Neuroscience: Auditory & Vestibular Systems

Front 1

Where Auditory & Vestibular Receptors are located?

What is the Bony Labyrinth?

Back 1

-inner ear & deep inside temporal lobe
-bony labyrinth (cavities in temporal bone) houses auditory & vestibular sense organs

Front 2

membranous labyrinth

Back 2

-lies w/ in bony labyrinth cavities
-consists of fine membrane (epithelia)
-membranes create series of interconnected sacs & ducts
-follows form of bony labyrinth but smaller
-filled w/ endolymph

Front 3

Where is perilymph found?

Back 3

-outside membranous labyrinth in space bw bony & membranous labyrinths

Front 4

What is found in each Labyrinth?

Back 4

(left & right)
-auditory apparatus (cochlea)
-vestibular apparatus: 3 semicircular canals/ducts & pair of otolith organs (utricle & saccule)

Front 5

Auditory System

Back 5

-complex sounds broken down into simple discrete vibrations
-translated into neural signals processed by brain

Front 6

Transmission of Sound Vibrations

Back 6

outer ear/external aud. meatus--> tympanic membrane (ear drum)--> ossicles (malleus, incus, stapes)-->oval window-->cochlea: contains organ of corti *hairs CN VIII

Front 7

eustachian tube:

Back 7

-air pressure in middle ear cavity equal to atmospheric pressure (connect middle ear to nasal cavity)

Front 8

Cochlea

Back 8

-spirals 2 1/2 turns in the inner ear in temporal bone
-3 fluid filled compartments (scalae):
1) scala vestibuli (perilymph): located on oval window side of cochlear duct
2) scala tympani (perilymph): continous w/ scala vestibuli @ helicotrema & located on round window side of cochlear duct
3)scala media: (cochlear duct) (endolymph) lies bw 2 other scala & extend into cochlea, houses organ of corti

Front 9

How does Cochlea work?

Back 9

-vibration of stapes & oval window puts pressure on fluid in scala vestibuli
-inward & outwards mvmt of round window of scala tympani
-pressure of scala vestibuli & tympani set up mvmt of scala media (cochlear duct)
-hair cells of organ of corti (sits on basilar membrane) stimulated by sound-wave vibration
-hair cell innervated by afferent/efferent axons of CN VIII

Front 10

Sensory Transduction

Back 10

Mxl vibration to neural impulses
1. sound transduced into mxl vibrations
2. sound enter inner ear canal (tympanic mem transmit thru middle ear cavity inner ear, cochlea)
3. pressure amplified by ossicles - act as piston (tympanic mem & oval window)
4. Additional amp in pressure gained bc area of tymp mem is greater than oval window, mxl vibration transduced into neural cavity in cochlea
-hair cells responsible for transduction process

Front 11

How hair cells are responsible for sensory transduction?

Back 11

-basilar mem move-->mvmt cause stereocilia to bend & angular displacement
-mxl deformation opens ion channels in hair cells
-flow inward current carried by cations generates receptor potential-->release NT-->depolarize peripheral CN VIII-->AP initiate bipolar neuron-->cell bodies (spiral ganglion) of CN VIII processes terminate in cochlear nuclei (medulla of brainstem)

Front 12

Response to Different Frequencies

Back 12

diff freq. of sound have peak amp @ diff points along mem.
thus sound freq will excite diff hair cells along cochlea:
-high frequency sounds have peak amp near base of cochlea (near oval window)
-low frequency sounds have peak amp near apex (near helicotrema)

Front 13

Hair Cells in the Organ of Corti

Back 13

1. hair cells along organ of corti are not identical & different electrochemical properties
2. at cochlea base have stereocilia that are short & stiff / hairs at apex stereocilia are 2x longer
3. diff structure cause cell to be freq. selectivity
4. single row of inner hair cells detect sound & excite most afferents of CN VIII
-3 rows of outer hair cells can alter length result in change in sensitivity of tuning of local region of corti

Front 14

Central Auditory Pathway

Back 14

8CSLIMA: info in CN VIII to aud. cortex
8=CN VIII; AP to...
C=Cochlear Nuclei; dorsal & ventral, in medulla
-tonotypically organized: based on frequenc
-most axons of cochlear decussate in lower pons (~20% don't, ascend ipsil)
S=Superior Olivary Complex (from ventral cochlear nuclei)
L=Lateral lemniscus (from ventral or dorsal cochlear nuclei)
I=Infereior Colliculus; fibers from lat. lem synapse here
M=med. genicualte; axons from inf. colliculus go here
-neurons send axons (aud. radiations) to prim. aud. cortex thru internal capsule
A=Primary Aud. Cortex (optic radiations), transverse gyri of Heschel

