10. ELECTRIC ORGANS
479
excitatory input to the pacemaker nucleus. This input seems too constant
to be due to casual stimulation of receptors resulting from movement,
and an activating system that is itself quite stable would seem to be
required to explain the data.
Pathways mediating the complete inhibition of discharge in Gymnotus, Hypopomus, and Sternopygus are also not worked out. The strong
excitatory drive to the medullary relay in Gymnotus and probably also
Hypopomus suggests that this nucleus is not involved. Probably the
inhibition operates on the pacemaker nucleus. If so, it might be considered surprising that in Gymnotus the discharge always starts and
stops abruptly with only a small degree of slowing below the basal
rate before and after complete cessation. However, similar observations
have been made during microelectrode recording from the pacemaker
nucleus in which activity at this level did indeed cease when discharge
ceased and restarted when renewed discharge was evoked by spinal
stimulation. These sudden changes in frequency are consistent with
calculations from the Hodgkin-Huxley equations, which predict that
maintained firing of the squid axon in response to a steady current
can occur only at rather high frequencies ( Stein, 1967).
Unlike Gymnotus, Sternopygus does not start up again by emitting
full-sized organ pulses; there is a gradual recovery of pulse amplitude
and synchronization. While this observation might represent a difference
in the pacemaker nucleus, it might also result from properties of lower
level synapses. If the spinal cord is sectioned in Gymnotus a single brief
stimulus to the cord will still cause a full-sized organ discharge. However, in Sternopygus moderately prolonged stimulation at about the
normal organ frequency is required to restore the discharges to their
full amplitude. Granted that transmission at the spinal relays is electrotonic, the most likely site of facilitation in Sternopygus is at the
nerve-electrocyte synapses. The absence of cessation of discharge in
the higher frequency species Eigenmannia and sternarchids is perhaps
a result of the fact that discharge does not stop following spinal section
but continues asynchronously.
The investigations to date have been largely restricted to elucidation
of what may be considcred the final common path for organ discharge.
A number of modifications to the original concept of final common
path appear necessary to describe control of electric organs. A group
of neurons at a particular level can fire as a single neuron, and the
common path may involve several levels of neurons. The highest level
determines the frequency of discharge; the lower levels act only as
relays and perhaps also do some signal shaping (or as in the mormyrids,
transformation of impulse number). The command path may bifurcate
479
excitatory input to the pacemaker nucleus. This input seems too constant
to be due to casual stimulation of receptors resulting from movement,
and an activating system that is itself quite stable would seem to be
required to explain the data.
Pathways mediating the complete inhibition of discharge in Gymnotus, Hypopomus, and Sternopygus are also not worked out. The strong
excitatory drive to the medullary relay in Gymnotus and probably also
Hypopomus suggests that this nucleus is not involved. Probably the
inhibition operates on the pacemaker nucleus. If so, it might be considered surprising that in Gymnotus the discharge always starts and
stops abruptly with only a small degree of slowing below the basal
rate before and after complete cessation. However, similar observations
have been made during microelectrode recording from the pacemaker
nucleus in which activity at this level did indeed cease when discharge
ceased and restarted when renewed discharge was evoked by spinal
stimulation. These sudden changes in frequency are consistent with
calculations from the Hodgkin-Huxley equations, which predict that
maintained firing of the squid axon in response to a steady current
can occur only at rather high frequencies ( Stein, 1967).
Unlike Gymnotus, Sternopygus does not start up again by emitting
full-sized organ pulses; there is a gradual recovery of pulse amplitude
and synchronization. While this observation might represent a difference
in the pacemaker nucleus, it might also result from properties of lower
level synapses. If the spinal cord is sectioned in Gymnotus a single brief
stimulus to the cord will still cause a full-sized organ discharge. However, in Sternopygus moderately prolonged stimulation at about the
normal organ frequency is required to restore the discharges to their
full amplitude. Granted that transmission at the spinal relays is electrotonic, the most likely site of facilitation in Sternopygus is at the
nerve-electrocyte synapses. The absence of cessation of discharge in
the higher frequency species Eigenmannia and sternarchids is perhaps
a result of the fact that discharge does not stop following spinal section
but continues asynchronously.
The investigations to date have been largely restricted to elucidation
of what may be considcred the final common path for organ discharge.
A number of modifications to the original concept of final common
path appear necessary to describe control of electric organs. A group
of neurons at a particular level can fire as a single neuron, and the
common path may involve several levels of neurons. The highest level
determines the frequency of discharge; the lower levels act only as
relays and perhaps also do some signal shaping (or as in the mormyrids,
transformation of impulse number). The command path may bifurcate