MECHANORECEPTORS AND BEHAVIOR 



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later), they developed the interesting hypothesis that a movement of each 

 hair cell bundle towards the kinocilium would excite the hair cell, whereas 

 movement in the opposite direction would be "inhibitory" (Figure 5). This 

 theory has found good support wherever it has been possible to correlate 

 physiological data with morphological observation. For example, in the 

 lateral-line organs some hair cells are excited by one direction of stimulus 

 and some by the opposite direction, and hair cells with kinocilia oriented in 

 both directions have been reported (Flock and Wersall 1962). 



Figure 5 The directional sensitivity of hair cells. Displacement of the ciliary bundle to- 

 ward the kinocilium (A) excites the hair cell, whereas displacement of the bundle away 

 from the kinocilium decreases hair-cell excitability. In the labyrinth of the ray (B) the 

 kinocilia (shown as "o") are on the outer face of the hair bundle (stereocilia group shown 

 as "•") in the ampulla of the horizontal canal. Rotation as shown causes the endolymph 

 to displace the hair bundle toward the kinocilium and is excitatory (B modified from 

 Lowenstein and Wersall 1969). 



Basic Processes in Hair-Cell Function— As elasmobranch hair cells are 

 not as large as those of amphibia or other species (Necturus hair cells are 80 

 fj.m by 15 jum— Frishkopf and Oman 1972) and are inconvenient for fine 

 electrophysiology, little detailed work has been done on their function. 

 Nevertheless, because it is probable that all hair cells operate in similar ways, 

 the current ideas on hair-cell function, developed from work on various 

 species, will be briefly summarised here (see also Russell 1976). 



It is generally accepted that the adequate stimulus for hair-cell excitation 

 is a shearing movement of the cupula (Trincker 1962), and several theories as 

 to how this actually excites the sense organ have been put forward (Flock 

 1971; Malcolm 1974; Hillman and Lewis 1971). 



An easily recorded extracellular potential, which is set up by a stimulus 

 and is always associated with mechanoreceptive hair cells, is the microphonic 

 potential that can be detected close to the sensory surface (Figure 6), ac- 

 companying the stimulus as a negative d.c. potential, but reversing in 

 polarity when the electrode is located beneath the hair-cell epithelium. This 

 potential, which depends on the integrity of the sensory epithelium, is now 

 believed to be a summation of coincident receptor potentials of a large 



