Phsiology of Hearing and Assesment

8/8/2019 Phsiology of Hearing and Assesment

1/30

Physiology of hearing

by

uma


2/30

HEARINGNEEDS Adequate stimulus-sound

Mechanical Conduction of sound

transduction of mechanical energy to

electrical impulses

central auditory processing of the impulses in

the auditory pathway


3/30

Sound

It is of successive waves of compression andrarefaction generated by vibrating object

Velocity of sound- 340m/s at the sea level

Propertiesof sound:

Physical dimension Perceptual dimension

Intensity Loudness

Frequency PitchComplexity timbre


4/30


5/30

Intensity: pressure exerted by a sound stimulus

Strength of sound ,units-decebels

1dB=1/10 ofBell, normal range= 0 dB-120 dB

Frequency :no.of waves/sec;units-Hz

Frequency range measured in octave scales i.e. each F.is double the previous one eg;250,500,1000

Frequency range=20-20000Hz

Complexsound : sound more than one frequency eg;human voice

Puretone: a single frequency sound eg:250Hz


6/30

Mechanical conduction

Pinna andE.A.C:collection of sound waves

Amplification of frequency 2000Hz-4000Hz

Tympanic membrane:partion wall between external

and middle ear.

As it moves to and fro, it is buckled in the regions

between the manubrium of malleus and the anterior

&posterior edges


7/30

Ossicles: malleus ,incus and stapes-

Axis of rotation of ossicles and the axis ofsuspension by their ligaments coincide with their

centre of rotational inertia At low frequencies, mass effect is small, so the

ligaments play imp role in maintaining theposition of ossicles

At higher frequencies vibrations become morecomplex with rotatory motion along the long andshort axis of the footplate


8/30


9/30

Internal ear


10/30

Impedence matching Middle ear serves as acoustic transformer to match the

large, low impedence TM to much smaller, high

impedence oval window It is done by two principles

1)hydraulic action ofTM: the area ofTM is much largerthan that of footplate, the avg ratio is 18.75:1 so theforces collected over the TM or concentrated on a smallerarea to increase the pressure at the Oval window

2)lever action of the ossicles: geometrical lenth of themalleus is approx 2.1 times more than that of incus so itmultiplies the force 2.1 times and the velocity isd

ecreased

2.1 times Thus Lever action increases the impedence ratio 4.4

times(2.1 x2.1)

Vibrations are coupled preferentially to only OWproducing differential pressure b/w two windows which is

required for movement of cochlear fluids


11/30


12/30

Transduction

Cochlea: it includes scala tympani,media andvestibuli.organ of corti is located in the scala media

Endolymph: formed by stria vascularis ,is like ICFcontains high K+ ion concentration(144Meq/l) and lowNa + ion concentration(16Meq/l)

Perilymph:it is not a simple ultra filtrate ofplasma.perilymph of scala vestibuli originates primarilyfrom the plasma and that of scala tympani from theplasma and the CSF

K+ion concentration in the scala vestibuli is some what

higher than in the scala tympani. The chemical andelectrical boarder b/w endo and peri lymph is thereticular lamina which include the transducing surfacesof the hair cells


13/30


14/30

Haircells :are of two types inner and outer

Outer hair cells:amplify the travelling wave to increase thecochleas sensitivity and frequency selectivity

The movements of footplate are transmitted to the

cochlear fluids and basement memb, setting of the

shearing force

Thus the stimulus is coupled to the steriocilia by means of

shear or motion between the tectoral memb and the

reticular lamina

Stereocilia on the top of hair cells are rigid move as a stiff

bundle.When cilia are deflected in the direction of

kinocilia,ion channels in the membrane are opened


15/30


16/30


17/30

Inner hair cells: single row, each has terminals

from 20 afferent nerve fibers

Potential grad

ient causes K+

entry into thehair cell from the endolymph there by

generation of action potentials occurs at the

bases of inner hair cells

Then the impulses are conducted to the brain

via ascending auditory pathway


18/30

Phase Locking ofInner Hair Cells

Auditory nerve connected to inner hair cell tends

to fire at the same phase of the stimulating

waveform.

