482
M. V. L. BENhTETT
mand nuclei. Probably this resetting occurs in the mormyrids by actual
invasion of the impulses, but mutual inhibition could also be a mechanism in other systems. For example, crossed inhibition like that between
Mauthner cells (Furukawa and Furshpan, 1963; A. A. Auerbach, unpublished data) would be equally effectivc in an electric organ system.
In the hatchetfish each Mauthner fiber activates the muscles depressing
both pectoral fins, and these cells thus constitute a bilateral command
system for the depressor muscles ( Auerbach and Bennett, 1969a,b).
Contraction of these muscles causes the animals to dart upward in an
escape reflex. Mutual excitation like that proposed for the mormyrid
pacemaker nuclei would not work for the Mauthner cells, because each
innervates the axial musculature on one side and near simultaneous
excitation could lead to a counter-productive attempt to flip the tail to
both sides simultaneously. Actually there is evidence that in the goldfish
each Mauthner fiber excites motoneurons on one side and inhibits them
on the other side but at a shorter latency (Diamond and Yasargil, 1969).
Simultaneous activity of both Mauthner fibers excites no axial motoneurons. If one Mauthner fiber fires, it causes muscle contraction on
one side and prevents the other Mauthner fiber from exciting the contralateral niotoneurons for about 100 msec.
Recognition of the requirement for speed of transmission in synchronized systems has been useful in predicting sites where electrically
mediated transmission has subsequently been found including neurons
controlling sonic muscles ( Bennett, 1966; Pappas and Bcnnett, 1966)
and oculoinotor neurons (Kriebel et al., 1969). In mediating activity
that is not very precisely synchronized, the speed of electrotonic
synapses might not be required. Examples are the bursting behavior
of respiratory neurons and various invertebrate cardiac ganglia. Electrotonic coupling does occur in a number of moderately synchronized
systems in both vertebrates (cf. Bennett, 1968b) and invertebrates
(Hagiwara et al., 1959, Willows and Hoyle, 1969). The extent to which
synchronization of slow systems is mediated by positive feedback and
the degree to which the positive feedback is mediated electrically remain
to be worked out. Electrotonical transmission appears to provide a
simpler and perhaps more efficient means of positive feedback between
cells (Bennett, 1968b).
One may question whether firing of an electric organ resembles
other decision processes. The final common path concept of a decision
requires extension even in some very simple systems. On the other
hand, it is reasonable to think that the features observed in simple
effector systems havc some more general relevance. Negative feedback
has been emphasized as a common property in neural control systems
M. V. L. BENhTETT
mand nuclei. Probably this resetting occurs in the mormyrids by actual
invasion of the impulses, but mutual inhibition could also be a mechanism in other systems. For example, crossed inhibition like that between
Mauthner cells (Furukawa and Furshpan, 1963; A. A. Auerbach, unpublished data) would be equally effectivc in an electric organ system.
In the hatchetfish each Mauthner fiber activates the muscles depressing
both pectoral fins, and these cells thus constitute a bilateral command
system for the depressor muscles ( Auerbach and Bennett, 1969a,b).
Contraction of these muscles causes the animals to dart upward in an
escape reflex. Mutual excitation like that proposed for the mormyrid
pacemaker nuclei would not work for the Mauthner cells, because each
innervates the axial musculature on one side and near simultaneous
excitation could lead to a counter-productive attempt to flip the tail to
both sides simultaneously. Actually there is evidence that in the goldfish
each Mauthner fiber excites motoneurons on one side and inhibits them
on the other side but at a shorter latency (Diamond and Yasargil, 1969).
Simultaneous activity of both Mauthner fibers excites no axial motoneurons. If one Mauthner fiber fires, it causes muscle contraction on
one side and prevents the other Mauthner fiber from exciting the contralateral niotoneurons for about 100 msec.
Recognition of the requirement for speed of transmission in synchronized systems has been useful in predicting sites where electrically
mediated transmission has subsequently been found including neurons
controlling sonic muscles ( Bennett, 1966; Pappas and Bcnnett, 1966)
and oculoinotor neurons (Kriebel et al., 1969). In mediating activity
that is not very precisely synchronized, the speed of electrotonic
synapses might not be required. Examples are the bursting behavior
of respiratory neurons and various invertebrate cardiac ganglia. Electrotonic coupling does occur in a number of moderately synchronized
systems in both vertebrates (cf. Bennett, 1968b) and invertebrates
(Hagiwara et al., 1959, Willows and Hoyle, 1969). The extent to which
synchronization of slow systems is mediated by positive feedback and
the degree to which the positive feedback is mediated electrically remain
to be worked out. Electrotonical transmission appears to provide a
simpler and perhaps more efficient means of positive feedback between
cells (Bennett, 1968b).
One may question whether firing of an electric organ resembles
other decision processes. The final common path concept of a decision
requires extension even in some very simple systems. On the other
hand, it is reasonable to think that the features observed in simple
effector systems havc some more general relevance. Negative feedback
has been emphasized as a common property in neural control systems