Physiology of Hearing & Balance

Dr. Archana Sudhir

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

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.

Main Components of the Hearing Mechanism

Divided into 4 parts (by function):

Outer Ear Middle Ear Inner Ear Central Auditory

Nervous System

Functions of the Outer Ear

Gathers sound waves Increases Pressure in a frequency

sensitive way. Aids in localization

Functions of the Middle Ear

Couple sound energy to the cochlea

Impedance matching Protects Cochlea Preferential

application of sound to one window.

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.

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.

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.

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

Structures of the Inner Ear

Bony Labyrinth Bony Cochlea Vestibule Semi Circular Canals

Membranous Labyrinth

Cochlea Duct Utricle & Saccule Semi Circular Canals

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

Movement of pressure waves through the cochlea

MOVEMENTS OF THE BASILAR MEMBRANE AND THE DEFLECTION OF THE STEREOCILIA.

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

Theories Of Hearing

Place theory of Helm holtz Telephone theory of Rutherford Volley theory of Wever Traveling wave theory of Bekesy

CENTRAL AUDITORY PATHWAYS

Auditory Cortex

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

Vestibular Apparatus

Vestibule Utricle Saccule

Semicircular canals - lateral, superior, posterior

Vestibular nerve

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

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

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)

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

Crista Ampullaris & Head Rotation

As head turns, the endolymph lags behind pushing the cupula and stimulating its hair cells

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

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

Ascending Vestibular Projections

Lateral & Superior

vestibular nuclei

Thalamus

Sensori Motor Cortex

Visual ProjectionsProprioceptive

Projections

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.

Vestibulo Ocular Reflexes

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.

ASSESMENT

HISTORY IDENTIFICATION OF PRESENCE/

ABSENCE OF VESTIBULAR COMPONENT.

1. VESTIBULO-SPINAL FUNCTION.

2. VESTIBULO – OCULAR FUNCTION.

VESTIBULO-SPINAL FUNCTION

ROMBERGS TEST UNTERBERGERS TEST

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

VESTIBULAR NYSTAGMUS

RHYTHMIC FAST AND SLOW PHASES NAMED AFTER FAST PHASE. 3 TYPES

1. SPONTANEOUS

2. POSITIONAL

3. INDUCED.

VESTIBULAR NYSTAGMUS

SPONTANEOUS NYSTAGMUS GRADE 1. GRADE 2. GRADE 3.

POSITIONAL NYSTAGMUS HALLPIKE MANOEUVRE

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

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