EXCITATION BY HYPERPOL ARIZING POTENTIALS 331 



Stimuli (Grundfest, 1958a). This is the presumed receptor for the electrical 

 guidance systems of fishes (Coates et al., 1954; Grundfest, 1957a; Bennett 

 and Grundfest, 1959; Lissmann, 1958). Indeed, the sensitivity of these recep- 

 tors for changes in the electric current must be very high (Coates et al., 1954). 

 In one form detection of a gradient of about 0-02 ju.V/cm has been reported 

 (Lissmann, 1958). These receptors may therefore be truly "specific" for 

 electricity. 



ELECTROPHYSIOLOGICAL PROCESSES IN SECONDARY 

 RECEPTOR SYSTEMS 



Secondary receptors are said to be confined to vertebrates (cf. Antrum, 

 1959), but as noted above, the Limulus lateral eye is probably a secondary 

 receptor system. The vertebrate organs of special senses, vision, taste, hearing 

 and equihbrium and the lateral-hne organs of fishes have specialized receptor 

 cells which act on other cells. In all but vision, the receptor cells make contact 

 with a nerve cell. The latter therefore is the final common path. In the eye the 

 rods and cones play upon horizontal and bipolar cells which in turn impinge 

 on neurons of the ganglion cell layer. The horizontal and bipolar cells do not 

 produce spikes (MacNichol and Svaetichin, 1958; Tomita et al, 1959; Brown 

 and Wiesel, 1959) and they may be regarded as operating in a manner similar 

 to that of the receptor cells. They may be all considered as elements in which 

 the conductile component of the generalized neuron is missing (Grundfest, 

 1958a). They should be electrically inexcitable (cf. Watanabe et al, 1960) 

 and might with propriety be regarded as neurosecretory cells (Grundfest, 

 1958b, 1961). 



The vertebrate taste buds, which are secondary receptor cells, generate 

 sustained depolarizing potentials without spikes (Fig. 5). The various cells 

 respond to different degrees to standard stimuli, and this indicates that the 

 specific sensations of taste arise out of differential patterns of many kinds of 

 cells. Presumably the cells excite activity of the common path neurons, but 

 whether by ephaptic means (cf. Grundfest, 1959a) or by release of a secretory 

 transmitter cannot be decided at present. 



In the case of the fish and cat retina, however, ephaptic transmission is 

 ruled out. No potentials have been recorded in conjunction with the activity 

 of the rods and cones. The absence of electrical sign may be due to some 

 experimental artifact since these receptor elements are small. However, an 

 artifact seems unlikely, since a number of investigators have looked for 

 responses in rods and cones without success. Perhaps, therefore, the photo- 

 chemical activity can excite the next cell (horizontal or bipolar) without itself 

 producing a potential. Certainly in the absence of potentials in the sensory 

 receptors ephaptic transmission to the neural element must be ruled out. 

 Transmission between the photochemical receptors and the intermediate 



