M. V. L. BENNETT
478
time down the main trunk of the axon (A,A'). The significance of the
synaptic delay can be well illustrated by recording the output of one
entire side of the organ when stimulating the nerve at different sites.
If the nerve is stimulated at its caudal end rather than its site of entry
into the organ, the differences in delay add to the differences in conduction time and a markedly asynchronous discharge results ( Fig. 78C').
The site of these compensatory delays is unknown; it could be in
presynaptic nerve branches and terminals or at the synapses themselves.
In the electric catfish it could also be in the stalks of electrocytes. However, no systematic differences in length of stalks have been observed
in teased preparations, and the variability of stalk length is quite great
in all parts of the organ. In both mormyrids and the electric catfish,
morphological investigations may reveal that terminals are longer and
thinner where delays are greater. In this case one would be less inclined
to consider synaptic delay as a factor in the compensatory mechanism.
C. Organization of Electromotor Systems
As discussed here the control of electric organs is reasonably well
described from one to several neural levels back into the nervous
system. Provided one can accept the proposition that the pacemaker
cells are autoactive, the analysis is reasonably complete for the very
regular basal discharge frequency found in many species. Certainly one
would like to know the ionic basis of the pacemaker activity, but in
respect to impulse traffic, the origins and pathways are well defined.
The pathways of afferents to the command nuclei in variable frequency
and intermittently discharging forms are by and large unknown, although most if not all sensory modalities can be excitatory and responses
can be of quite short latency (e.g., Fig. 3B). The slight modifications
of frequency in some high and constant frequency species also require
investigation. The relative fixity of jamming avoidance responses suggests
fairly direct electrosensory inputs, while the brief modulations that
perhaps have a signaling function probably involve much more complex
pathways (Bullock, 1970). The involvement of higher centers is also
indicated by the fact that changes in organ discharge rate can be
operantly and respondently conditioned ( Mandriota et al., 1965, 1968)
as well as serve for signaling ( Black-Cleworth, 1970; Moller, 1971).
An interesting problem is raised by the ability of Gymnorhamphichthys
and some Hypopomus to maintain organ discharge frequency constant
at low, high, and even intermediate levels. This ability suggests that
there are neurons that maintain a tonic, asynchronous but quite constant
478
time down the main trunk of the axon (A,A'). The significance of the
synaptic delay can be well illustrated by recording the output of one
entire side of the organ when stimulating the nerve at different sites.
If the nerve is stimulated at its caudal end rather than its site of entry
into the organ, the differences in delay add to the differences in conduction time and a markedly asynchronous discharge results ( Fig. 78C').
The site of these compensatory delays is unknown; it could be in
presynaptic nerve branches and terminals or at the synapses themselves.
In the electric catfish it could also be in the stalks of electrocytes. However, no systematic differences in length of stalks have been observed
in teased preparations, and the variability of stalk length is quite great
in all parts of the organ. In both mormyrids and the electric catfish,
morphological investigations may reveal that terminals are longer and
thinner where delays are greater. In this case one would be less inclined
to consider synaptic delay as a factor in the compensatory mechanism.
C. Organization of Electromotor Systems
As discussed here the control of electric organs is reasonably well
described from one to several neural levels back into the nervous
system. Provided one can accept the proposition that the pacemaker
cells are autoactive, the analysis is reasonably complete for the very
regular basal discharge frequency found in many species. Certainly one
would like to know the ionic basis of the pacemaker activity, but in
respect to impulse traffic, the origins and pathways are well defined.
The pathways of afferents to the command nuclei in variable frequency
and intermittently discharging forms are by and large unknown, although most if not all sensory modalities can be excitatory and responses
can be of quite short latency (e.g., Fig. 3B). The slight modifications
of frequency in some high and constant frequency species also require
investigation. The relative fixity of jamming avoidance responses suggests
fairly direct electrosensory inputs, while the brief modulations that
perhaps have a signaling function probably involve much more complex
pathways (Bullock, 1970). The involvement of higher centers is also
indicated by the fact that changes in organ discharge rate can be
operantly and respondently conditioned ( Mandriota et al., 1965, 1968)
as well as serve for signaling ( Black-Cleworth, 1970; Moller, 1971).
An interesting problem is raised by the ability of Gymnorhamphichthys
and some Hypopomus to maintain organ discharge frequency constant
at low, high, and even intermediate levels. This ability suggests that
there are neurons that maintain a tonic, asynchronous but quite constant