Front 15

Unilateral Lesion of Central Aud. Pathway

Back 15

-Cause more marked hearing loss on side opposite the lesion

Front 16

Fxns of parts of Central Auditory Pathway

Back 16

Primary Aud. Cortex: conscious hearing of sound

Superior Olivary Nucleus & Inf. Colliculus: orientation/localization of sound

Reticular Formation: CNS activation/alertness

Front 17

Descending Auditory Pathway

Back 17

-descending efferent pathway from cortex to cochlea via CN VIII
-can excite/inhibit cochlea activity
inhibition: cause suppress extraneous noise & sharpen clarity of hearing

Front 18

Middle Ear Muscles

Back 18

1. tensor tympani muscle: inserts into malleus (CN V)
-fxn: dampen vibration of tympanic mem to loud nosie

2. Stapedius Muscle: inserts into stapes (CN VII)
-fxn: dampen vibration of stapes in response to loud noise

Front 19

Conductive Deafness

Back 19

-caused by interruption of passage of sound waves thru external/middle ear
-foreign body: external aud. meatus
-Otosclerosis: most frequent, fixiation of stapes, excessive bone growth of bony labyrinth
-Otitis media: inflammation of middle ear (meningitis, brain abscecces)

Front 20

Sensorineural Deafness

Back 20

-nerve deafness/preceptive deafness
-causes:
A. diesase of cochlea, CN VIII, or central aud. connections
B. drugs or toxins
C. rubella infection
D. Presbycusis
E. Acoustic neuroma (a schwannoma) tumor CN VIII
-may be located in cerebellopontine angle

Front 21

Hyperacusis

Back 21

-damage to CN V or tensor tympani muscle
-prevent muscle from dampen sound, increased acuity of hearing & hypersensitivity to low freq
-CN VII damagae: prevent stapedius muscle from dampen sound

Front 22

Tinnitus

Back 22

-abnormal sound (ringing, hissing, roaring, buzzing, or humming)
-causes:
A. Acoustic neuroma
B. Irritation of spiral organ of Corti in cochlea
C. Irritation of CN VIII
D. Overproduction of endolymph

Front 23

Meniere's Syndrome (Disease)

Back 23

-due to overproduction of endolymph or inability to absorb endolymph
-excessive quantity of endolymph= endolyphatic hydrops
-symptoms: tinnitus & vestibular disturbances (dizziness, vertigo, nausea, vomiting, nystagmus)

Front 24

Unilateral Lesion of Auditory Pathway

Back 24

bilateral diminuation of hearing, more marked on contralateral side (ascend. fibers mostly crossed)

Front 25

Lesions in Central Auditory Path

Back 25

do not lead to total deafness unless bilateral

Front 26

Lesions in auditory cortex

Back 26

-do not disrupt perception of sound frequency
-affect ability to localize sound in space
-impair process auditory info

Front 27

Vestibular System

Back 27

-detects position & motion of head in space by integrating info from peripheral receptors located in inner ear
-provides info to NS important to 2 areas:
1. reflex control of eye mvmts (VOR)
2. reflex control of upright posture (VSR)

Front 28

Semicircular Canals

Back 28

-oriented so can detect angular acceleration in 3 planes (horizontal, ant, post) & direction of head mvmt
-both ends of canals terminate in utricle & an enlargement called ampulla
-contains the vestibular hair cells: in region of ampulla where eptih. is thickened (specialized receptors)
-cupula: gelatinous mass suround hair cells

Front 29

Sensory Transduction Process in Semicircular Canals

Back 29

1. sensory transduction based on inertia
2. head moved, bony semicircular canals turn w/ it
3. fluid filling endolymph, not attach, retains orig. position (left behind)
4. moving ampulla pushed against stationary fluid, create pressure gradient (cupula)
-cause bending of hair, stim depend of direction of hair, & release of NT to CN VIII
5. inertia overcome, canal fluid return to resting. hair cells stim only during changes in rate of motion

Front 30

Depolarization/Hyperpolarization of Vestibular Hair Cells

Back 30

-hair cell consist of single kinocilium (tall) & shorter sterocilia
-bending of hair toward kinocilium=depolarization (increase fire rate of CN VIII)
-bend away kinocilium=hyperpolarization(decrease fire rate of CN VIII)
-semiciruclar canals on either side of head work in concert

