Auditory System: Anatomy, Physiology, and Sound Perception - Prof. Ramalingam Chellappa, Study notes of Computer Science

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Auditory System

Slides adapted fromRutgers University

(http://qneuro.rutgers.edu )

and

Imperial college

(http://www.doc.ic.ac.uk/~phwl/teaching/mm/Index.html)

The Nature of

Sound

(P. 149)

Sound

: vibratory energy caused by movement of physical objects

•^

Rate of vibration is called

frequency

• What we hear is

pitch

(high or low)

• We hear 20-20,000 Hz

(cycles/sec)

•^

Size (intensity) of vibration is

amplitude

• What we experience is loudness– Measured in

decibels

(dB) (too loud too long = hearing loss)

Sound

as

mechanical waveenergy requires amedium such as airor water in which tomove.

Speed of Sound - How fast sound pulsetravels. All sound travels at thesame speed in a given mediumIn air, the speed of sound is 344m/S. Pitch.

This is

perception.

A

high frequency sound is heardas a high pitch.

A B

Species - Frequency RangeHumans

20 - 20,000 Hz

Bats

100,000 Hz

Frequency - The number of sound pulses thattravel past a fixed point withina second. A is a lower frequencysound than B. Unit is Hz.

Any sound can be broken down

into component frequencies

The word “butter”

Frequency

Time

The Ear

Inside the Cochlea

Copyright © 2002 Wadsworth Group. Wadsworth is an imprint of theWadsworth Group, a division of Thomson Learning

Organ of corti

Arrays of Inner Ear Hair Cells •^

The outer hair cells are more numerous than inner hair cells, but theydo not send a larger electrical response to the auditory cortex;– there are often 100-120 cilia per outer hair cell, arranges in rows of V-

formations; only the tallest cilia extend into the tectorial membrane

•^

The function of the outer cells seems to be to amplify and sharpenbasilar membrane vibration through an efferent-guided

motile

response

that pushes and pulls against it

The hair cells of the organ of Cortiare arranged in four rows along thelength of the basilar membrane.There may be 16,000 - 20,000 suchcells (about 3000 inner hair cells,with 40-60 cilia each)source:

http://hyperphysics.phy-

astr.gsu.edu/hbase/sound/corti.html#c

Tip Links

-^

Shorter cilia are attached to theirlonger neighbors by tip links– Made of actin, like cilia themselves

-^

Without bending, the ion gate“trapdoor” is open about 20% of thetime.– inward flow of ions is matched by

outward flow of ions, maintainingresting potential (no transmitterrelease)

-^

When cilia are bent (due to

endolymph

flow or membrane shearing), relativelymore positively charged ions can enter,causing depolarization, which resultsin turn in neurotransmitter release

Transducing Protein for Hearing

•^

TRPA1 (transient receptorpotential), a mechanically-gated protein channel for ions,responds 1,000 times fasterthan the opening of similarchannels in the eye in responseto light.

-^

The same protein was known tobe involved in pungent odortransduction (mustard,cinnamon, wasabi) and forpainful cold.

•D. Corey et al

, Nature

, Dec 2004

Place

Theory

Different parts of the basilar membrane respond most to different

frequencies: high frequency at base; low frequency at apex.

Cochlea andFrequency

•^

There is a

tonotopic

map of frequencies to different locations in the

cochlea. This tonotopic mapping continues in the auditory cortex

-^

The basilar membrane responds to complex tones by responding to theindividual composite tones of which it is composed.–

The complex wave (middle) is composed of three frequencies, each of whichhas its own reaction in the basilar membrane