Stimulation of Cochlear Hair Cells

To produce a sensation of sound, vibration of the auditory ossicles leads to vibration of the basilar membrane on which the hair cells rest. A simple mechanical model of the ear (fig. 16.14) makes it easy to see how this happens. The stapes pushes on the perilymph of the scala vestibuli; the perilymph pushes the vestibular membrane down; the vestibular membrane pushes on the endolymph of the cochlear duct; and the endolymph pushes the basilar membrane down. (The vestibular membrane is omitted from the diagram for simplicity; it has no significant effect on the mechanics of the cochlea.) The basilar membrane puts pressure on the perilymph of the scala tympani below it, and the secondary tympanic membrane bulges outward to relieve this pressure. In short, as the stapes goes in-out-in, the secondary tympanic membrane goes out-in-out, and the basilar membrane goes down-up-down. It is not difficult to see how this happens— the only thing hard to imagine is that it can happen as often as 20,000 times per second!

The vestibular membrane separates the perilymph of the scala vestibuli from the endolymph of the cochlear duct. In order for the hair cells to function properly, the tips of their stereocilia must be bathed in endolymph. Endolymph has an exceptionally high K+ concentration, which creates a strong electrochemical gradient from the

Outer ear

Middle ear

Inner ear

Stapes Incus Malleus Sound

Tympanic membrane

Auditory tube

Outer ear

Middle ear

Inner ear

Stapes Incus Malleus Sound

Tympanic membrane wave

Figure Mcgraw Hill Saladin

Oval window

Basilar membrane

Secondary tympanic membrane

Figure 16.14 Mechanical Model of the Ear. Each inward movement of the tympanic membrane pushes inward on the auditory ossicles of the middle ear and fluid of the inner ear. This pushes down on the basilar membrane, and pressure is relieved by an outward bulge of the secondary tympanic membrane. Thus the basilar membrane vibrates up and down in synchrony with the vibrations of the tympanic membrane.

Why would high air pressure in the middle ear reduce the movements of the basilar membrane of the inner ear?

Oval window

Basilar membrane

Secondary tympanic membrane

Figure 16.14 Mechanical Model of the Ear. Each inward movement of the tympanic membrane pushes inward on the auditory ossicles of the middle ear and fluid of the inner ear. This pushes down on the basilar membrane, and pressure is relieved by an outward bulge of the secondary tympanic membrane. Thus the basilar membrane vibrates up and down in synchrony with the vibrations of the tympanic membrane.

Why would high air pressure in the middle ear reduce the movements of the basilar membrane of the inner ear?

wave

Saladin: Anatomy & I 16. Sense Organs I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition tip to the base of a hair cell. This gradient provides the potential energy that ultimately enables a hair cell to work.

The tectorial membrane is especially important in cochlear mechanics. Remember that the stereocilia of the outer hair cells have their tips embedded in it, and those of the inner hair cells come very close to it. The tectorial membrane is anchored to the modiolus, which holds it relatively still as the basilar membrane and hair cells vibrate up and down. Movement of the basilar membrane thus bends the hair cell stereocilia back and forth.

At the tip of each stereocilium of the inner hair cells is a single transmembrane protein that functions as a mechanically gated ion channel. A fine, stretchy protein filament called a tip link extends like a spring from the ion channel of one stereocilium to the side of the stereocilium next to it (fig. 16.15). The stereocilia increase in height progressively, so that all but the tallest ones have tip links leading to taller stereocilia beside them. When a taller stereocilium bends away from a shorter one, it pulls on the tip link and opens the ion channel of the shorter stereocil-ium. The channel is nonselective, but since the predominant ion of the endolymph is K+, the primary effect of this gating is to allow a quick burst of K+ to flow into each hair cell. This depolarizes the hair cell while the channel is open, and when the stereocilium bends the other way its channel closes and the cell becomes briefly hyperpolar-ized. During the moments of depolarization, a hair cell releases a neurotransmitter that stimulates the sensory

Depolarization Hair Cells

Unstimulated Stimulated

Figure 16.15 Potassium Gates of the Cochlear Hair Cells.

Each stereocilium has a gated K channel at its tip. Vibrations of the cochlea cause each stereocilium to bend and, with its tip link, pull open the K+ channel of the adjacent stereocilium. The inflow of K+ depolarizes the hair cell.

Unstimulated Stimulated

Figure 16.15 Potassium Gates of the Cochlear Hair Cells.

Each stereocilium has a gated K channel at its tip. Vibrations of the cochlea cause each stereocilium to bend and, with its tip link, pull open the K+ channel of the adjacent stereocilium. The inflow of K+ depolarizes the hair cell.

Chapter 16 Sense Organs 603

dendrites synapsing with its base. Each depolarization thus generates action potentials in the cochlear nerve.

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  • matta
    When hair cell stimulated?
    2 years ago

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