physiology of hearing & balance

8/6/2019 Physiology of Hearing & Balance

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Physiology ofHearing & Balance

Dr. Archana Sudhir


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The Nature of Sound

Sound is any audible

vibration of molecules

Vibrating objectpushes air molecules

into zones of

compression

separated by zones ofrarefaction


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Properties of Sound

Frequency the number of waves that

pass a given point in a given time

Pitch

perception of different frequencies(we hear from 2020,000 Hz)

Intensity The power transmitted by a

wave through an unit area.

Loudness The perception of intensity.


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Main Components ofthe Hearing

MechanismDivided into 4 parts

(by function):

Outer Ear

Middle Ear

Inner Ear

Central AuditoryNervous System


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Functions ofthe Outer Ear

Gathers sound waves

Increases Pressure in a frequency

sensitive way.

Aids in localization


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Functions of the Middle Ear

Couple sound energy

to the cochlea

Impedance matching

Protects Cochlea

Preferential

application of soundto one window.


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Impedance Transformer

Large area ofTM incomparison to small areaof foot plate (pressureincreases inversely to the

ratio of these areas) Ossicular lever ratio

(Malleus is 1.3 timeslonger than incus)

Buckling action ofTM

Ligaments suspendingossicles.


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Impedance Efficiency

Middle ear converts low pressure highdisplacement movements of the ear druminto high pressure low displacement

movements needed for the cochlear fluidmovement.

50% of sound energy from TM gets

transmitted and absorbed in the cochlea. Without middle ear only 1% of sound

energy will be absorbed by the cochlea.


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Role of Middle Ear Muscles

Tensor tympani attaches to the neck ofmalleus. It pulls the drum medially.

Stapedius muscle attaches to the posterior

aspect of neck of stapes. Contraction of these muscles increase the

stiffness of ossicular chain thus blunting

low frequencies. Stapedius contracts in response to loud

sounds and acts as an in built ear plug.


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Bone Conduction

Bone vibration conducted through ext

canal

Skull vibration

ossicles lag behind. Differential distortion of bony cochlea

Direct vibration of osseous spiral lamina

Skull vibration via CSF to endolymph


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Structures ofthe Inner Ear Bony Labyrinth

Bony Cochlea

Vestibule Semi Circular Canals

Membranous

Labyrinth

Cochlea Duct Utricle & Saccule

Semi Circular Canals


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Organ of Corti

16,000 hair cells have 30-100 stereocilia(microvilli )

Microvilli make contact with tectorial membrane (gelatinous membrane

that overlaps the spiral organ of Corti)

Basal sides of inner hair cells synapse with 1st order sensory neurons

whose cell body is in spiral ganglion


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Movement of pressure waves through the cochlea


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MOVEMENTS OFTHE BASILAR MEMBRANE

AND THE DEFLECTION OFTHE STEREOCILIA.


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Potassium Gates of Cochlear Hair Cells

Stereocilia bathed in high K+ concentration creatingelectrochemical gradient from tip to base

Stereocilia of OHCs have tip embedded

in tectorial membrane which is anchored

Movement of basilar membrane bends

stereocilia

Bending pulls on tip links

and opens ion channels

K+ flows in -- depolarizing

it & causing release ofneurotransmitter stimulating

sensory dendrites at its base


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Theories Of Hearing

Place theory of Helm holtz

Telephone theory ofRutherford

Volley theory of Wever

Traveling wave theory of Bekesy


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CENTRAL AUDITORY PATHWAYS


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Auditory Cortex


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APPLIED PHYSIOLOGY

EAC BLOCK - 30db HL

TM PERFORATION - 26db HL

TM PERFORATION WITH OSSICULAR INTERRUPTION -

26.5 +7.3+ 5=38.3dbHL

TOTAL LOSS OFTM WITH OSSICULAR INTERRUPTION -

26.5 +7.3+ 16.2=50dbHL

OSSICULAR INTERRUPTION WITH INTACTTM -

38+15=54dbHL

OSSICULAR INTERRUPTION WITH INTACTTM WITH

CLOSED OVAL WINDOW - 60dbHL


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Vestibular Apparatus Vestibule

