CODING MECHANISMS OF ELECTRORECEPTOR FIBERS IN SOME ELECTRIC FISH

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Abstract
Properties of electroreceptor were analyzed by recording impulse discharge of single lateral line nerve fibers of Staetogenes (electric organ discharge of 40-60 cyc./sec), Gymnotus (40-60 cyc./sec), Eigenmannia (250-400 cyc./sec), and Sternopygus (60-100 cyc/sec). The intensity of the potential field produced by the electric organ pulse is coded by a single lateral line nerve fiber principally by the gradation of number of impulses in a train produced by each pulse in fishes having a low frequency discharge of the electric organ such as in Staetogenes and Gymnotus. However in fishes of a high frequency electric organ discharge such as in Eigenmannia the coding is performed by grading the probability of impulse initiation after each pulse of the electric organ. In Sternopygus both mechanisms are operative. Small conducting or dielectric objects in the water just over the receptor respectively suppress or enhance the nerve response and anterior or posterior to the receptor area gives rise to the opposite type of responses. The threshold for an imposed electric pulse to produce one nerve impulse is about 10 mV/cm along the long axis of the fish and is the same for a pulse of any duration above 5 msec. in Staetogenes and Gymnotus. The threshold for an imposed a.c. field is about mV/sec. in Eigenmannia is higher when the frequency of a.c. departs far from that of the electric organ discharge.

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