56514243 physiology-of-hearing-balance
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Transcript of 56514243 physiology-of-hearing-balance
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Physiology of Hearing & Balance
Dr. Archana Sudhir
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The Nature of Sound
Sound is any audible vibration of molecules
Vibrating object pushes air molecules into zones of compression separated by zones of rarefaction
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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 20–20,000 Hz) Intensity – The power transmitted by a
wave through an unit area. Loudness – The perception of intensity.
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Main Components of the Hearing Mechanism
Divided into 4 parts (by function):
Outer Ear Middle Ear Inner Ear Central Auditory
Nervous System
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Functions of the 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 sound to one window.
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Impedance Transformer Large area of TM in
comparison to small area of foot plate (pressure increases inversely to the ratio of these areas)
Ossicular lever ratio (Malleus is 1.3 times longer than incus)
Buckling action of TM Ligaments suspending
ossicles.
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Impedance Efficiency
Middle ear converts low pressure high displacement movements of the ear drum into high pressure low displacement movements needed for the cochlear fluid movement.
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 of
malleus. 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 of the 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 OF THE BASILAR MEMBRANE AND THE DEFLECTION OF THE STEREOCILIA.
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Potassium Gates of Cochlear Hair Cells Stereocilia bathed in high K+ concentration creating
electrochemical 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 of neurotransmitter stimulating sensory dendrites at its base
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Theories Of Hearing
Place theory of Helm holtz Telephone theory of Rutherford 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 OF TM WITH OSSICULAR INTERRUPTION -
26.5 +7.3+ 16.2=50dbHL OSSICULAR INTERRUPTION WITH INTACT TM -
38+15=54dbHL OSSICULAR INTERRUPTION WITH INTACT TM WITH
CLOSED OVAL WINDOW - 60dbHL
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Vestibular Apparatus
Vestibule Utricle Saccule
Semicircular canals - lateral, superior, posterior
Vestibular nerve
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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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The Saccule and Utricle
Saccule & utricle chambers containing macula
patch of hair cells with their stereocilia & one kinocilium buried in a gelatinous otolithic membrane weighted with granules called otoliths
otoliths add to the density & inertia and enhance the sense 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 nuclei
Dieter
Vestibulo Spinal Tract, Reticulo Spinal 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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Vestibular Reflexes
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 Ocular Reflexes
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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 VESTIBULAR COMPONENT.
1. VESTIBULO-SPINAL FUNCTION.
2. VESTIBULO – OCULAR FUNCTION.
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VESTIBULO-SPINAL FUNCTION
ROMBERGS TEST UNTERBERGERS TEST
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VESTIBULO-OCULAR FUNCTION
NYSTAGMUS INVOLUNTARY DEVIATION OF EYES AWAY
FROM DIRECTION OF GAZE FOLLOWED BY A RETURN OF THE EYES TO THEIR ORIGINAL POSITION.
3 TYPES1. CENTRAL2. OCULAR3. VESTIBULAR
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VESTIBULAR NYSTAGMUS
RHYTHMIC FAST AND SLOW PHASES NAMED AFTER FAST PHASE. 3 TYPES
1. 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 labyrinths simultaneously
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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