ANATOMY OF AUDITORY SYSTEM - PowerPoint Presentation

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ANATOMY OF AUDITORY SYSTEM

Ear

The ear converts changes in pressure in the air to changes in the electrical activity of neurons.

The

human ear can detect sound frequencies between 20 and 20 000 Hz.

Anatomists distinguish the outer ear, the middle ear, and the inner ear

The outer ear

It includes the

pinna

,

and auditory canal

1.

Pinna

:

It is also known as auricle.

It

is The familiar structure of flesh and cartilage attached to each side of the head.

By

altering the reflections of sound waves, the

pinna

helps us locate the source of a sound.

It only plays a minor role in hearing

Even if it is removed, hearing remains

uneffected

It helps to distinguish the direction of sound -

localisation

Rabbits

’

large movable

pinnas

enable them to localize sound sources even more precisely.

The

depression of auricle, which forms the orifice of external auditory

meatus

, is called

concha

.

2. Auditory canal

External auditory

meatus

starts from the

concha

extends inside as a slightly curved canal, with a length of about 55 mm.

From

the

pinna

, sound waves pass through the auditory canal, they strike the tympanic membrane, or eardrum, in the middle ear.

The middle ear

It is an air filled cavity. Middle

ear consists of the following structures:

1

.

tympanic membrane

2. Auditory

ossicles

3.

Auditory muscles

4.

Eustachian tube.

1. The

tympanic membrane

Also known as ear drum

It vibrates

at the same frequency as the sound waves that strike it.

Tympanic

membrane is a thin, semitransparent

membrane

, which separates the middle ear from external auditory

meatus

.

The chief quality of eardrum is its ability to move to and fro- elasticity

Periphery

of the membrane is fixed to tympanic

sulcus

in the surrounding bony ring, by means of

fibrocartilage

.

The

tympanic membrane, or eardrum, forms the boundary between the outer ear and middle ear.

2. Ear

ossicles

It helps to amplify sound and in its conduction

Auditory

ossicles

are:

i.

Malleus

ii.

Incus

iii. Stapes.

The

three

ossicles

bridging the middle ear are the

malleus

(hammer),

incus

(anvil), and

stapes

(stirrup).

The

purpose of these bones is to transfer sound energy from the outside air to the fluid in the inner ear without losing too much of it.

Malleus

It is

otherwise called hammer.

It

has a handle, head and neck.

Hand

is called

manubrium

and it is attached to tympanic membrane.

Neck

extends from handle to the head.

Head

or

capitulum

articulates with the body of

incus

.

Incus

It is also known as anvil.

It looks like a premolar tooth.

Incus

has a body, one long process and one short process.

Stapes

It is also called stirrup.

It is the smallest bone in the body.

It has a head, neck, anterior

crus

, posterior

crus

and a footplate.

Head articulates with

incus

.

Footplate fits into oval window.

3.

AUDITORY MUSCLES

Two skeletal muscles are attached to

ossicles

:

i

. Tensor tympani

ii

.

Stapedius

.

Tensor tympani

It

is

larger of the two muscles of tympanic cavity.

It is attached to tympanic

membrane through

malleus

.

Tensor

tympani muscle pulls and keeps the tympanic membrane stretched or tensed constantly.

This

constant stretching of tympanic membrane is essential for the transmission of sound waves, which may reach any part of the tympanic membrane.

Paralysis

of tensor tympani causes hearing impairment.

Stapedius

It is

the smallest skeletal muscle in human body with a length of just over 1 mm.

Stapedius

prevents excess movements of stapes.

4. Eustachian tube or the auditory tube

It

is

the flattened canal extending from the anterior wall of middle ear to

nasopharynx

.

Eustachian

tube connects middle ear with posterior part of nose and forms the passage of air between middle ear and atmosphere.

So

, the pressure on both sides of tympanic membrane is equalized.

The Inner Ear

It begin on the other side of oval window.

The

inner ear contains two sets of fluid-filled cavities embedded in the temporal bone of the skull. One set, known as the semicircular canals, is part of the vestibular system. The other set is known as the cochlea (

“

snail”

in Greek).

Semicircular canals and vestibular sacs are not involved in hearing. Their major role is to help in maintaining body balance and posture.

Cochlea

The

fluid-filled cochlea contains

specialised

receptor cells that respond to the vibrations transmitted to the inner ear.

The

cochlea is about 32 mm long and 2 mm in diameter.

When

rolled up like a snail shell, the human cochlea is about the size of a pea.

The cochlea is divided into three parallel chambers.

Scala

vestibuli

- vestibular canal

Scala

media – cochlear duct

Scala

tympani – tympanic canal

Two of the chambers, the vestibular canal and the tympanic canal, are connected to each other near the apex, which is the part of the cochlea most distant from the oval window.

These two chambers contain a fluid known as

perilymph

, which is similar to cerebrospinal fluid.

The third chamber, the cochlear duct, contains a very different type of fluid, known as

endolymph

.

The

endolymph

is rich in potassium and low in sodium.

The fluids (and chambers) are separated by two membranes.

Reissner

’

s membrane separates the vestibular canal and the cochlear duct.

The basilar membrane separates the tympanic canal and the cochlear duct.

At the base of the cochlea, at the boundary between the middle and inner ears, the oval window covers the vestibular canal.

The

tympanic canal is covered by another membrane, known as the round window.

Because

the vestibular and tympanic canals are connected, pressure applied to the oval window by the stapes travels through the

perilymph

and pushes the round window out into the middle ear

.

Within the cochlear duct is a specialized structure known as the

organ of

Corti

, which is responsible for translating vibrations in the inner ear into neural messages.

The

organ of

Corti

, consisting of rows of hair cells, rests on the basilar membrane.

Over

the top of the hair cells, and actually attached to some of them, is the

tectorial

(roof) membrane.

The

tectorial

membrane is attached to the cochlear duct at only one side and can move independently from the basilar membrane.

Several

structural features of the basilar membrane are relevant to its response to sound.

The

membrane is about five times wider at its apex (far end) than at its base (beginning).

In

addition, the basilar membrane is about 100 times stiffer at its base than at its apex.

These

structural differences are similar to the range of size and flexibility found in the different strings of a guitar.

When

vibration produces pressure changes within the cochlea, the

basilar

membrane responds with a wavelike motion, similar to the motion of a rope or whip that is snapped.

high-

frequency sounds will cause a peak vibration of the basilar membrane near its base, whereas low-frequency sounds will cause a peak vibration closer to its apex.

The movement of the basilar membrane is sensed by the hair cells attached to the organ of

Corti

.

Out

of the approximately 15,500 hair cells in each human inner ear, about 3,500 of them are known as inner hair cells, which are the actual auditory receptors.

The

inner hair cells are located near the connection between the

tectorial

membrane and cochlear duct.

The

remaining 12,000 hair cells are known as outer hair cells, which appear to amplify sound.

Both

have

hairlike

cilia extending from their tops, but only the cilia from outer hair cells are attached to the

tectorial

membrane

Although

there are many more outer hair cells in the ear, only 5 percent of the auditory nerve (cranial nerve VIII) fibers connect with outer hair cells. The remaining 95 percent of auditory nerve fibers connect with the inner hair cells.