1. The Ear

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

3. The Ear

4. The Ear Auricle (Pinna) Collects sound Helps in sound localization
Most efficient in directing high frequency sounds to the eardrum

5. 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)

6. The Ear Mastoid Process Bony ridge behind the auricle
Provides support to the external ear and posterior wall of the middle ear cavity

7. The Ear Tympanic Membrane Thin membrane
Forms boundary between outer and middle ear
Vibrates in response to sound
Changes acoustical energy into mechanical energy

8. 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

9. 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

10. The Ear Stapedius Muscle Connects the stapes to the middle ear wall
Contracts in response to loud sounds; known as the Acoustic Reflex

11. The Ear Structure of The Inner Ear
Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy

12. Structures of the Inner Ear (Cont.)
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

13. The Ear
Organ of Corti
The end organ of hearing; contains stereocilia and hair cells.

14. The Ear Hair Cells Frequency-specific
High pitch sounds = base of cochlea
Low 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

15. The Ear Vestibular System Consists of three semi-circular canals
Shares fluid with the cochlea
Controls balance
No part in hearing process

16. 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

17. 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!

18. 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.

19. The Ear The path of sound
ear 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.

20. The Ear Cochlear 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.

21. The Ear

22. The Ear

23. 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.

24. The Ear
Inner Hair Cells