Special Senses-Sense of Hearing...complete lecture

8/12/2019 Special Senses-Sense of Hearing...complete lecture

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SENSE OF HEARING


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Sense of Hearing

Mechanism by which EAR

Receive sound waves

Discriminate their frequencies

Transmit auditory information to central nervoussystem


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ANATOMY OF EAR1. The outer ear

- pinna

- ear canal

- eardrum

2. The middle ear

- three ossicle bones;

(malleus, incus,stapes)

- two major muscles

(stapedial muscle,tensor

tympani)

- Eustachian tube

3. The inner ear

- cochlea (hearing)

- vestibular system(balance)

4. The central auditorysystem


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OUTER EAR

Three parts ofouter ear

1) Pinna

2) Ear canal

3) Ear drum

Major functionof outer ear

1)amplification

2) soundlocalization


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MIDDLE EAR


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Three main parts ofmiddle ear

(1)Three Ossicle

bones:- Malleus(1), Incus(3),

Stapes(6)

Function)Impedancematching

(2)Two muscles

- Stapedial muscle(5)

- Tensor tympani(9)

Function)Protection(attenuation reflex)

(3)Eustachian tube(8)

Function)Equalizer ofair pressure


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TRANSMISSION OF SOUND

WAVES THROUGH MIDDLE EAR The tip end of handle of

malleus is attached tocenter of tympanicmembrane and at thisattachment is constantlypulled by tensor tympanimuscle which keep tympanicmembrane tensed

Sound vibrations on anyportion of tense tympanicmembrane is transmitted tomalleus and then to incusand then to stapes foot plate

on oval window window, setsthe fluid of inner ear intomotioneventually excitinghearing receptors.


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IMPEDANCE MATCHING

Sound transmission in fluid (inner ear) requiresmuch higher pressure than sound transmission in air(outer ear)

Therefore, the tympanic membrane and ossicular

system provide impedance matching between thesound waves in air and the sound vibrations in the

fluid of the cochlea.

The amplitude of movement of the stapes faceplate

with each sound vibration is as much as the

amplitude of the handle of the malleus


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This increases theforce of movement about 1.3

times.

The surface area of the tympanic membraneis about 55 square millimeters, whereas the

surface area of the stapes averages 3.2 square

millimeters. This 17-fold difference times the 1.3-fold ratio

of the lever system causes about 22 times as

much total force to be exerted on the fluid ofthe cochlea as is exerted by the sound waves

against the tympanic membrane.


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ATTENUATION REFLEX

When loud sounds aretransmitted through the

ossicular system and from

there into the central

nervous system, a reflexoccurs after a latent period

of only 40 to 80

milliseconds to cause

contraction of the

stapedius muscle and, to a

lesser extent, the tensor

tympani muscle


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The tensor tympani musclepulls the handle

of the malleus inward while the stapedius

muscle pulls the stapes outward. These two

forces oppose each other and thereby causethe entire ossicular system to develop

increased rigidity


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IMPORTANCE

1. Toprotect the cochlea from damaging vibrationscaused by excessively loud sound

2. To mask low-frequency sounds in loud

environments. allows a person to concentrate onsounds above 1000 cycles per second

3. Decrease a persons hearing sensitivity to his or

her own speech.


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INNER EAR


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FUNCTIONAL ANATOMY OF

COCHLEA

The cochlea is a systemof 3 coiled tubes

(1) scala vestibuli(contains perilymph)

(2) scala media (contains

endolymph)(3) scala tympani.

(contains perilymph)

Reissners membrane

Basilar membrane

Organ of Corti

Helicotrema


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BASILAR MEMBRANE Fibrous membrane Separates the scala

media from the scalatympani

20,000 to 30,000 basilar

fibers Projects from modiolus,

toward the outer wall

Stiff, elastic, reed likestructures

Fixed at basal end butfree at distal end

Vibrates at free end


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The lengths of the basilar fibers increaseprogressively

beginning at the oval window (base) of the cochleato the helicotrema (apex)

The diameters of the fibers, however, decrease frombase

to apex

The stiff, short fibers near the oval window of the

cochlea vibrate best at a very high frequency

Long, limber fibers near the tip of the cochlea vibrate

best at a low frequency.


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the

Cochlea

When stapes footplatemoves inwards againstthe oval window, roundwindow must bulgeoutwards

Sound waves causebasilar membrane to bendin direction of roundwindow

As a result basilar fibersdevelop elastic tension,

initiates a fluid wave thattravels along the basilarmembrane towards theapex


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Different sound frequencies on

basilar membrane


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is for discrimination of sound waves

of different frequencies

High frequency at the base Low frequency at the apex

Intermediate frequency at the intermediatedistance between two extremes

Spatial organization of nerve fibers in cochlearpathway from cochlea to cerebral cortex

Specific brain neurons are activated by specificsound frequencies

So nervous system detects different soundfrequencies according to position of stimulatedpart of basilar membrane


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VOLLEY OR FREQUENCY

PRINCIPLE

Low frequencysounds 20 to 2000cycles per secondssend volleys of nerve

impulsessynchronized at samefrequency throughcochlear nerve tocochlear nuclei, which

distinguish thedifferent frequenciesof volleys.


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Sound frequencies are discriminated from oneanother on the basis of

Place of maximum stimulation of nerve fibers from

the organ of corti lying on the basilar membrane.

