10. ELECTRIC ORGANS
367
and enters cells (E-H). The amplitude decreases as the electrode
crosses innervated faces but not as it crosses uninnervated faces because of their relatively high and low resistances, respectively.
The neural mediation of these responses is indicated by the latency
which remains about 1 msec even if very suprathreshold stimuli are
used. If spike generating membrane were present, a much shorter
latency could be obtained (an example is given in Fig. 18). Neural
mediation is confirmed by pharmacological data and the effects of
denervation (see below). Responses like those of Fig. 7 result from
single nerve fibers for their amplitude varies in an all-or-none manner
as stimulus strength is varied.
Other experiments involving passage of current during responses
show that the resistance of the innervated faces is decreased during
neurally mediated activity but is not affected by depolarization alone.
Thus during activity the resistance of the innervated face moves toward
that of the uninnervated face in the kind of impedance matching discussed in the preceding section. The resting resistances of the two faces
correlate with the degree of surface elaboration seen in the fine structural studies; the lower resistance membrane is much more elaborated
( Mathewson et al., 1961). (The negligible voltage drop across the
innervated faces need not obtain in responses of the entire organ when
all the cells are active in series and much larger currents flow. In organ
discharge there would also be a smaller potential across the innervated
face because of its internal resistance. )
The PSPs of the electrocytes are cholinergic as they presumably are
in the muscle fibers of origin. They are greatly reduced by the blocking
agent curare and prolonged by the anticholinesterase eserine. The cells
are depolarized by acetylcholine, presumably the actual transmitter,
and the related compound carbamylcholine. Denervated cells retain
their ability to respond to these drugs, but the response to electrical
stimulation disappears confirming its neural mediation.
The microelectrode experiments demonstrate that unitary responses
of the electrocytes are essentially of the same duration, shape, and sign
as the organ discharges. If 150 cells in series give rise to a 5 V pulse
about 30 mV per cell is required, a value exceeded somewhat in the
microelectrode experiments. The discrepancy is explicable as a result
of greater current flow during synchronous activity and failure of some
of the cells to fire. Furthermore, cell counts, organ discharges, and PSP
amplitudes may have varied in the different animals used for the
measurements. Although most of the presynaptic fibers are active during
a response (see Section 111), some do fail to fire on occasion as indicated
by the variable response amplitude, and a few inactive cells in series
367
and enters cells (E-H). The amplitude decreases as the electrode
crosses innervated faces but not as it crosses uninnervated faces because of their relatively high and low resistances, respectively.
The neural mediation of these responses is indicated by the latency
which remains about 1 msec even if very suprathreshold stimuli are
used. If spike generating membrane were present, a much shorter
latency could be obtained (an example is given in Fig. 18). Neural
mediation is confirmed by pharmacological data and the effects of
denervation (see below). Responses like those of Fig. 7 result from
single nerve fibers for their amplitude varies in an all-or-none manner
as stimulus strength is varied.
Other experiments involving passage of current during responses
show that the resistance of the innervated faces is decreased during
neurally mediated activity but is not affected by depolarization alone.
Thus during activity the resistance of the innervated face moves toward
that of the uninnervated face in the kind of impedance matching discussed in the preceding section. The resting resistances of the two faces
correlate with the degree of surface elaboration seen in the fine structural studies; the lower resistance membrane is much more elaborated
( Mathewson et al., 1961). (The negligible voltage drop across the
innervated faces need not obtain in responses of the entire organ when
all the cells are active in series and much larger currents flow. In organ
discharge there would also be a smaller potential across the innervated
face because of its internal resistance. )
The PSPs of the electrocytes are cholinergic as they presumably are
in the muscle fibers of origin. They are greatly reduced by the blocking
agent curare and prolonged by the anticholinesterase eserine. The cells
are depolarized by acetylcholine, presumably the actual transmitter,
and the related compound carbamylcholine. Denervated cells retain
their ability to respond to these drugs, but the response to electrical
stimulation disappears confirming its neural mediation.
The microelectrode experiments demonstrate that unitary responses
of the electrocytes are essentially of the same duration, shape, and sign
as the organ discharges. If 150 cells in series give rise to a 5 V pulse
about 30 mV per cell is required, a value exceeded somewhat in the
microelectrode experiments. The discrepancy is explicable as a result
of greater current flow during synchronous activity and failure of some
of the cells to fire. Furthermore, cell counts, organ discharges, and PSP
amplitudes may have varied in the different animals used for the
measurements. Although most of the presynaptic fibers are active during
a response (see Section 111), some do fail to fire on occasion as indicated
by the variable response amplitude, and a few inactive cells in series