bony labyrinth
Tortuous channels in temporal bone
Three regions: vestibule, semicircular canals, and cochlea
Filled with perilymph – similar to CSF
Membranous labyrinth
Series of membranous sacs and ducts
Filled with potassium-rich endolymph
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Blue structure – series of ducts
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Central egg-shaped cavity of bony labyrinth
Contains two membranous sacs
1.
Saccule is continuous with cochlear duct
2.
Utricle is continuous with semicircular canals
These sacs
House equilibrium receptor regions (maculae)
Respond to gravity and changes in position of head
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Three canals (anterior, lateral, and posterior) that each define ⅔ circle
Lie in three planes of space
Membranous semicircular ducts line each canal and communicate with utricle
Ampullae of each duct houses equilibrium receptor region called the crista
ampullaris
Receptors respond to angular (rotational) movements of the head
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A spiral, conical, bony chamber
Size of split pea
Extends from vestibule
Coils around bony pillar (modiolus)
Contains cochlear duct, which houses spiral organ (organ of Corti) and
ends at cochlear apex
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Cavity of cochlea divided into three chambers
Scala vestibuli—abuts oval window, contains perilymph
Scala media (cochlear duct)—contains endolymph
Scala tympani—terminates at round window; contains perilymph
Scalae tympani and vestibuli are continuous with each other at helicotrema
(apex)
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The "roof" of cochlear duct is vestibular membrane
External wall is stria vascularis – secretes endolymph
"Floor" of cochlear duct composed of
Bony spiral lamina
Basilar membrane, which supports spiral organ
The cochlear branch of nerve VIII runs from spiral organ to brain
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EM viewed from tectorial membrane
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Sound is
Pressure disturbance (alternating areas of high and low pressure)
produced by vibrating object
Sound wave
Moves outward in all directions
Illustrated as an S-shaped curve or sine wave
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Frequency
Number of waves that pass given point in given time
Pure tone has repeating crests and troughs
Wavelength
Distance between two consecutive crests
Shorter wavelength = higher frequency of sound
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Amplitude
Height of crests
Amplitude perceived as loudness
Subjective interpretation of sound intensity
Normal range is 0–120 decibels (dB)
Severe hearing loss with prolonged exposure above 90 dB
Amplified rock music is 120 dB or more
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Pitch
Perception of different frequencies
Normal range 20–20,000 hertz (Hz)
Higher frequency = higher pitch
Quality
Most sounds mixtures of different frequencies
Richness and complexity of sounds (music)
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Sound waves vibrate tympanic membrane
Ossicles vibrate and amplify pressure at oval window
Cochlear fluid set into wave motion
Pressure waves move through perilymph of scala vestibuli
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Waves with frequencies below threshold of hearing travel through
helicotrema and scali tympani to round window
Sounds in hearing range go through cochlear duct, vibrating basilar membrane
at specific location, according to frequency of sound
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Waves with frequencies below threshold of hearing travel through
helicotrema and scali tympani to round window
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Sounds in hearing range go through cochlear duct, vibrating basilar membrane
at specific location, according to frequency of sound
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Fibers near oval window short and stiff
Resonate with high-frequency pressure waves
Fibers near cochlear apex longer, more floppy
Resonate with lower-frequency pressure waves
This mechanically processes sound before signals reach receptors
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Cells of spiral organ
Supporting cells
Cochlear hair cells
One row of inner hair cells
Three rows of outer hair cells
Have many stereocilia and one kinocilium
Afferent fibers of cochlear nerve coil about bases of hair cells
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Stereocilia
Protrude into endolymph
Longest enmeshed in gel-like tectorial membrane
Sound bending these toward kinocilium
Opens mechanically gated ion channels
Inward K+ and Ca2+ current causes graded potential and
release of neurotransmitter glutamate
Cochlear fibers transmit impulses to brain
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Stereocilia
Protrude into endolymph
Longest enmeshed in gel-like tectorial membrane
Sound bending these toward kinocilium
Opens mechanically gated ion channels
Inward K+ and Ca2+ current causes graded potential and
release of neurotransmitter glutamate
Cochlear fibers transmit impulses to brain
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Impulses from cochlea pass via
spiral ganglion to cochlear nuclei
of medulla
From there, impulses sent
To superior olivary nucleus
Via lateral lemniscus to Inferior
colliculus (auditory reflex center)
From there, impulses pass to
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medial geniculate nucleus of
thalamus, then to primary auditory
cortex
Auditory pathways decussate so that
both cortices receive input from
both ears
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Pitch perceived by impulses from specific hair cells in different positions along
basilar membrane
Loudness detected by increased numbers of action potentials that result when
hair cells experience larger deflections
Localization of sound depends on relative intensity and relative timing of
sound waves reaching both ears
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Vestibular apparatus
Equilibrium receptors in semicircular canals and vestibule
Vestibular receptors monitor static equilibrium
Semicircular canal receptors monitor dynamic equilibrium
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Sensory receptors for static equilibrium
One in each saccule wall and one in each utricle wall
Monitor the position of head in space, necessary for control of posture
Respond to linear acceleration forces, but not rotation
Contain supporting cells and hair cells
Stereocilia and kinocilia are embedded in the otolith membrane studded with
otoliths (tiny CaCO3 stones)
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Maculae in utricle respond to horizontal movements and tilting head side to
side
Maculae in saccule respond to vertical movements
Hair cells synapse with vestibular nerve fibers
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Hair cells release neurotransmitter continuously
Movement modifies amount they release
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Bending of hairs in direction of kinocilia
Depolarizes hair cells
Increases amount of neurotransmitter release
More impulses travel up vestibular nerve to brain
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Bending away from kinocilium
Hyperpolarizes receptors
Less neurotransmitter released
Reduces rate of impulse generation
Thus brain informed of changing position of head
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Sensory receptor for rotational acceleration
One in ampulla of each semicircular canal
Major stimuli are rotational movements
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Sensory receptor for rotational acceleration
One in ampulla of each semicircular canal
Major stimuli are rotational movements
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Each crista has supporting cells and hair cells that extend into gel-like mass
called ampullary cupula
Dendrites of vestibular nerve fibers encircle base of hair cells
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Hair Cell Transduction
Mechanosensitive Ion Channels are gated by Cilia displacement; they are
associated with tonic release of Glutamate at rest but levels can either
increase or decrease depending on direction of Cilia deflection.
TOWARD TALLEST CILIA: Channels open when tip links are stretched causing
influx of K⁺ and Depolarization. This opens Voltage-Gated Ca²⁺ channels at the
Basolateral surface of the Hair Cell triggering ↑ Glutamate release
TOWARD SHORTEST CILIA: Tip link relax = ↓ Glutamate release
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Bending of hairs in the opposite direction causes
Hyperpolarizations, and fewer impulses reach the brain
Thus brain informed of rotational movements of head
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Equilibrium information goes to
reflex centers in brain stem
Allows fast, reflexive responses to
imbalance
Impulses travel to vestibular
nuclei in brain stem or
cerebellum, both of which
receive other input
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Equilibrium information goes to
reflex centers in brain stem
Allows fast, reflexive responses to
imbalance
Impulses travel to vestibular
nuclei in brain stem or
cerebellum, both of which
receive other input
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Three modes of input for balance
and orientation:
Vestibular receptors
Visual receptors
Somatic receptors
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