11. ELECTRORECEPTION
555
logical data establish projections of electroreceptors to the eminentia
granulosa in gymnotids (Enger and Szabo, 1965) and to the lateral line
lobes in gymnotids and elasmobranchs (Enger and Szabo, 1965;
Nicholson et al., 1969). Single units in the lateral line lobe of gymnotids
can signal the presence of objects by an alteration in the number of
spikes following an electric organ discharge (Enger and Szabo, 1965).
No responses to electrosensory inputs have been found in the corpus
cerebelli of elasmobranchs, and the suggestion has been made that this
part of the cerebellum is involved in control of movement rather than
electroreception ( Nicholson et al., 1969).
An interesting aspect of the mormyrid electrosensory system is that
the neural command signal that fires the electric organ also projects to
many parts of the cerebellum and hind brain. This signal presumably
prepares the afferent pathways for incoming signals from the electroreceptors. Both inhibitory and facilitatory effects on central responses to
afferent volleys can be observed when electroreceptors are activated in
specific time relation to the command signal to excite the organ (Bennett
and Steinbach, 1969; B. Zipser and M. V. L. Bennett, unpublished data).
These effects are independent of actual organ discharge and occur in
the curarized animal. They indicate that the fish “knows” when it is
going to discharge its electric organ. At least in principle, the fish can
measure the latency of the receptor response. In a passive sensory system only differences in latency between receptors can be determined.
D. Behavioral Responses to Voltage Gradients and Conductance
Changes
A number of different kinds of behavioral responses that apparently
involve electroreception have been observed in electric fish. Many
gymnotids move away from a metal rod that is brought close to them,
whereas they tend to ignore visually similar insulators. Individuals of
more aggressive species such as Gymnotus and Electrophorus may attack metal objects, A number of catfish and sharks also respond to metal
rods, much more strongly than to insulators (M. V. L. Bennett, unpublished data; Dijkgraaf, 1963, 1968). In electric fish the differential responsiveness may involve active electroreception, and as already noted
alterations in the nerve volleys evoked by organ discharge can be produced by metallic objects (Fig. 37). In addition inhomogeneities in
metals cause eddy currents in surrounding water, and these fields can
be adequate to stimulate tonic receptors as has been observed in gymnotids and dogfish (Fig. 39). Presumably the latter mode of action is
555
logical data establish projections of electroreceptors to the eminentia
granulosa in gymnotids (Enger and Szabo, 1965) and to the lateral line
lobes in gymnotids and elasmobranchs (Enger and Szabo, 1965;
Nicholson et al., 1969). Single units in the lateral line lobe of gymnotids
can signal the presence of objects by an alteration in the number of
spikes following an electric organ discharge (Enger and Szabo, 1965).
No responses to electrosensory inputs have been found in the corpus
cerebelli of elasmobranchs, and the suggestion has been made that this
part of the cerebellum is involved in control of movement rather than
electroreception ( Nicholson et al., 1969).
An interesting aspect of the mormyrid electrosensory system is that
the neural command signal that fires the electric organ also projects to
many parts of the cerebellum and hind brain. This signal presumably
prepares the afferent pathways for incoming signals from the electroreceptors. Both inhibitory and facilitatory effects on central responses to
afferent volleys can be observed when electroreceptors are activated in
specific time relation to the command signal to excite the organ (Bennett
and Steinbach, 1969; B. Zipser and M. V. L. Bennett, unpublished data).
These effects are independent of actual organ discharge and occur in
the curarized animal. They indicate that the fish “knows” when it is
going to discharge its electric organ. At least in principle, the fish can
measure the latency of the receptor response. In a passive sensory system only differences in latency between receptors can be determined.
D. Behavioral Responses to Voltage Gradients and Conductance
Changes
A number of different kinds of behavioral responses that apparently
involve electroreception have been observed in electric fish. Many
gymnotids move away from a metal rod that is brought close to them,
whereas they tend to ignore visually similar insulators. Individuals of
more aggressive species such as Gymnotus and Electrophorus may attack metal objects, A number of catfish and sharks also respond to metal
rods, much more strongly than to insulators (M. V. L. Bennett, unpublished data; Dijkgraaf, 1963, 1968). In electric fish the differential responsiveness may involve active electroreception, and as already noted
alterations in the nerve volleys evoked by organ discharge can be produced by metallic objects (Fig. 37). In addition inhomogeneities in
metals cause eddy currents in surrounding water, and these fields can
be adequate to stimulate tonic receptors as has been observed in gymnotids and dogfish (Fig. 39). Presumably the latter mode of action is