Front 31

Otolith Organs

Back 31

-the utricle & saccule
-provide NS w/ 2 types of info: linear acceleration & head position w/ respect to gravitational forces
-hair cells located w/in utricle & saccule, supported by macula in each chamber
-in horizontal plane=macula of utricle
-in vertical plane=macula of saccule
-hair cells of utricle respond most to linear acceleration & tilting head forward & backward
-hair cells of saccule respond most to vertical displacement
-hairs in gelatinous substance where otoliths (otoconia: tiny stones, calcium carbonate) embedded

Front 32

Central Vestibular Pathway

Back 32

1. CN VIII (vestibular component) composed of bipolar neurons w/ cell bodies in vestibular ganglia
-central portion of axons terminate/synapse in vestibular nuclei in medulla:
A. lateral vestibular nucleus (deiter's nucleus)
B. Medial Vestibular nucleus
C. Superior Vestibular nucleus
D Inferior Vestibular nucleus

Front 33

Superior Vestibular Nucleus

Back 33

Input: ampullae of semicircular canals, maculae in utricle & saccule, cerebellum

Pathway (efferent fibers):
1. axon run ascend component (ipsi & contra) MLF & terminate in nuclei of CN III, IV, VI (extrinsic eye muscles)
2. axons travel to cerebellum (inf. peduncle or dorsal pontine reticular formation)

fxn: peripheral response to vestibular stimulation, vestibulo-ocular reflex, coordination of head & eye mvmt to maintain stable gaze w/ head moves

Front 34

Lateral Vestibular Nuclei

Back 34

Input: afferents from macula of utricle (some cerebellum)

Pathways (efferent fibers):
-to SC-->form ipsil. lateral vestibulospinal tract
-to thalamus--> cortex

Fxn: maintain postureal balance (vestibulospinal reflexes) & perception of balance

Front 35

Medial Vestibular Nuclei

Back 35

Input: afferents from ampullae of semicircular canals & cerebellum

Pathway (efferent fibers):
-form bilateral medial vestbulospinal tract
-occulomotor nuclei CN III, IV, VI

Fxn: coordinate eye, neck, & head mvmt

Front 36

Inferior Vestibular Nucleus

Back 36

Input: afferent from maculae of utricle & saccule & cerebellum

Pathway (efferent fibers):
-go to cerebellum & reticular formation (LVST/MVST)
-numerous crossing fibers to contralateral vestibular nuclei

Fxn: integrates vestibular singals from 2 sides of body w/ info from cerebellum & reticular formation

Front 37

Vestibulo-Ocular Reflex

Back 37

VOR
1. help maintain a stable visual imag eon the retina while head is moving
2. some cells in both medial & superior vestibular nuclei located in medulla participate
3. fxn of nuclei tested by turning head to L/R. Eyes rotate in opp direction, helps maintain visual field in horizontal plane

Front 38

Tests of Vestibular Fxn

Back 38

Rotation Test
-rotation around vertical axis causes nystagmus
-response: horizontal nystagmus, dysmetria, & vertigo

Front 39

Caloric Irrigation Test

Back 39

-external aud. meatus irrigated w/ cold/warm water
-mvmt of endolymph stim. vestibular nerve induce nystagmus
-response: cold: nystagmus to opp side / hot: nystagmus to same side
COWS

Front 40

Vertigo

Meniere's Disease (Inner Ear Disease)

Back 40

-sense of irregular motion/whilrling
-illusion of mmt when none

-increased production of endolymph or decreased absorption
-episodic vertigo attacks, tinnitus, hearing loss, neusea, vomiting, feel full, pressure in ear
-horizontal nystagmus: past pointing, fall toward affected side

Front 41

Labyrinthitis

Motion Sickness

Back 41

-caused by bacterial/viral infection or toxicity
-symptoms same as menier's

-conflict in sensory input from visual and vestibular receptors

Front 42

Benign Paroxysmal Positional Vertigo

Back 42

-common cause of recurrent vertigo
-elicited by certain head positions
-cause by 1. cupulothiasis: dislocation of utricular macular otoliths to cupula of post. semicircular duct or 2. Canaliths: otoliths that have fallen into semicircular canals, cause abnormal mvmt of endolymphs, can also stick to gelatinous cupula in ampulla (false stimulation)

Front 43

Vestibular Schwannoma

Cochlear Implantation

Back 43

-acoustic neuroma
-hallmark of neurofibromatosis 2; an autosomal dominant disease charact. by formation of mult tumors in CNS & PNS

-symptoms after cochlear implantation possible
-vestibular dysfxn (vertigo & imbalance) may also occur after implantation of cochlear prosthesis to improve hearing