(depends upon positions along the basilar

membrane that are most stimulated)


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Loudness (amplitude/intensity)

Perception of loudness is related to

1- sound pressure level in db

2- duration of sound

POWER LAW:Psychophysical perception is log linear with

stimulus magnitude


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DETERMINATION OF LOUDNESS

1) Amplitude of vibration of basilar membrane andhair cells increases leading to excitation of

nerve endings at more rapid rates.

2) Spatial summation: more and more hair cells

stimulated leading to transmission throughmany nerve fibers rather than few.

3) Stimulation of outer hair cells


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Function of organ of corti

Receptor organ

Single row of internal

hair cells

3 or 4 rows of outer

hair cells

90-95% cochlear

nerve endings

terminate on innerhair cells (detection of

sound)


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Reticular

lamina

Stereocilia

Tectorialmembrane

Triangular

rods of

Corti

Basilar

fibers


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Hair Cell Receptor Potentials and Excitation


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Hair Cell Receptor Potentials and Excitation

of Auditory Nerve

Fibers


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Endocochlear Potential

An electrical potential of about +80 millivoltsexists all the time between endolymph and

perilymph, with positivity inside the scala media

and negativity outside

Generated by continual secretion of positivepotassium ions into the scala media by the stria

vascularis.


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Central auditory pathway


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AUDITORY CORTEX

Supratemporal portionof superior temporalgyrus

Primary auditory cortexexcited by projections

from medial geniculatebody

Secondary auditorycortex (auditoryassociation area)excited by impulsesfrom primary auditorycortex and thalamicassociation area


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Low frequency

anteriorly

High frequency

posteriorly

Discriminates sound

frequencies

Detection of direction Sudden onset of

sound


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Importance of auditory cortex

Discrimination of tonal and sequential sound patterns Destruction of both primary auditory cortices leads to

high reduction of ones sensitivity for hearing

Destruction of one side only slightly reduces hearing

in the opposite ear because of many crossoverconnections in auditory pathway

However, it does affect ones ability to localize thesource of a sound, because comparative signals inboth cortices are required for the localization

Lesions that affect the auditory association areas onlyleads to unable to interpret the meaning of the soundheard.


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Determination of sound direction

(1) The time lag between the entry of sound into one ear andits entry into the opposite ear

(2) The difference between the intensities of the sounds inthe two ears.

The first mechanism functions best at frequencies

below 3000 cycles per second The second mechanism operates best at higher

frequencies

The two mechanisms cannot tell whether the sound isemanating from in front of or behind the person or fromabove or below. This discrimination is achieved mainly by

thepinnae of the two ears. The shape of the pinnachanges the quality of the sound entering the ear,depending on the direction from which the sound comes.


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Centrifugal Signals from the Central Nervous

System to Lower Auditory Centers

Retrograde pathways (inhibitory) from cortex tocochlea which allows the person to direct his or

her attention to sounds of particular qualities

while rejecting sounds of other qualities


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Conduction deafness:

Anatomical site: Middle ear (ossicular chain), tympanic membrane or

external ear.

Causes: Ear wax, infection (otitis media), foreign body, tumor, tympanicmembrane perforation, genetic causes.

Weber test: Sound localizes to affected ear (ear with conductive loss).

Rinnes test: Negative, Bone conduction> air conduction (bone/air gap)

Treatment: In most of the cases hearing loss is reversible. Antibiotics,

antifungal, surgical repair, cochlear implants.

Perceptive deafness:

Anatomical site: Root cause lies in vestibulocochlear nerve, the inner

ear (damage to hair cells) or central processing centers of brain.

Causes: Noise, trauma, infections, congenital, aging.

Weber test: Sound localizes to normal ear.

Rinnes test: Positive, air conduction> bone conduction (both air and

bone conduction are decreased equally, but difference between them

unchanged).

Treatment: Loss permanent. Hearing aids


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Audiogram:

Is plotted on the basis of interrelationship ofsound intensity with frequency.


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The Vestibular System

VestibularLabyrinthOtolith organs

Utricle and

SacculeSemicircular

canals Anterior,

Posterior andLateral orhorizontal


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Function of vestibular apparatus

Is the sensory organ for detecting sensations ofequilibrium.

Accurate control requires accurate information

Sensory inputs:

1- vestibular system

2- visual system

3- proprioceptive system

4- cutaneous sensations


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Otolith organs

Both utricle and sacculecontain a sensory areainside known asMACULA

Macula of utricle lies in

horizontal plane anddetermine orientation ofhead when head isupright

Macula of saccule liesin vertical plane anddetermine orientation ofhead when person islying down


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Function of Otolith organs

Static equilibrium Linear acceleration


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Semicircular Ducts

ANTERIOR POSTERIOR

LATERAL

Three planes of space

Ampulla Endolymph

Crista ampullaris

Cupula

Hair cells Stereocilia

Kinocilia


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Crista ampullaris


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Crista ampullaris

RrRotationof head

Bending of cupu

Stereocilia bend Receptor cell fires

Synapse activated

Stimulation of nerve endings Dynamic equilibrium sense


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Function of semicircular ducts

Angular acceleration Rapid intricate

changing body

movements

Predictive role


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VESTIBULAR PATHWAY:

First order neurons in Scarpas ganglion 2ndorder neurons in vestibular nuclei at junction

of pons and medulla

Then pathway ascends to parietal cortex, area for

equilibrium


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Neural connections

Cerebellum Motor nuclei of CN 3,4 & 6

Reticular formation

Spinal cord

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Special Senses-Sense of Hearing...complete lecture