Sound Amplification And Attenuation

The elaborate system of fluid-filled membrane ducts within the cochlea serves to amplify sound vibrations and to provide maximum acuity while at the same time protecting receptor cells from potential damage from high-intensity sounds. The tips of the stereocilia are embedded in a gelatinous meshwork called the tectoral membrane, which is connected to the reticular lamina at one end and to a bony protuberance called the modiolus at the other end (see Fig. 5). This arrangement produces a shearing action on the stereocilia during displacements of the basilar membrane, maximizing the transfer of small to medium movements of fluid while offering some protection against damage from exaggerated movements.

A second amplification mechanism is provided by special properties of the outer hair cells. Depolarization hardest bones in the body are the ear ossicles. The malleus ("hammer") attaches to the back of the tympanic membrane (eardrum) and to the incus ("anvil"), which attaches to the stapes ("stirrup"). The foot of the stapes fits onto the membrane covering the oval window, which is hidden behind the stapes. (C) Air movements vibrate the large tympanic membrane, which vibrates the ear ossicles, and their lever action vibrates the fluid under the small oval window. The incompressible fluid of the inner ear bulges the membrane of the round window with each vibration. The air pressure on the tympanic membrane is amplified 20 times because of the lever action and the difference in area of the tympanic membrane and the oval window.

A. Cochlea with stapes displaced

A. Cochlea with stapes displaced

scala media

B. Structures of the middle ear oval window into scala vestibuli

B. Structures of the middle ear oval window into scala vestibuli

FIGURE 3 Structure of the cochlea. (A) Cochlea with stapes displaced; scala media filled with endolymph is shown in blue. (B) Cochlea with a small wedge removed showing the three canals or scala; the dotted line indicates the relative positions of the basilar membrane within the cochlea. (C) Enlargement of the cochlear ducts shows the hair cells extending into the endolymph.

FIGURE 3 Structure of the cochlea. (A) Cochlea with stapes displaced; scala media filled with endolymph is shown in blue. (B) Cochlea with a small wedge removed showing the three canals or scala; the dotted line indicates the relative positions of the basilar membrane within the cochlea. (C) Enlargement of the cochlear ducts shows the hair cells extending into the endolymph.

of the cell by potassium in response to a sound wave stimulates intracellular motor proteins, causing the cell to lengthen. Because the hair cell is sandwiched between the basilar membrane and the tectoral membrane, a lengthening of the cell increases the distance between the two membranes. Subsequently, the basilar membrane bends more and shearing forces generated by the tectoral membrane are increased. The net result is an amplification of the response (cochlear amplification) as the subsequent series of sound waves pass over the activated area.

Inner hair cells lack these motor proteins and do not change their length in response to stimulation. They also differ from outer hair cells in the extent to which they are innervated. Though outnumbered 5 to 1 by outer hair cells, the inner hair cells are much more richly innervated. Peripheral fibers from approximately 10 separate spiral ganglia neurons are committed to the innervation of each inner hair cell. The opposite is true for outer hair cells, which are innervated in groups of 10 or more by a single spiral ganglia neuron. Of the spiral ganglia cells, 95% innervate inner hair cells, and the remaining 5% innervate outer hair cells. Thus, the major source of auditory information is the inner hair cell, with perhaps the major function of the outer hair cells being limited to cochlear amplification.

One of the major protective mechanisms for the auditory system, called the attenuation reflex, involves not the cochlea per se, but other elements of the middle ear. The tensor tympani muscle is attached to the bone of the middle ear and to the malleus. Similarly, the stapedium muscle attaches to the stapes. Both muscles are innervated by brain stem structures that receive input from the auditory pathway as it projects to higher centers. In response to a loud sound, particularly at lower frequencies, motor neurons from the brain stem cause these muscles to contract, thus stabilizing the ossicles and reducing sound conductance to the inner ear. Sound attenuation serves to adapt the ear to continuous sound at high frequency, preventing saturation of the receptors and increasing the dynamic response range of the auditory system. It also serves to protect the structures of the inner ear against potential damage by overstimulation. Because the relay pathway is polysynaptic and involves central neurons, there is a response delay of 50 to 500 ms. Thus, there is no protection against very sudden increases in sound intensity, such as a shotgun blast.

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