The Ear. Components of hearing mechanism - Outer Ear - Middle Ear - Inner Ear - Central Auditory Nervous System.

The Ear

The Ear

Components of hearing mechanism- Outer Ear- Middle Ear- Inner Ear- Central Auditory Nervous System

The Ear

The Ear

Auricle (Pinna) Collects sound Helps in sound

localization Most efficient in

directing high frequency sounds to the eardrum

The Ear

External Auditory Canal Approximately 1¼ inch in

length “S” shaped Lined with cerumen glands Outer 1/3rd cartilage; inner

2/3rds mastoid bone Increases sound pressure

at the tympanic membrane by as much as 5-6 dB (due to acoustic resonance)

The Ear

Mastoid Process Bony ridge behind the

auricle Provides support to the

external ear and posterior wall of the middle ear cavity

The Ear

Tympanic Membrane Thin membrane Forms boundary

between outer and middle ear

Vibrates in response to sound

Changes acoustical energy into mechanical energy

The Ear

The Ossicular Chain A: Malleus B: Incus C: Stapes

Ossicles are smallest bones in the body

Act as a lever system Footplate of stapes enters

oval window of the cochlea

The Ear

Eustachian Tube Lined with mucous

membrane; connects middle ear to back of the throat (nasopharynx)

Equalizes air pressure Normally closed except

during yawning or swallowing

Not a part of the hearing process

The Ear

Stapedius Muscle Connects the stapes to the

middle ear wall Contracts in response to loud

sounds; known as the Acoustic Reflex

The Ear

Structure of The Inner Ear Cochlea - Snail-shaped organ

with a series of fluid-filled tunnels; converts mechanical energy into electrical energy

Oval Window – located at the footplate of the stapes; when the footplate vibrates, the cochlear fluid is set into motion

Round Window – functions as the pressure relief port for the fluid set into motion initially by the movement of the stapes in the oval window

Structures of the Inner Ear (Cont.)

The Ear

Organ of Corti The end organ of

hearing; contains stereocilia and hair cells.

The Ear

Hair Cells Frequency-specific

High pitch sounds = base of cochleaLow pitch sounds = apex of cochlea

When the basilar membrane moves, a shearing action between the tectorial membrane and the organ of Corti causes hair cells to bend

The Ear

Vestibular System Consists of three semi-

circular canals Shares fluid with the cochlea Controls balance No part in hearing process

The Ear

Central Auditory System 8th Cranial Nerve or “Auditory Nerve” carries

signals from cochlea to brain Fibers of the auditory nerve are present in the

hair cells of the inner ear Auditory Cortex: Temporal lobe

of the brain where sound is

perceived and analyzed

The Ear

How Sound Travels Through The Ear …

Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves strike the tympanic membrane, causing it to vibrate like a drum, and changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. (The middle ear components mechanically amplify sound). The stapes moves in and out of the oval window of the cochlea creating a fluid motion. The fluid movement within the cochlea causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent via the Auditory Nerve to the brain, where sound is interpreted!

The Ear

Transduction of sound into an auditory

perception Sound is a propagating pressure wave. Perception of sound involves the electrical activity

of neurons in the auditory cortex of the brain. The transduction process is the means by which

the pressure waves in air (a mechanical stimulus) is converted into neural activity (action potentials).

This process involves a number of stages, some of which involve conduction and impedance matching.

The Ear

The path of soundear canal → vibrate tympanic membrane → vibrate ossicles

(3 bones: Malleus, Incus, Stapes) → vibrate oval window of

cochlea → create waves in cochlea fluid → create standing

waves in basilar membrane → movement of hair cells

generates electrical activity through mechanically

gated ionic channels → hair cells stimulate the

auditory nerve → series of action potentials up to

the auditory cortex.

The EarCochlear Mechanics

Basilar membrane : The spectral analyser• Basilar membrane (BM) is approx 33mm long in humans• Apex of BM is wide and relatively loose• Base of BM is thinner and more stiff• Variations in length and stiffness provides BM with a continuum of resonant frequencies along its length: low frequencies at apex and high frequencies at base• A wave with a particular frequency produces a maximum displacement at a particular portion of the basilar membrane: tonotopic organization• BM is heavily damped beyond the resonant frequency• Travelling wave velocity is in range 1-20m/sec and is frequency dependent (velocity is reduced apically for low frequencies)• High frequency waves vibrate the basal part of the basilar membrane, dissipate energy and then die out.• Lower frequency waves travel further towards apex before dying out.

The Ear

The Ear

The Ear

Mechanism of Hearing by Organ of Coti

• Vibration of the basilar membrane produces shear

forces that bend the stereocilia (hairs protruding

from the hair cells) against the tectorial membrane

• Movement of the stereocilia either cause the hair

cell to depolarise or hyperpolarise, depending

upon the direction of movement

• Changes in the membrane potential of the hair cell

generate an AP in the nerve fibre attached to the

hair cell.

The Ear

Inner Hair Cells