M. V. L. BENNETT
472
well synchronized. The oculomotor neurons do not simply relay the
signal; they also “decide” whether or not a sufficiently large command
volley is present to call for a discharge, and this decision is not always
unanimous in spite of the coupling between cells.
Often firing of the command neurons is in pairs of spikes, and PSPs
in the oculomotor neurons show two components. Apparently, a few
oculomotor neurons fire twice in normal activity and give rise to the
delayed component on the falling phase of some discharges (Fig. 6 ) .
4. RAJrnS
In the skate the electric organ is probably controlled by a midline
nucleus in the medulla since stimuli applied in this region evoke organ
discharge (Szabo, 1955, 1961b). There is no evidence as to whether this
nucleus is a relay or pacemaker, or whether the neurons are electrotonically coupled. This is a particularly interesting example because the
discharge is repetitive and probably asynchronous. While it seems likely
that positive feedback between neurons is involved in the decision to
discharge the organ, the speed of electrical transmission is apparently
not required. If feedback is still mediated electrically, it would suggest
that electrical coupling may be involved in other slow systems as well.
5. MORMYRIDS
In the mormyrids control of organ discharge is more complex than in
the fish discussed up to this point (Bennett et al., 1967a; Bennett, 1968a).
There is some evidence that the pacemaker nucleus is bilateral and that
and organ discharge can be initiated by a command signal arising on
either side (Fig. 73). This activity is relayed through a midline nucleus
in the medulla to the spinal neurons and then to the electric organ.
Although each pacemaker nucleus is able to initiate a command volley
independently, either because it is spontaneously active or because it
receives tonic excitatory inputs, pacemaker activity on the two sides is
coordinated. An impulse arising in what may be considered the dominant
nucleus at that time propagates from the medullary relay “antidromically” into the other pacemaker nucleus, the subordinate one. (Transmission at the pacemaker synapses is apparently electrotonic and allows
“antidromic” propagation. ) Thus both nuclei are excited, and the pacemaker potential of both nuclei is returned to the level of hyperpolarization that immediately follows spike activity; pacemaking in both nuclei
is reset. The cells of the two nuclei then begin to depolarize again, but
those in the dominant nucleus reach the firing level first, again resetting
the activity of both nuclei.
472
well synchronized. The oculomotor neurons do not simply relay the
signal; they also “decide” whether or not a sufficiently large command
volley is present to call for a discharge, and this decision is not always
unanimous in spite of the coupling between cells.
Often firing of the command neurons is in pairs of spikes, and PSPs
in the oculomotor neurons show two components. Apparently, a few
oculomotor neurons fire twice in normal activity and give rise to the
delayed component on the falling phase of some discharges (Fig. 6 ) .
4. RAJrnS
In the skate the electric organ is probably controlled by a midline
nucleus in the medulla since stimuli applied in this region evoke organ
discharge (Szabo, 1955, 1961b). There is no evidence as to whether this
nucleus is a relay or pacemaker, or whether the neurons are electrotonically coupled. This is a particularly interesting example because the
discharge is repetitive and probably asynchronous. While it seems likely
that positive feedback between neurons is involved in the decision to
discharge the organ, the speed of electrical transmission is apparently
not required. If feedback is still mediated electrically, it would suggest
that electrical coupling may be involved in other slow systems as well.
5. MORMYRIDS
In the mormyrids control of organ discharge is more complex than in
the fish discussed up to this point (Bennett et al., 1967a; Bennett, 1968a).
There is some evidence that the pacemaker nucleus is bilateral and that
and organ discharge can be initiated by a command signal arising on
either side (Fig. 73). This activity is relayed through a midline nucleus
in the medulla to the spinal neurons and then to the electric organ.
Although each pacemaker nucleus is able to initiate a command volley
independently, either because it is spontaneously active or because it
receives tonic excitatory inputs, pacemaker activity on the two sides is
coordinated. An impulse arising in what may be considered the dominant
nucleus at that time propagates from the medullary relay “antidromically” into the other pacemaker nucleus, the subordinate one. (Transmission at the pacemaker synapses is apparently electrotonic and allows
“antidromic” propagation. ) Thus both nuclei are excited, and the pacemaker potential of both nuclei is returned to the level of hyperpolarization that immediately follows spike activity; pacemaking in both nuclei
is reset. The cells of the two nuclei then begin to depolarize again, but
those in the dominant nucleus reach the firing level first, again resetting
the activity of both nuclei.