10. ELECTRIC ORGANS
467
requires less than 0.2 msec, which is more rapid than could be achieved
by chemically mediated transmission ( Fig. 74A). [The delay is greater
when one cell is directly stimulated, for the unstimulated cell is much
less depolarized when the impulse arises ( Fig. 74B) .]
2. WEAKLY ELECTRIC GYMNOTIDS
In weakly electric gymnotids the command system has more than
one neural level (Bennett et al., 1967~). The organ frequency is set by
what has been termed the pacemaker nucleus, which is in the medulla.
(From a functional point of view it would be difficult for synchronous
activity to arise within a group of neurons that are as widely separated
as the spinal neurons innervating the electrocytes. ) The pacemaker
nucleus lies in the midline and contains some 30 to 200 cells depending
on the species. It activates a relay nucleus also in the midline of the
medulla, and this nucleus in turn activates the spinal neurons. There are
50 or so medullary relay neurons and some hundreds to thousands of
spinal neurons. A single spike occurs at each level before each organ
pulse (Fig. 75). In the pacemaker neurons there is a gradually rising
depolarization between spikes (Fig. 75A,C). This depolarization is
similar to pacemaker potentials in other tissues, and the cells appear to
be spontaneously active. In the relay cells the potential between spikes
is quite flat and the discharges arise abruptly from a level base line (Fig.
75B,D). These cells are clearly activated from a higher level which is in
fact the pacemaker nucleus.
Fig. 72. Morphological basis of electrotonic coupling: axosomatic and axodendritic electrotonic synapses on giant electromotor neurons of the electric catfish.
( A ) Toluidine blue stained thick section that passes through the nuclei of the
two giant cells. The central canal (arrow) is just ventral to the cells. A number
of small dendrites come off the somata, but there is no apparent direct connection
between the cells. ( B ) Electron micrograph of two axosomatic synapses (a,a’) on
the cell. At the upper one ( a ) the myelin sheath is seen to terminate in the plane
of section. There is a region of close apposition of axon and soma membranes
at this synapse (between arrows), but probably not a t the other one. Terminal
a’ contains many vesicles, but there are relatively few in a. The former may be
one of the chemically transmitting inhibitory synapses that occur on these cells.
Near these endings there is a relatively large amount of extracellular space ( e )
filled with granular material. ( C ) Higher magnification of an axodendritic synapse.
The axonal side is to the left and a presynaptic vesicle ( v ) is seen in it. A central
dark line is formed at the close membrane apposition extending between the
arrows. ( D ) An axodendritic synapse like that in C, but the close membrane
apposition is cut more tangentially and striations appear with a periodicity of
about 100A. ( E ) Diagram of current flow where cells are coupled by way of
presynaptic fibers. The cell on the left is more depolarized. All sections of osmic
acid fixed material. Modified from Bennett et al. (1967b).
467
requires less than 0.2 msec, which is more rapid than could be achieved
by chemically mediated transmission ( Fig. 74A). [The delay is greater
when one cell is directly stimulated, for the unstimulated cell is much
less depolarized when the impulse arises ( Fig. 74B) .]
2. WEAKLY ELECTRIC GYMNOTIDS
In weakly electric gymnotids the command system has more than
one neural level (Bennett et al., 1967~). The organ frequency is set by
what has been termed the pacemaker nucleus, which is in the medulla.
(From a functional point of view it would be difficult for synchronous
activity to arise within a group of neurons that are as widely separated
as the spinal neurons innervating the electrocytes. ) The pacemaker
nucleus lies in the midline and contains some 30 to 200 cells depending
on the species. It activates a relay nucleus also in the midline of the
medulla, and this nucleus in turn activates the spinal neurons. There are
50 or so medullary relay neurons and some hundreds to thousands of
spinal neurons. A single spike occurs at each level before each organ
pulse (Fig. 75). In the pacemaker neurons there is a gradually rising
depolarization between spikes (Fig. 75A,C). This depolarization is
similar to pacemaker potentials in other tissues, and the cells appear to
be spontaneously active. In the relay cells the potential between spikes
is quite flat and the discharges arise abruptly from a level base line (Fig.
75B,D). These cells are clearly activated from a higher level which is in
fact the pacemaker nucleus.
Fig. 72. Morphological basis of electrotonic coupling: axosomatic and axodendritic electrotonic synapses on giant electromotor neurons of the electric catfish.
( A ) Toluidine blue stained thick section that passes through the nuclei of the
two giant cells. The central canal (arrow) is just ventral to the cells. A number
of small dendrites come off the somata, but there is no apparent direct connection
between the cells. ( B ) Electron micrograph of two axosomatic synapses (a,a’) on
the cell. At the upper one ( a ) the myelin sheath is seen to terminate in the plane
of section. There is a region of close apposition of axon and soma membranes
at this synapse (between arrows), but probably not a t the other one. Terminal
a’ contains many vesicles, but there are relatively few in a. The former may be
one of the chemically transmitting inhibitory synapses that occur on these cells.
Near these endings there is a relatively large amount of extracellular space ( e )
filled with granular material. ( C ) Higher magnification of an axodendritic synapse.
The axonal side is to the left and a presynaptic vesicle ( v ) is seen in it. A central
dark line is formed at the close membrane apposition extending between the
arrows. ( D ) An axodendritic synapse like that in C, but the close membrane
apposition is cut more tangentially and striations appear with a periodicity of
about 100A. ( E ) Diagram of current flow where cells are coupled by way of
presynaptic fibers. The cell on the left is more depolarized. All sections of osmic
acid fixed material. Modified from Bennett et al. (1967b).