The Ear

For Medical and Science students (level 2)

Devised by Tim Jacob

SOUND
Sound is the compression and rarefaction of air, or, in other words, alternating air pressure. The distance between the pressure peaks is called the wavelength. The frequency of the sound is {wavelength^-1 x speed of sound} . The frequency determines the pitch of the sound. Humans can detect sound in the frequency range 20 to 20,000Hz (Hz = cycles, or waves, per second).

TRANSDUCTION of SOUND into ELECTRICAL EVENTS
The numbers in the diagram below indicate the sequence of events in the detection and transduction of sound waves.

THE EAR

1. Sound waves enter the external ear and are directed towards the tympanic membrane.
2. Air molecules under pressure cause the tympanic to vibrate. Low frequency sound waves produce slow vibrations and high frequency sounds produce rapid vibrations. These move the malleus on the other side of the membrane.
3. The handle of the malleus strikes the incus causing it to vibrate.
4. The vibrating incus moves the stapes in and out and vibrates the oval window. The total force of the sound wave is transferred to the oval window, but, because the oval window is much smaller the force per unit area is increased 15-20 times. Additional mechanical advantage is gained from the leverage in the middle ear bones. This is necessary because the fluid in the inner ear is more difficult to move than air and thus sound must be amplified.
5. The sound waves that reach the inner ear through the oval window set up pressure changes that vibrate the perilymph in the scala vestibuli.
6. Vibrations in the perilymph are transmitted across the vestibular membrane to the endolymph of the cochlear duct, and also up the scala vestibuli and down the scala tympani. The vibrations are transmitted to the basilar membrane (see diagram opposite) which in turn vibrates at a particular frequency, depending upon the position along its length {high frequencies vibrate the window end where the basilar membrane is narrow and thick, and low frequencies vibrate the apical end where the membrane is wide and thin}.
7. The cilia of the hair cells, which contact the overlying tectorial membrane, bend as the basilar membrane vibrates, this opens ion channels and causes the entry of ions into the hair cell and a generator potential develops. If large enough, the generator potential causes transmitter release from the hair cells which excites the afferent nerve.Displacement of the stereocilia in the direction of the tallest stereocilia (called the kinocilium in hair cells of the vestibular system and immature auditory system) is excitatory and in the opposite direction is inhibitory. One theory suggests a mechanical link to ion channels which opens a "trap door" as it is pulled taut. This is a little fanciful and it is thought that the ion channels are located at the base of the stereocilia but are indeed mechanoreceptors, in that they respond to mechanical pressure. The endolymph surrounding a hair cell is K+ rich and so K+ (and calcium) enter the hair cell, causing a depolarisation.
8. The action potentials are transmitted along the cochlear branch of the vestibulocochlear nerve, activating auditory pathways in the central nervous system, eventually terminating in the auditory area of the temporal lobe of the cerebral cortex.
9. Finally, the vibrations in the scala tympani are dissipated out of the round window, into the middle ear.

FREQUENCY CODING
The basilar membrane is narrow and stiff at the window end and wide and flexible at the apical end. This natural topographical difference in structure results in different regions vibrating at different resonant frequencies. The end near the stapes (window end) vibrates at high frequencies whereas the apical end vibrates at low frequencies. Information about the vibration at different locations along the basilar membrane is relayed to the auditory cortex by the nerves synapsing with the hair cells at those locations. The auditory cortex is therefore said to be tonotopically mapped, i.e. the basilar membrane is represented point for point on the auditory cortex.

Not all cells in the auditory cortex respond to simple tones. Complex signals like clicks, voices, whistles, which contain many frequencies, excite many different regions of the basilar membrane simultaneously and there are specific cells in the auditory cortex that respond to these stimuli.

AUDITORY PATHWAY

Afferent nerves from the cochlear (spiral) ganglion terminate in the cochlear nucleus in the brainstem. Axons from neurons in the cochlear nucleus project to the superior olive, inferior colliculus, medial geniculate nucleus (of the thalamus) and the auditory cortex (Brodman areas 41 and 42). Note that auditory input projects to both sides of the cortex. The superior olive and the inferior colliculus send efferent fibres back to the stapedius and tensor tympani muscles respectively. These muscles are concerned with protecting the middle ear bones from overload.

For additional information:

Ear structure South Bank University - detailed info on structure , [Special Senses Menu] , [Bionic Ear]

Reading list

Basic text:

• "Neurophysiology", R.H.S. Carpenter, pub. Edward Arnold, 2nd ed. 1990, 3rd ed. 1995. ISBN 0 340 50634 2.
• "Signals and Perception; the fundamentals of human sensation", ed. David Roberts, pub. Open University, Palgrave Macmillan, 2002.

More detail:

• "The Senses" by H.B. Barlow and J.D. Mollon, Cambridge University Press, 1982, ISBN (pbk) 0 5221 28714 6
• "An introduction to the physiology of hearing" by J.O. Pickles, Academic Press, 1988, ISBN 0 12 554754 (pbk).

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last update 2nd December 2002