10. ELECTRIC ORGANS
419
caudally running section of the fiber. There are first moderately enlarged
nodes, then narrowed ones, then very enlarged ones.
The organ discharge of Sternarchus consists of diphasic pulses that
are initially head positive (Figs. 3D and 45). As would be expected from
the neural origin of the electric organ, the discharge is unaffected by a
dose of curare that is sufficient to paralyze the animal completely.
The contribution of the single cells to the organ discharge is disclosed
by microelectrode experiments. Just anterior to where the fiber enters
the organ, a large impulse is recorded in the fiber during the headpositive phase of organ discharge (Fig. 46A). This impulse is smaller
more anteriorly and the peak is delayed; thus, like the cells of Sternopygus, these cells are not isopotential. The response in this part of the fiber
accounts for the head-positive phase of the discharge because current
runs anteriorly along the interior of the cell at this time. In the
posteriorly running section of the fiber the impulse is larger anteriorly
(Fig. 46B). This activity occurs during the head-negative phase of the
organ discharge and current flows in the cell in such a way as to give
rise to the head-negative phase. The anatomy and recordings indicate
that impulses coming from the spinal cord propagate along the electrocyte in the organ and that the activity of the anteriorly running part
excites the posteriorly running part. Because the impulse is smaller at
the distal end of each section of the fiber, it is likely that these regions
are inexcitable like the uninnervated faces of eel electrocytes. The
current paths and intracellular potentials during one discharge cycle are
diagrammed in Fig. 47.
There is evidence that the inactive regions of the cells act as a series
capacity rather than as a series resistance. The effects when one face of an
electrocyte acts as a capacity are discussed in detail in respect to
Gymnarchus, for which the experimental evidence is more complete
(Section 11, D, 3 ) . The most important effect is that the organ discharge
Fig. 43. Electric organ of Sternarchus. Formalin fixed material, dissected and
stained with methylene blue. Dorsal to the top, anterior to the right. ( A ) The
electric organ ( 0 ) following removal of its connective tissue sheath. Single longitudinally running electrocytes are visible. Nerves to the organ ( n ) and to more ventral
tissues ( s ) are also seen. (B) Following dissection away of all electrocytes from
nerves caudal to those shown and of most fibers entering the organ from the more
caudal nerve in the figure. Three descending fibers of this nerve run approximately
parallel in their origin position (arrows); two others are deflected anteriorly; the
central part of a sixth is broken off shortly after entry into the organ (arrow). The
more anterior nerve is undissected; its fibers diverge in the organ. Most of them lie
medial to the fibers from the more caudal nerve. ( C ) Higher magnification of the
left part of B. The five numbered fibers can be traced from the top left to the lower
right. The smaller fibers of the ventral segmental nerve ( s ) are visible. Fiber 4
passes medial to this nerve. Calibrations on the lower right.
419
caudally running section of the fiber. There are first moderately enlarged
nodes, then narrowed ones, then very enlarged ones.
The organ discharge of Sternarchus consists of diphasic pulses that
are initially head positive (Figs. 3D and 45). As would be expected from
the neural origin of the electric organ, the discharge is unaffected by a
dose of curare that is sufficient to paralyze the animal completely.
The contribution of the single cells to the organ discharge is disclosed
by microelectrode experiments. Just anterior to where the fiber enters
the organ, a large impulse is recorded in the fiber during the headpositive phase of organ discharge (Fig. 46A). This impulse is smaller
more anteriorly and the peak is delayed; thus, like the cells of Sternopygus, these cells are not isopotential. The response in this part of the fiber
accounts for the head-positive phase of the discharge because current
runs anteriorly along the interior of the cell at this time. In the
posteriorly running section of the fiber the impulse is larger anteriorly
(Fig. 46B). This activity occurs during the head-negative phase of the
organ discharge and current flows in the cell in such a way as to give
rise to the head-negative phase. The anatomy and recordings indicate
that impulses coming from the spinal cord propagate along the electrocyte in the organ and that the activity of the anteriorly running part
excites the posteriorly running part. Because the impulse is smaller at
the distal end of each section of the fiber, it is likely that these regions
are inexcitable like the uninnervated faces of eel electrocytes. The
current paths and intracellular potentials during one discharge cycle are
diagrammed in Fig. 47.
There is evidence that the inactive regions of the cells act as a series
capacity rather than as a series resistance. The effects when one face of an
electrocyte acts as a capacity are discussed in detail in respect to
Gymnarchus, for which the experimental evidence is more complete
(Section 11, D, 3 ) . The most important effect is that the organ discharge
Fig. 43. Electric organ of Sternarchus. Formalin fixed material, dissected and
stained with methylene blue. Dorsal to the top, anterior to the right. ( A ) The
electric organ ( 0 ) following removal of its connective tissue sheath. Single longitudinally running electrocytes are visible. Nerves to the organ ( n ) and to more ventral
tissues ( s ) are also seen. (B) Following dissection away of all electrocytes from
nerves caudal to those shown and of most fibers entering the organ from the more
caudal nerve in the figure. Three descending fibers of this nerve run approximately
parallel in their origin position (arrows); two others are deflected anteriorly; the
central part of a sixth is broken off shortly after entry into the organ (arrow). The
more anterior nerve is undissected; its fibers diverge in the organ. Most of them lie
medial to the fibers from the more caudal nerve. ( C ) Higher magnification of the
left part of B. The five numbered fibers can be traced from the top left to the lower
right. The smaller fibers of the ventral segmental nerve ( s ) are visible. Fiber 4
passes medial to this nerve. Calibrations on the lower right.