10. ELECTRIC ORGANS
473
In the mormyrids there is not simply a single impulse at each level
of the electric organ control system (Fig. 73). PSPs from what is presumably a single spike in either pacemaker nucleus initiate a two-spike
discharge in the medullary relay (the second spike propagates back to
the dominant pacemaker and both spikes propagate to the subordinate
pacemaker). Firing in “doublets” appears to be a property intrinsic to
the cell membrane rather than a result of a long-lasting PSP because the
cells give a two-spike discharge in response to a brief intracellularly
applied current pulse. The two medullary relay volleys descend the
spinal cord and cause two PSPs in spinal neurons that innervate the
electrocytes. These PSPs are chemically mediated. The first PSP initiates
a three-spike discharge. Firing in triplets appears intrinsic to the cell
membrane of the spinal cells as does the doublet firing of the medullary
cells. However, the third spike is somewhat labile and the second PSP
from the medullary relay guarantees its occurrence. The three electromotor neuron spikes are propagated out to the synapses on the stalks
of the electrocytes, but only a single postsynaptic spike is produced. The
first PSP is very small, the second is greatly facilitated (increased in
amplitude) but still subthreshold, and the third is facilitated sufficiently
to reach threshold. The PSPs are not recorded external to the fish because the synapses are on stalks far from the body of the cells and there
is little or no longitudinal current associated with them. ( A similar
firing pattern could be detected externally if the cells were diffusely
innervated over one face.) The significance of this peculiar sequence
of signal transformations is unknown. It also occurs in Mormyrus and
Mormyrops in some species of which the discharge is longer lasting
and the requirement for synchronization is not so great.
Both medullary and spinal neurons are closely coupled electrotonically, and an impulse in one cell propagates to all the other cells of that
nucleus. (An antidromic impulse in an axon generally fails to invade
the cell body, at least in part because of the low input resistance resulting from the close coupling of the cells.) The neurons are connected
by thick dendrodendritic bridges in both nuclei (Fig. 71), and coupling
by way of presynaptic fibers probably occurs in the medullary relay
as well.
Interpretation of the peculiar multiple firing in the mormyrid control
system is not likely to be aided by comparison to Gymnurchus. Light
microscopy suggests that there are four interconnected nuclei in the
medulla, but they have not been studied physiologically ( Szabo, 1961b ) .
All that is known is that a single volley descends the spinal cord, and
a single PSP in the electrocytes initiates each organ discharge, An interesting aspect of the mormyrid control system is that the command
473
In the mormyrids there is not simply a single impulse at each level
of the electric organ control system (Fig. 73). PSPs from what is presumably a single spike in either pacemaker nucleus initiate a two-spike
discharge in the medullary relay (the second spike propagates back to
the dominant pacemaker and both spikes propagate to the subordinate
pacemaker). Firing in “doublets” appears to be a property intrinsic to
the cell membrane rather than a result of a long-lasting PSP because the
cells give a two-spike discharge in response to a brief intracellularly
applied current pulse. The two medullary relay volleys descend the
spinal cord and cause two PSPs in spinal neurons that innervate the
electrocytes. These PSPs are chemically mediated. The first PSP initiates
a three-spike discharge. Firing in triplets appears intrinsic to the cell
membrane of the spinal cells as does the doublet firing of the medullary
cells. However, the third spike is somewhat labile and the second PSP
from the medullary relay guarantees its occurrence. The three electromotor neuron spikes are propagated out to the synapses on the stalks
of the electrocytes, but only a single postsynaptic spike is produced. The
first PSP is very small, the second is greatly facilitated (increased in
amplitude) but still subthreshold, and the third is facilitated sufficiently
to reach threshold. The PSPs are not recorded external to the fish because the synapses are on stalks far from the body of the cells and there
is little or no longitudinal current associated with them. ( A similar
firing pattern could be detected externally if the cells were diffusely
innervated over one face.) The significance of this peculiar sequence
of signal transformations is unknown. It also occurs in Mormyrus and
Mormyrops in some species of which the discharge is longer lasting
and the requirement for synchronization is not so great.
Both medullary and spinal neurons are closely coupled electrotonically, and an impulse in one cell propagates to all the other cells of that
nucleus. (An antidromic impulse in an axon generally fails to invade
the cell body, at least in part because of the low input resistance resulting from the close coupling of the cells.) The neurons are connected
by thick dendrodendritic bridges in both nuclei (Fig. 71), and coupling
by way of presynaptic fibers probably occurs in the medullary relay
as well.
Interpretation of the peculiar multiple firing in the mormyrid control
system is not likely to be aided by comparison to Gymnurchus. Light
microscopy suggests that there are four interconnected nuclei in the
medulla, but they have not been studied physiologically ( Szabo, 1961b ) .
All that is known is that a single volley descends the spinal cord, and
a single PSP in the electrocytes initiates each organ discharge, An interesting aspect of the mormyrid control system is that the command