10. ELECTRIC ORGANS
471
from one cell to another. As shown in Fig. 76A,B an impulse in one
pacemaker cell does not cause a descending volley in the spinal cord;
it only advances the phase of firing. Thus, there is some spread of depolarization to the other cells but not a propagated impulse (unless the
cells are very near to firing). The coupling is sufficiently close to synchronize the cells, and, when all the cells are firing in the vicinity of a
cell penetrated by a microelectrode, it is extremely difficult to block
that cell’s activity by hyperpolarizing current. While a spike in any one
cell affects its neighbors but little, many cells firing together can produce
a large and very suprathreshold depolarization in an inactive cell.
As it happens, the medullary relay neurons are also electrotonically
coupled in most gymnotids that have been studied. Coupling is by way
of the pacemaker fibers afferent to them. At this level the coupling does
not serve to keep the cells from firing out of phase, for each cell is
always excited once per organ discharge by the large descending volley
from the pacemaker nucleus. Presumably the coupling serves to synchronize relay cell firing, either because asynchrony was present in the
initial pacemaker volley or because it has arisen in transmission from the
pacemaker nucleus. The spinal relay neurons have been adequately
studied only in the electric eel (Bennett et al., 1964; Bennett, 1968a).
These cells, too, are electrotonically coupled and by way of the descending fibers ending on them (Pappas and Bennett, 1 W ) . Preliminary
experiments indicate that the spinal relay neurons are similarly organized
in Gymnotus. Again, coupling would tend to increase synchronizatio?.
Sherrington (1906) called the motoneuron the final common path for
muscle fiber activity. In the gymnotids the final common path for
electric organ discharge extends three neurons back into the nervous
system. Each level involves a number of neurons and a certain amount
of signal shaping may go on in the relays, but these are only small
extensions of the Sherrington concept.
3. Asmoscomrs
In the stargazer the command signal is initiated in what is probably
a midline nucleus in the medulla and is relayed in the very large oculomotor nucleus (Bennett, 1968a, and unpublished data). Neurons are
electrotonically coupled in both command and relay nuclei (Fig. 70).
In the relay both dendrodendritic synapses and the presynaptic fibers
mediate coupling. The organ discharges are somewhat variable in size
and apparently do not always involve every oculomotor neuron (Fig. 6).
The experimental evidence indicates that the command nucleus is not
471
from one cell to another. As shown in Fig. 76A,B an impulse in one
pacemaker cell does not cause a descending volley in the spinal cord;
it only advances the phase of firing. Thus, there is some spread of depolarization to the other cells but not a propagated impulse (unless the
cells are very near to firing). The coupling is sufficiently close to synchronize the cells, and, when all the cells are firing in the vicinity of a
cell penetrated by a microelectrode, it is extremely difficult to block
that cell’s activity by hyperpolarizing current. While a spike in any one
cell affects its neighbors but little, many cells firing together can produce
a large and very suprathreshold depolarization in an inactive cell.
As it happens, the medullary relay neurons are also electrotonically
coupled in most gymnotids that have been studied. Coupling is by way
of the pacemaker fibers afferent to them. At this level the coupling does
not serve to keep the cells from firing out of phase, for each cell is
always excited once per organ discharge by the large descending volley
from the pacemaker nucleus. Presumably the coupling serves to synchronize relay cell firing, either because asynchrony was present in the
initial pacemaker volley or because it has arisen in transmission from the
pacemaker nucleus. The spinal relay neurons have been adequately
studied only in the electric eel (Bennett et al., 1964; Bennett, 1968a).
These cells, too, are electrotonically coupled and by way of the descending fibers ending on them (Pappas and Bennett, 1 W ) . Preliminary
experiments indicate that the spinal relay neurons are similarly organized
in Gymnotus. Again, coupling would tend to increase synchronizatio?.
Sherrington (1906) called the motoneuron the final common path for
muscle fiber activity. In the gymnotids the final common path for
electric organ discharge extends three neurons back into the nervous
system. Each level involves a number of neurons and a certain amount
of signal shaping may go on in the relays, but these are only small
extensions of the Sherrington concept.
3. Asmoscomrs
In the stargazer the command signal is initiated in what is probably
a midline nucleus in the medulla and is relayed in the very large oculomotor nucleus (Bennett, 1968a, and unpublished data). Neurons are
electrotonically coupled in both command and relay nuclei (Fig. 70).
In the relay both dendrodendritic synapses and the presynaptic fibers
mediate coupling. The organ discharges are somewhat variable in size
and apparently do not always involve every oculomotor neuron (Fig. 6).
The experimental evidence indicates that the command nucleus is not