10. ELECTRIC ORGANS
475
milliseconds’ increase in the minimum latency at which the main organ
can be activated. Probably the control system has evolved its dual
functioning from an earlier stage in which there was only a single
weakly electric organ. An intriguing problem is how pacemaker activity
for the two organs is controlled.
B. Synchronization of Electrocyte Activity
Given that the command nucleus has reached a decision to fire the
electric organ, that is, has itself fired, the individual electrocytes must
be excited synchronously. This is a significant problem because the
different parts of the electric organ may be quite far apart. Nerve
tracts running directly to the different parts of the organ could not
fire the cells synchronously unless they were much more rapidly conducting than any known nerves.
Two basic mechanisms are known to contribute to synchronization;
both involve utilization of conduction time in nerve fibers or electrocyte
stalks to equalize overall latency of the command signal in reaching
the electrocytes. In one mechanism the nerve fibers run more or less
directly to the more distant part of the organ, but take a more devious
path to the nearer parts, thus tending to equalize path length and
thereby conduction time (Fig. 77A). In the second mechanism shorter
paths to nearer parts of the electric organ are of lower conduction
velocity, again tending to equalize conduction time (Fig. 77B; fibers
conducting more slowly are indicated as being of smaller diameter).
Lower conduction velocity may also be found in only a portion of the
path to the electrocytes; slowly conducting collaterals may branch off
from the main rapidly conducting path to innervate the nearer parts
of the organ (Fig. 77C).
Equalization of path length is an obvious feature of a number of
electric organs where the different nerves enter the electric organ and
run for some distance before giving off branches that return to end
near the point of entry. Compensatory differences in conduction velocity
apparently occur in the stalk system of mormyrid electrocytes where
differences in conduction distance are easily visualized and where
synchronization between parts of the body of the cell is very precise
(Fig. 61). As would be expected, the shorter paths involve stalks that
are smaller in diameter. Probably most systems use a combination of
the two mechanisms.
In the electric eel differences in conduction time down the spinal
cord are compensated for by both increased delay at the spinal relays
475
milliseconds’ increase in the minimum latency at which the main organ
can be activated. Probably the control system has evolved its dual
functioning from an earlier stage in which there was only a single
weakly electric organ. An intriguing problem is how pacemaker activity
for the two organs is controlled.
B. Synchronization of Electrocyte Activity
Given that the command nucleus has reached a decision to fire the
electric organ, that is, has itself fired, the individual electrocytes must
be excited synchronously. This is a significant problem because the
different parts of the electric organ may be quite far apart. Nerve
tracts running directly to the different parts of the organ could not
fire the cells synchronously unless they were much more rapidly conducting than any known nerves.
Two basic mechanisms are known to contribute to synchronization;
both involve utilization of conduction time in nerve fibers or electrocyte
stalks to equalize overall latency of the command signal in reaching
the electrocytes. In one mechanism the nerve fibers run more or less
directly to the more distant part of the organ, but take a more devious
path to the nearer parts, thus tending to equalize path length and
thereby conduction time (Fig. 77A). In the second mechanism shorter
paths to nearer parts of the electric organ are of lower conduction
velocity, again tending to equalize conduction time (Fig. 77B; fibers
conducting more slowly are indicated as being of smaller diameter).
Lower conduction velocity may also be found in only a portion of the
path to the electrocytes; slowly conducting collaterals may branch off
from the main rapidly conducting path to innervate the nearer parts
of the organ (Fig. 77C).
Equalization of path length is an obvious feature of a number of
electric organs where the different nerves enter the electric organ and
run for some distance before giving off branches that return to end
near the point of entry. Compensatory differences in conduction velocity
apparently occur in the stalk system of mormyrid electrocytes where
differences in conduction distance are easily visualized and where
synchronization between parts of the body of the cell is very precise
(Fig. 61). As would be expected, the shorter paths involve stalks that
are smaller in diameter. Probably most systems use a combination of
the two mechanisms.
In the electric eel differences in conduction time down the spinal
cord are compensated for by both increased delay at the spinal relays