10. ELECTRIC ORGANS
461
potential propagates along a single membrane. Still separate cells are
always separately innervated. One exception to the rule of central control
of organ discharge is known, the chin organ of Adontosternarchus. Since
this organ consists of a single layer of parallel elements, the spontaneous
activity of the cells tends to be synchronized and the discharge frequency
is set in the organ itself without central control (see Section 11, D, 1, g).
As might be expected, this organ often operates at a somewhat different
frequency from the main organ.
Control of electric organ discharge can be divided into two problems,
how the fish “decides” to discharge its organ, and, having reached the
decision, how it activates the different generating elements synchronously. The “decision” to discharge the organ is reached, depending on
the kind of fish, by a small group of neurons in the higher spinal cord,
medulla, or perhaps midbrain ( Bennett et al., 1967a,b,c; Bennett, 1968a).
These cells make up the “command nucleus,” and when they fire synchronously, the “command signal” to discharge the organ is initiated.
This activity is then transmitted to the electrocytes either directly or
through one or more neural relays.
Neurons of the command nucleus are probably spontaneously active
in the continually discharging forms; that is, they are pacemaker neurons
in a manner analogous to pacemaker cells in the heart. In species that
discharge only intermittently, the command neurons receive excitatory
inputs and perhaps inhibitory inputs as well. When these inputs reach
threshold, the neurons “decide” that the organ will be discharged. A very
significant feature of the pacemaker or command cells is that they are
coupled to each other by means of “electrotonic synapses.” The coupling
is the basis of the highly synchronous firing that is observed in command
neurons of the electric organ systems. The electrotonic synapses provide
resistive pathways between cells that for this reason behave as if they are
part of the same core conductor; potentials spread between the cells in the
same way as they spread electrotonically along an axon. Two important
properties of electrotonic synapses are relevant to their functioning in
electric organ control. Current can flow in either direction across the
synapses, and current begins to flow without delay when pre- and postsynaptic potentials differ. These properties differentiate electrotonic
synapses from chemically transmitting synapses in which transmission is
basically in one direction and postsynaptic current is delayed with respect
to presynaptic impulses (by about ?h msec at room temperature). Electrotonically mediated PSPs are delayed with respect to presynaptic potentials because of the capacity of the postsynaptic cells, but generally the
delay is very short compared to the delay at chemically transmitting
synapses (Bennett, 1966).
The electrotonic synapses mediate rapid-acting positive feedback be-
461
potential propagates along a single membrane. Still separate cells are
always separately innervated. One exception to the rule of central control
of organ discharge is known, the chin organ of Adontosternarchus. Since
this organ consists of a single layer of parallel elements, the spontaneous
activity of the cells tends to be synchronized and the discharge frequency
is set in the organ itself without central control (see Section 11, D, 1, g).
As might be expected, this organ often operates at a somewhat different
frequency from the main organ.
Control of electric organ discharge can be divided into two problems,
how the fish “decides” to discharge its organ, and, having reached the
decision, how it activates the different generating elements synchronously. The “decision” to discharge the organ is reached, depending on
the kind of fish, by a small group of neurons in the higher spinal cord,
medulla, or perhaps midbrain ( Bennett et al., 1967a,b,c; Bennett, 1968a).
These cells make up the “command nucleus,” and when they fire synchronously, the “command signal” to discharge the organ is initiated.
This activity is then transmitted to the electrocytes either directly or
through one or more neural relays.
Neurons of the command nucleus are probably spontaneously active
in the continually discharging forms; that is, they are pacemaker neurons
in a manner analogous to pacemaker cells in the heart. In species that
discharge only intermittently, the command neurons receive excitatory
inputs and perhaps inhibitory inputs as well. When these inputs reach
threshold, the neurons “decide” that the organ will be discharged. A very
significant feature of the pacemaker or command cells is that they are
coupled to each other by means of “electrotonic synapses.” The coupling
is the basis of the highly synchronous firing that is observed in command
neurons of the electric organ systems. The electrotonic synapses provide
resistive pathways between cells that for this reason behave as if they are
part of the same core conductor; potentials spread between the cells in the
same way as they spread electrotonically along an axon. Two important
properties of electrotonic synapses are relevant to their functioning in
electric organ control. Current can flow in either direction across the
synapses, and current begins to flow without delay when pre- and postsynaptic potentials differ. These properties differentiate electrotonic
synapses from chemically transmitting synapses in which transmission is
basically in one direction and postsynaptic current is delayed with respect
to presynaptic impulses (by about ?h msec at room temperature). Electrotonically mediated PSPs are delayed with respect to presynaptic potentials because of the capacity of the postsynaptic cells, but generally the
delay is very short compared to the delay at chemically transmitting
synapses (Bennett, 1966).
The electrotonic synapses mediate rapid-acting positive feedback be-