In addition to afferent fibers, the organ of Corti is supplied by efferent fibers, most of which terminate on the outer hair cells. These cochlear efferent fibers originate in the superior olivary nucleus of the brainstem and are often called olivocochlear fibers. The length of the outer hair cells varies; this characteristic suggests that changes in outer hair cell length may affect the sensitivity, or “tuning,” of the inner hair cells. The cochlear efferent fibers may control outer hair cell length. Such a mechanism could conceivably influence the sensitivity of the cochlea and the way that the brain recognizes sound. Other efferent fibers that end on cochlear afferent fibers may be inhibitory, and they may help improve frequency discrimination.
Sound is transduced by the organ of Corti. Sound waves that reach the ear cause the tympanic membrane to oscillate, and these oscillations are transmitted to the scala vestibuli by the ossicles. This creates a pressure difference between the scala vestibuli and the scala tympani (see
Fig. 8.17B ) that serves to displace the basilar membrane and, with it, the organ of Corti (see
Fig. 8.18A and B ). Because of the shear forces set up by the relative displacement of the basilar and tectorial membranes, the stereocilia of the hair cells bend. Upward displacement bends the stereocilia toward the tallest cilium, which depolarizes the hair cells; downward deflection bends the stereocilia in the opposite direction, which hyperpolarizes the hair cells.
Acommoncauseofdeafnessisthedestructionofhaircellsbyloudsounds.Haircellscanbedestroyed,forexample,byexposuretoindustrialnoiseorbylisteningtoloudmusic.Typically,haircellsincertainpartsofthecochleaareselectivelydamagedbyexposuretohighlevelsofsoundatparticularfrequencies(aspredictedbytheplacetheory),andthushearingmaybelostoveradiscretefrequencyrange.Presbycusis,orthelossofhigh-frequencyhearingwithage,isprobablyincreasedbythelossofhaircellsasaresultoflong-termnoiseexposureinurbanenvironments.
In view of the wide range of frequencies and amplitudes of sound stimuli, it is no surprise that hair cell transduction must provide for a fast response. The fast response to deflection of the cilia is based on direct opening of ion channels by so-called tip links that connect the tip of each stereocilium with the shaft of the next taller one (see
CHAPTER 8 The Special Senses •Fig. 8.18 DetailoftheorganofCortiatrest(A) andwithupwardmovementofthebasilarmembrane(B). Theupwardmovementcausesthestereociliatobendbecauseofshearforcesproducedbyrelativedisplacementofthehaircellsandthetectorialmembrane.C, Diagramofahaircellwithtiplinkconnectionsbetweenthehaircellciliatoshowhowshearforcesopenmechanoreceptorchannelsand depolarizethehaircell.
Fig. 8.18C ). With deflection, the tip links are subjected to a lever action that transiently opens the channels, allows the entry of K+ (because of the high [K+] and high potential in endolymph), and depolarizes the hair cell. Several mechanisms have been proposed to account for the equally important rapid adaptation necessary for a high-frequency response. A “spring” response by the tip links would allow the attachment point of the tip link to be moved along the stereocilium’s shaft to reset the mechanical leverage of the tip link. In addition, it has been observed that Ca++ can enter and bind to the open channel, change it to require greater opening force, and thereby reduce the statistical probability of opening.