For Low frequency stimuli Nt will be released in

pockets and concentratedduring the

depolarising phase of the hair cells response

It is seen only at low frequency stimuli. By 3-5 Hzfrequency the AC response is so small that phase

locking of action potential is negligible


19/30

Cental auditory pathway E

xtend

s from cochlear nuclei to the aud

itory area in thecerebral cortex

First order neurons: bipolar cells connecting the haircells of organ of corti to the dorsal and ventral cochlearnuclei. Cell bodies in the spiral ganglion are housed with

in the bony spiral lamina

These auditory division of fibers are arranged in atonotopic fashion which is maintained in the cochlearnuclei also

Low frequency fibers are represented ventrolaterally andhigh frequency fibers dorsomedially

Fibers near the base of the cochlea will respond best to ahigh frequency and fibers near the apex to a low

frequency


20/30

Second order neurons: starts from cochlear nuclei and

project to inferior colliculus as dorsal pathway, as ventral

pathway to the ipsilateral and contra lateral SOC.

Superior olivary complex: it contains an S-shaped lateral

olivary nucleus, disk shaped medial olivary nucleus and

the medial nucleus of trapezoid body together withsmaller peri olivary nuclei

The neurons from the SOC pass through the lateral

lemniscus in the mid brain

Neurons that localize the sound with inter aural delay arelocated in MOC

Neurons that detect differences in the sound intensity are

located in LOC


21/30


22/30

Inferior colliculus: contains central nucleus which

receives the major auditory input and outer dorsalcortex and external lateral cortex

It is involved in sound localization and auditory moto

responses eg:controling the middle ear muscles incase of noise

Third order neurons: connects inferior colliculus to

medial geniculate body which has dorasl,ventral and

medial divisions

Ventral division-transmits information on auditory

discrimination to the auditory cortex


23/30

Dorsal division-sends axons to the auditory

association cortex for maintaining and

d

irectingauditory attention

Medial division- functions as a multi sensory

arousal system

Fourth order neurons: connect MGB to the cerebral

cortex

These neurons pass through the sub lenticular

portion of the internal capsule

Primary auditory cortex(areas 41,42 of broadmen):

it is the gyrus of heschl on the upper part of the

superior temporal gyrus in the sylvian fissure


24/30

Theories of hearing Place theory:it states that perception of sound

depends upon the selective vibration s of a specific

place on the basilar membrane. frequency analysis

is carried out in the inner ear

Temporal/timing theory: perception of sounddepends on the temporal patterns with which

neurons respond to the sound in the cochlea

As the basilar membrane vibrates, each clump ofhair cells along its length is deflected in time

More intense the vibration is the more hair cells

ared

eflected

&more likely they are to cause nerve


25/30

Telephone theory(Rutherford):Perception is

related to the rate of firing of individual nerve

fibers

Hair cells transform sound vibrations to the

nerve in similar frequency&amplitude.so there isno analysis of the complex vibrations in the

cochlea which occurs in the brain

Volley theory:higher frequencies are perceivedby the place mechanisams,low frequencies by

the telephone mechanism and the intermediate

frequencies by both


26/30

Travelling wave theory: sound waves from the

OV produce a wave which travels along thebasilar memb to the apex of the cochlea, called

classical traveling wave

As it moves there is increase in the amplitude.the point of maximum amplitude on the BM

depends upon the frequency of the sound

high frequency prod

uced

travelling wave peaksnear the waves and low frequency wave

stretched further up to the apex and finally dies

away


27/30

Oto acoustic emissionsLow intensity sounds produced by the movements of

OHC of the cochlea

Can be recorded & measured by placing a

microphone receiver in the deep EAC

OAC can be recor

ded

in all normal hearing personsirrespective of age& consciousness. OAC are of two

types;spontaneous&evoked

OAC in response to high frequency sound stimulus

originate from the basal turn of the cochlea& have a

shorter latency

Absence ofOAC indicates structurally damaged or

non functional OHC eg:acoustic trauma,ototoxic


28/30


29/30

References

Scott brown 7th edition

Pl dhingra

Anirban Biswas

Internet


30/30

Documents

Phsiology of Hearing and Assesment