10. ELECTRIC ORGANS
391
The excitability properties of the innervated face lead to an unusual
sequence of conductance changes during the response (Morlock et al.,
1969). At the start of the spike, depolarization causes sodium activation
and increased conductance and further depolarization; depolarization
also decreases the conductance of anomalously rectifying channels, but
the net result is increased conductance, Sodium inactivation ensues,
and the total conductance decreases to below the resting value while
the membrane potential falls from the spike peak; at a sufficiently low
potential anomalous rectification reverses and conductance rises to the
resting level. This diphasic sequence of conductance changes, increase
followed by decrease, contrasts to the monophasic increase seen in
nerve (Cole and Curtis, 1939; Tasaki and Freygang, 1955).
One might ask why the innervated membrane should not just have
a high resting resistance instead of anomalous rectification. One possible
reason is to provide a conductance in inactive cells for flow of current
generated by active cells in series with them. As will be discussed further
in Section 111, A, the cells are not all active at the same time except in the
largest discharges. The voltage of these responses is accounted for by
synchronous activity of some 6000 cells in series each producing over
100 mV.
b. Gymnotus. This species is weakly electric; its organ discharge is a
fraction of a volt recorded in water and not much more than a volt
recorded in air. Its body shape is similar to that of the eel, but the electric
organ is much smaller (Fig. 1). When undisturbed and resting it normally emits pulses at about 50/sec. The pulses are approximately triphasic, initially head negative, and about 1 msec in duration (Fig. 3B,
B’) . Mechanical stimuli, light, electric fields, and resistance changes can
cause moderate transient accelerations of the discharge, and when the
animal is feeding, the discharge frequency can briefly exceed 200/sec
( Fig. 3, Lissmann, 1958; Bennett and Grundfest, 1959; Black-Cleworth,
1970). When swimming around its tank the fish maintains a fairly steady
frequency somewhat above the resting level. Gymnotus is also capable
of ceasing its discharge completely for brief periods, a response that may
sometimes represent hiding or “listening.” Both accelerations and cessations can be involved in communication between other members of the
same species and a relatively potent stimulus for inducing cessation of
firing is weak electric pulses at a frequency close to that of the organ
discharge.
The electric organ runs longitudinally from just behind the chin to the
tip of the caudal filament. The organ lies dorsal to the anal fin but
extends beyond it rostrally to the cleithrum as well as caudally (Fig. 1).
The organ on one side consists of about four longitudinal columns of
391
The excitability properties of the innervated face lead to an unusual
sequence of conductance changes during the response (Morlock et al.,
1969). At the start of the spike, depolarization causes sodium activation
and increased conductance and further depolarization; depolarization
also decreases the conductance of anomalously rectifying channels, but
the net result is increased conductance, Sodium inactivation ensues,
and the total conductance decreases to below the resting value while
the membrane potential falls from the spike peak; at a sufficiently low
potential anomalous rectification reverses and conductance rises to the
resting level. This diphasic sequence of conductance changes, increase
followed by decrease, contrasts to the monophasic increase seen in
nerve (Cole and Curtis, 1939; Tasaki and Freygang, 1955).
One might ask why the innervated membrane should not just have
a high resting resistance instead of anomalous rectification. One possible
reason is to provide a conductance in inactive cells for flow of current
generated by active cells in series with them. As will be discussed further
in Section 111, A, the cells are not all active at the same time except in the
largest discharges. The voltage of these responses is accounted for by
synchronous activity of some 6000 cells in series each producing over
100 mV.
b. Gymnotus. This species is weakly electric; its organ discharge is a
fraction of a volt recorded in water and not much more than a volt
recorded in air. Its body shape is similar to that of the eel, but the electric
organ is much smaller (Fig. 1). When undisturbed and resting it normally emits pulses at about 50/sec. The pulses are approximately triphasic, initially head negative, and about 1 msec in duration (Fig. 3B,
B’) . Mechanical stimuli, light, electric fields, and resistance changes can
cause moderate transient accelerations of the discharge, and when the
animal is feeding, the discharge frequency can briefly exceed 200/sec
( Fig. 3, Lissmann, 1958; Bennett and Grundfest, 1959; Black-Cleworth,
1970). When swimming around its tank the fish maintains a fairly steady
frequency somewhat above the resting level. Gymnotus is also capable
of ceasing its discharge completely for brief periods, a response that may
sometimes represent hiding or “listening.” Both accelerations and cessations can be involved in communication between other members of the
same species and a relatively potent stimulus for inducing cessation of
firing is weak electric pulses at a frequency close to that of the organ
discharge.
The electric organ runs longitudinally from just behind the chin to the
tip of the caudal filament. The organ lies dorsal to the anal fin but
extends beyond it rostrally to the cleithrum as well as caudally (Fig. 1).
The organ on one side consists of about four longitudinal columns of