Utricle

Saccule

Semicircularcanals

- lateral, superior, posterior

Vestibularnerve


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Equilibrium

Static equilibrium is perception of head

orientation perceived by macula

Dynamic equilibrium is perception of motion

or acceleration linear acceleration perceived by macula

angular acceleration perceived by crista


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T

he Saccule and Utricle Saccule & utricle

chambers containing

macula patch of hair cells with their

stereocilia & one kinociliumburied in a gelatinousotolithic membraneweighted with granules

called otoliths otoliths add to the density &

inertia and enhance thesense of gravity and motion

Otoliths


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Macula of Saccule and Utricle

With the head erect, stimulation is minimal, but when the head is tilted,

weight of membrane bends the stereocilia (static equilibrium)

Linear acceleration is detected since heavy otolith lags behind (one type of

dynamic equilibrium)


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Crista Ampullaris of Semicircular Ducts

Crista ampullaris consists of hair cells buried in a mound of

gelatinous membrane

Orientation of ducts causes different ducts to be stimulated by

rotation in different planes


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Crista Ampullaris & Head Rotation

As head turns, the endolymph lags behind

pushing the cupula and stimulating its hair cells


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Equilibrium Projection Pathways

Unmyelinated plexus at the base of

sensory epithelium gives rise to primary

vestibular neuron

Central processes of primary vestibular

neurons synapses with vestibular

nucleus of pons, cerebellum


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Vestibular Nuclei

Cristae of SCC &

Cerebellum

Superior

vestibular nuclei

Bechterew

Medial

Longitudinal

Fasciculus

Cerebellum &

Utricular Macula

Lateral

vestibular nucleiDieter

Vestibulo Spinal

Tract, ReticuloSpinal Tract

Cristae Cerebellum Medial vestibular

nuclei Schwalbe

Medial

Longitudinal

Fasciculus

Utricular &

Sacular Maculae

Descending

vestibular nuclei

Cerebellum &

Reticular

Formation


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Ascending Vestibular Projections

Lateral &

Superior

vestibular nuclei

Thalamus

Sensori Motor

Cortex

Visual ProjectionsProprioceptive

Projections


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VestibularReflexes

Vestibulo-spinal Helps maintain center of gravity

Vestibulo-ocular Helps maintain stability of visual field

Vestibulo-collic: Helps to maintain stability of the head during movement

of the torso.


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Vestibulo OcularReflexes


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CLINICAL RELEVENCE

GIDDINESS

1. NON CORRECTABLE VISUAL IMPAIRMENT.

2. NEUROPATHY.

3. VESTIBULAR DYSFUNCTION.

4. CERVICAL SPONDYLOSIS.

5. ORTHOPAEDIC DISTURBANCES.

6. CARDIAC DISORDERS.

7. NEUROLOGICAL DEFICITS.


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ASSESMENT

HISTORY

IDENTIFICATION OF PRESENCE/

ABSENCE OF

VEST

IBULAR

COMPONENT.1. VESTIBULO-SPINAL FUNCTION.

2. VESTIBULO OCULARFUNCTION.


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VESTIBULO-SPINAL FUNCTION

ROMBE

RGS

TES

T

UNTERBERGERS TEST


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VESTIBULO-OCULARFUNCTION

NYSTAGMUS INVOLUNTARY DEVIATION OF EYES AWAY

FROM DI

REC

TION O

FGAZE

FOLLOWED B

YARETURN OFTHE EYES TO THEIR ORIGINAL

POSITION.

3 TYPES1. CENTRAL

2. OCULAR

3. VESTIBULAR


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VESTIBULAR NYSTAGMUS

RHYTHMIC

FAST AND SLOW PHASES

NAMED AFTERFAST PHASE.

3 TYPES1. SPONTANEOUS

2. POSITIONAL

3. INDUCED.


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VESTIBULAR NYSTAGMUS

SPONTANEOUS NYSTAGMUS GRADE 1.

GRADE 2.

GRADE 3.

POSITIONAL NYSTAGMUS HALLPIKE MANOEUVRE


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INDUCED NYSTAGMUS

ROTATIONAL TESTS Nystagmus Induced by accelerating and

decelerating rotating chair, tests both labyrinthssimultaneously

CALORIC TESTS COWS- cold water opposite side, warm water

same side, direction of nystagmus

Extent of caloric response indicates function of

labyrinth


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Electronystagmograghy

Positive potential between the cornea

and retina recorded as eyes move from

straight ahead gaze

Test includes different head positions,

eyes open, closed and caloric tests


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physiology of hearing & balance

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