Anatomy and physiology of the peripheral and central auditory system

Lecture 2©

Dr. Ghulam SaqulainM.B.B.S., D.L.O, F.C.P.S

Head of Department of E.N.TCapital Hospital

THEORIES OF HEARING

Precise means of hearing remains unknown

Most theories attempt to explain how we perceive pitch

Hearing theories:

Helmholtz's resonance theory Place theory Traveling wave theory Frequency theory Resonance-volley theory

Helmholtz's Resonance Hearing Theory

Cochlear structures consists of many tiny resonators Each tuned to a specific frequency

The ear performs a Fourier analysis; breaking each complex sound into its components

Each resonator responded most vigorously to its tuned frequency with less amplitude to adjacent frequencies

Propose the placement of high frequency fibers at the basal end of the cochlea and lower frequencies near the apex

Place Theory of Hearing

Every tone that could be heard was assigned to its own specific place in the cochlea (i.e. piano keys)

Assumes that there is neural representation for every place on the basilar membrane

Good in explaining some phenomenon (i.e. masking & sharp pitch discrimination)

Explains loudness in terms of amplitude of movement of BM A loud sound creates greater amplitude than a softer sound Greater amplitude increases the number of impulses transmitted

by the nerve fibers

Drawbacks: didn’t explain why pitch discrimination is very poor close to auditory threshold

Unacceptable hearing theory

Traveling Wave Theory

Each inward and outward movement of footplate of the stapes, there is a downward and upward movement of the BM

Produced by the disturbance of endolymph The wave moves down the cochlear duct from base to apex with

the max. amplitude for HF tones occurring at the basal end and that for the LFs at the apical end

HFs excite only fibers in the basal turn of the cochlea LFs excite fibers all along the BM

Frequency determines: The distance of the motion of traveling wave The rate of BM vibration (related to frequency & period)

Frequency Hearing Theory

Analysis happens in a retrocochelar area

Auditory nerve transmits a pattern that corresponds directly with the input signal

Explain loudness in terms of the amount of spread along the BM The greater the amplitude of the input signal, the larger the surface

area of BM stimulated

i.e. 100 Hz tone make the auditory nerve fires 100 times per sec

But the auditory nerve can fire up to about 400 times per sec, the theory doesn’t explain the perception of tones above this frequency

Volley Theory of Hearing

A series of impulses is sent along the auditory nerve

The sum represents a reproduction of the vibrations of the BM

During the refractory (rest) period of one set of neurons, another set is actively firing

Explains only frequencies up to 4000 Hz

Resonance Volley Theory of Hearing

Combines the two following theories:

Place theory suggests spatial representation and explains HF perception

Volley theory suggests temporal dimension and explains LF interpretation

HYPOTHESIS FOR HAIR CELL TRANSDUCTION

Mechanical motion of hair cells (Shearing); converting a sound source into a form of energy that can be transmitted by the auditory Nerve

A. Mechanical Hypothesis: the pressure that moves the hair cells stimulates the nerve endings directly

B. Chemical Hypothesis: the hair cells are deformed, a neurotransmitter substance is released that stimulates the nerve endings

C. Electrical Hypothesis: cochlear potential stimulates the nerve endings

FREQUENCY ANALYSIS IN THE COCHLEA

Lowest frequencies (about 20 Hz) apical end of cochlea

2000 Hz to 20,000 Hz from midpoint of BM to the basal end of cochlea near the OW

Below 2000 Hz on the other half of the BM

Why human ears have excellent frequency discrimination capabilities?

Auditory N fibers are sharply tuned to specific frequencies

Characteristic Frequency /Best Frequency: Frequency that can

increase the firing rate of a neuron above its spontaneous firing rate

Psychophysical tuning curve (PTC)

Created to study the tuning mechanism of cochlea Used to measure cochlear frequency resolving abilities

Tuning becomes sharper (narrower bandwidth) as frequency is

increased (traveling wave peaks closer to the basal turn) Less sharply tuned than the auditory nerve The slope of tuning curve is much steeper above the stimulating

frequency than below it When the PTC is widened in damaged ears, this can explain speech

discrimination difficulties those pts. have

VESTIBULAR SYSTEM

This system regulates balance. It is also within the inner ear. SEMI-CIRCULAR CANALS (Three of them, all in

different planes) determine movement in three planes.

Within each semi-circular canal is endolymph and hair cells, which connect to nerves that go to the cerebellum.

When you move in one direction, like sliding across the room, the fluid slashes like a cup of coffee, and it makes the hair cells move.

Anatomy and Function of the Otoliths

Figure 16.21b