Week 11/12 - Hearing, Deafness and Hearing Loss Management

Sound

Audible air pressure fluctuations

Sound intensity

Results from the source amplitude of the soundwave and the distance traveled from the sound source

Hearing threshold

The faintest sound a human ear can detect

Decibel range (dBs)

Unit of sound measuring sound pressure level; is logarithmic with each intensity level with a factor of 10 change

Loudness

The human perception of sound pressure level

Ear location

Goes deep into the temporal bone and closely approaches the brain

Outer ear

Amplifies incoming sound and protects the ear

Middle ear

Amplifies signal and converts audio signal to mechanical signal

Inner ear

Analyzes signal and converts mechanical signal to neuro-electrical signal to the brain via the auditory nerve

Pinna

Focuses soundwaves and conducts vertical localization

Auditory canal

Amplifies sound and protects the eardrum from particulates, damage, temperature

Tympanic membrane (eardrum)

Extremely sensitive membrane that detects tiny vibrations of soundwaves and transmits to air-filled tympanic cavity

Ossicles

Three small bones that transmit vibrations from the tympanic membrane to the membrane-covered oval window (malleus, incus, stapes)

Malleus

Hammer that transmits signals from the eardrum

Incus

Anvil that transmits signals from the hammer

Stapes

Stirrup that transmits signals from the anvil to the inner ear

Semicircular canals

Important for balance

Cochlea

Coiled (snail-like) fluid-filled chambers separated by the basilar membrane, contains hair cells that transmit nerve impulses to the auditory nerve

Hair cells

Microscopic cells, when bent trigger nerve impulses that are routed through the auditory nerve

Organ of Corti

Actual hearing sensing organ where stereocilia hair cells are bent by the shearing movement of the tectorial membrane, converts mechanical energy to an electrical signal transmitted via the auditory nerve

Stereocilia

Microscopic hair cells in the Organ of Corti that sense soundwaves by detecting cell deformation

Sound sensing process

1. Inside the cochlea, sound waves cause the basilar membrane to vibrate up and down. 

2. Creates a shearing force between the basilar membrane and the tectorial membrane

3. Causes the hair cell stereocilia to bend back and forth. 

4. Leads to internal changes within the hair cells that create electrical signals. 

5. Auditory nerve fibers rest below the hair cells and pass these signals on to the brain (auditory cortex)

Tonotopic organization

From the oval window to the innermost part of the cochlea, the base detects the highest frequency while the apex detects the lowest frequency

Encoding in the cochlea

Basilar membrane tapered in width and in thickness along 3.5 cm length, where tension and density change with position

High frequencies in cochlea

Base near oval window; narrow and stiff

Low frequencies in cochlea

Apex at helicotrema; large and floppy at most curled-up end

Functional columns

Information organization in the primary auditory cortex where neurons vertically organized respond optimally to sounds in the same frequency range

Tonotopic organization

Information organization in the primary auditory cortex where more anterior regions detect lower frequencies and more posterior regions detect higher frequencies

Brainstem localization

The exception to the tonotopic organization of auditory system structures