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