FIGURE 1 Vestibular and auditory organs in the inner ear. (A) Anterior view of the right ear: The external and middle ear are separated by the tympanic membrane or eardrum. The air-filled middle ear contains three tiny ear ossicles that transduce sound vibrations from the tympanic membrane to the oval window in the inner ear and to the cochlea. The vestibular apparatus occupies the vestibule of the inner ear. The vestibular and auditory nerves join to form cranial nerve VIII as it projects to the medulla. (B) Vestibular organs of the right ear: Three semicircular canals and two otolith organs (the utricle and the saccule) are formed by membranous sacs filled with endolymph (blue) and surrounded by perilymph (gray). The cupula within the semicircular canals, and the macula in the otolith organs represent areas of specialized epithelium containing sensory hair cells.
cilium called the kinocilium (Fig. 2). Stereocilia vary in length; the longest is next to the kinocilium, and each successive one is shorter. Rows of stereocilia are connected tip to tip by extracellular filaments that act as minute elastic springs.
When stereocilia are deflected toward the kinocilium (or the tallest microvilli, in the case of auditory receptors), tension on the connecting filaments is increased, opening potassium channels within the cell membrane and depolarizing the hair cell. Ordinarily, an increase in potassium conductance would hyperpolarize a cell, but the tips of hair cells are immersed in a special fluid, endolymph, containing an unusually high potassium concentration (150 mmol/L) and low sodium concentration (1 mmol/L) compared to that found in the cytoplasm (140 mmol/L and 9 mmol/L, respectively). Thus, opening of potassium channels causes an inward flow of potassium. The resulting depolarization affects the entire hair cell and causes release of the neurotransmitter, glutamate, from synapses at the base of the cell. Glutamate depolarizes the adjacent afferent neuron, which generates a series of action potentials.
When stereocilia are deflected away from the kino-cilium, the hair cell is hyperpolarized. This happens because, in the normal resting state with stereocilia in an upright position, there is some minimal tension on the connecting fibers, causing some (perhaps 10%) of the potassium channels to remain open. With displacement of stereocilia away from the kinocilium, resting tension is relaxed, potassium channels close, and the cell hyperpo-larizes. As a result, less glutamate is released and fewer action potentials are generated in the afferent nerve. Movement in an orthogonal direction from the kino-cilium results in no change in tension on the connecting fibers and thus no change in frequency of action potentials in the afferent nerve.
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