Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Hearing and Equilibrium
Hair cells

Structure

All Hair Cells of the Inner ear share a similar structure (Fig. 9-8). Each hair Cell is surrounded by supporting cells. With the exception of the outer hair cells of the cochlea, the basal region of these hair cells is embraced by dendrites of afferent Neurons. Projecting from the apical surface of each hair cell are 30-150 rod-like processes, or hairs. Except in the cochlea, one of these processes—the kinocilium—is a true yet immotile cilium containing nine peripheral microtubule doublets and a central pair of microtubules (see Chapter 1). It is one of the longest processes, featuring a club-like Swelling at its tip. Kinocilia are lost by cochlear hair cells in adult mammals. The other type of processes, stereocilia, are present in all hair cells without exception. They possess a core formed by parallel Actin filaments coated with various Myosin isoforms. The bundle of processes on each hair cell is arranged in an orderly fashion: the height of the stereocilia progressively increases toward the kinocilium; in the perpendicular direction, within the same row, the stereocilia are of uniform height.

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Fig. 9-6. Schematic representation of the cristae ampullares. The cupula at the apex of the crest spans the ampulla and is capable of bending. Due to the inertia of the endolymph during angular acceleration, it shifts in the direction opposite to the rotation. This causes bending of the processes and, consequently, Changes in the permeability and Membrane Potential of the hair cells.

Electrical Responses

The resting membrane potential of hair cells is approximately - 60 mV. When the stereocilia are deflected toward the kinocilium, the membrane potential decreases to - 50 mV. Deflection of the stereocilia in the opposite direction hyperpolarizes the cells. Deflection of the processes perpendicular to this axis produces no change in membrane potential. Deflection in an oblique direction causes depolarization or hyperpolarization of the hair cells, proportionally depending on whether the stereocilia move toward or away from the kinocilium. Thus, the hair-like processes implement a mechanism that transduces the direction and magnitude of deflection into changes in the hair cell membrane potential.

Fig. 9-7. Main vestibular Pathways of the Brainstem, posterior view. The Cerebellum and Cerebral Cortex have been removed.

Generation of action potentials in afferent nerve fibers

As noted above, the processes of hair cells are surrounded by endolymph, whereas their basal portion is bathed by perilymph. This arrangement is essential for the normal generation of action potentials. Perilymph is derived primarily from Blood Plasma. Although The transport of mannitol and sucrose from the plasma into the perilymph of the scala tympani is slower compared to their transport into the perilymph of the scala vestibuli, and there are other minor differences between the fluids in these two compartments, both types of perilymph closely resemble extracellular fluid. Endolymph, produced by the stria vascularis, has a high K⁺ concentration and a low Na⁺ concentration (Fig. 9-9). The cells of the stria vascularis are characterized by high levels of Na⁺-K⁺-ATPase. In addition, these cells feature a unique voltage-dependent K⁺ pump, which generates a potential difference of 85 mV between the scala media and either the scala vestibuli or the scala tympani.

Fig. 9-8. Left: structure of a hair cell from the frog saccule, illustrating its relationship with the otolithic membrane (OM); K - kinocilium; C - stereocilium; ВК - hair cell with afferent (A) and efferent (E) nerve fibers; ОЛ - otolith; ПК - supporting cell (reproduced with permission from Hillman DE: Morphology of peripheral and central vestibular systems. In: Llinás R, Precht W [editors]: Frog Neurobiology. Springer, 1976). Right: scanning electron micrograph of the hair cell processes from the macula of the frog saccule. The otolithic membrane has been removed. The short processes surrounding the hair cell are microvilli of the supporting cells (AJ Hudspeth).

Very fine processes, known as tip links (Figs. 9-10), connect the apex of each stereocilium to the lateral surface of an adjacent, taller stereocilium, which at this contact site contains mechanosensitive cation channels. When shorter stereocilila are deflected toward the taller ones, the open time of these channels increases. It is believed that the tension state of each channel is regulated by an "adaptation motor" formed by myosin of the taller stereocilium. Deflection of the stereocilia in the opposite direction decreases the channel's open time. The function of this channel may be served by the epithelial Na⁺ channel $\alpha$-subunit, which itself forms a low-selectivity cation channel (see Chapter 1), with amiloride bound to the contact site between the shorter and taller stereocilia. These channels are relatively non-specific cation channels, and since they are bathed in endolymph with an elevated K⁺ ion concentration, ions enter the hair cell through the open channels, causing membrane depolarization. The influx of Ca²⁺ triggers the release of a neurotransmitter, which depolarizes the membrane of one or more afferent neurons in contact with the hair cell. Although the neurotransmitter has not yet been definitively identified, glutamate is the most likely candidate.

K⁺ ions that enter the hair cell through mechanosensitive cation channels undergo recycling (see Fig. 9-9). First, they enter a supporting cell, from which they diffuse Through Gap Junctions into other supporting cells. Finally, they reach the stria vascularis, from where they are secreted back into the endolymph, completing the cycle.