Lecture 04 - Ears 1 Flashcards Preview

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Flashcards in Lecture 04 - Ears 1 Deck (19):

Subdivisions of the ear

3 parts
1. external (pinna, ext. auditory meatus) - funnels sound
2. middle (drum, air-filled cavity behind drum)
-3 bones transfer drum vibrations to oval window and fluid behind oval window
-auditory tube only opening of middle ear to outside world
3. inner (fluid filled sacs inside bony sac behind oval window)
-holds sensory hair cells for hearing and balance
-vestibular and cochlear nerves --> CN8


hair cells

innervated by CN 8



Pouch 1 - middle ear space and eustacian tube
Cleft 1 - ext. acoustic meatus
***membrane btw pouch and cleft 1 - ear drum
Branchial arch 1 - malleus and incus
Branchial arch 2 - stapes
otic placode invaginates to form otic vesicle, which gives rise to inner ear structures, including hair cell receptors and ganglion cells that form CN8


Ext. Ear innervation

Sensory innervation (touch, pressure, 2pt, pain temp) is complex
-from branches of mandibular division (V3, ariculotemporal)
-from branch of cervical plexus (greater auricular nerve)
-from small branches of facial (CN7) and vagus (CN10)

cranial nerve branches of 5, 7, 10 terminate in trigeminal nuclear complex of brainstem
-touch/pressure - chief nucleus of 5
-pain/temp - spinal trigeminal nucleus of 5


External auditory meatus

S-shaped from top down
-layered, slightly cone-shaped ear drum at end with malleus attached to backside


Ear drum

Healthy drum is smooth, pink and translucent
-innervation of drum is complex (outer surface is V3, 7, 10, inner surface is 9)
-Auditory tube is path for infection nasopharynx
-eardrum reddens (otitis media)
-can get fluid build up
-too much pressure = drum rupture



air molecules/particles in constant random motion, bumping into each other and exerting small pressure variations
-pressure wave transmitted through air with each particle only moving a small distance
-pressure wave pushes on ear drum
-sinusoidal wave of pressure moves ear drum in and out


Features of pure tones

Single frequency
-period = 1/f (time it takes to complete one cycle of wave
-distance between successive peaks is wavelength
-sound transmitted at velocity dependent on density and elasticity of substance


Characteristics of sound

Amplitude = loudness (decibels)
frequency = pitch
complexity = Timbre



Humans can hear over a huge range of intensities
-hearing sensitivity not the same at all frequencies --> best in speech frequency range
-in range, 16Hz to 16kHz heard (conversation = middle)
-aging affects high frequency hearing (presbicusis)


Middle ear

-narrow, air-filled, mucous membrane-lined chamber in temporal bone between ext. acoustic meatus and inner ear
-eustachian tube connects outside world to middle ear to nasopharynx
-Branch of CN9 (tympanic) innervates lining of middle ear and carries parasympathetics for the otic ganglion
-facial CN7 runs in back wall of middle ear and gives off chordae tympani that runs through middle ear space carrying taste and parasymp fibers
-3 bones malleus, incus and stapes = hammer, anvil and stirrup, to transfer vibrations of drum to fluid of inner ear
-two muscles: tensor tympani - malleus (CN5), stapedius muscle - stapes (CN7)

***major function to AMPLIFY airborne sound wave so it can be transferred to fluid of inner ear to activate hair cells (overcome impedance mismatch)


Inner ear

cochlea (hearing) and vestibular apparati (balance)
-cochlea = membranous duct sitting inside bony duct (coiled like a seashell - 2.5 turns)
-hair cells for hearing sit inside membranous labyrinth


Membranous labyrinth

separates bony labyrinth into 3 spaces:
1. scala vestibuli -> and tympani filled with perilymph
2. scala tympani
3. scala media - has hair sells --> filled with endolymph (high K, low Ca)

formed by reissner's membrane (separate from vestibuli)
stria vascularis pasted to bony wall and secretes endolymph
basilar membrane separates from scala tympani and auditory hairs cell sit here
tectorial membrane sits over hair cells



Sound wave transmitted to fluid via oval window --> scala vestibuli --> scala tympani via helicotrema --> round window bulge into middle ear --> pressure differential across membranous labyrinth (move up and down)

***only part of the length of the basilar membrane vibrates to a given frequency


Basilar membrane

width and stiffness varies from one end (base) to the other (apex)
-change resonant properties
-stiff narrow base vibrates at high freq
-apex frequency of vibration systematically becomes lower


Hair cell activation

-stereocilia bending initiated by up and down movment of basilar membrane
-tectorial membrane also moves up and down but in a slightly skewed direction so movement causes shearing effect on stereocilia and bending them
-stereocilia bending toward tallest cilia (kinocilium) --> depol
-depol opens VG Ca channels in hair cell body and Ca influx causes transmitter release
---> excites auditory nerve fibers


Auditory nerve fibers (ANFs)

3 rows of outer hair cells
1 row of inner hair cells along basilar membrane
-inner hair cell function transduces info about auditory environment to CNS (cochlear nucleus) via ANF
-95% ANF to inner (myelin), 5% to outer (unmyelinated)
-ANFs and their spikes carry info about particular sound frequency (diff ANF have diff best freqs)


Outer hair cells

cells in brain (from sup. olive) that send axons to cochlea to regulate activity primary target OHCs
-OHC function to help IHC do their job of encoding sound
-change length at very rapid frequency
-helps basilar membrane be more resonant and increases sensitivity (amplifies vibration and enhance IHC stim)
OHC = cochlear amplifier


Otoacoustic Emissions (OAEs)

-sounds which arise in ear canal when ear drum receives vibrations transmitted backgrounds from cochlea
-by product of cochlear amplifier process
-OHC energy propagates back thru bones and drum acts like speaker
-measure of cochlear function