M. V. L. BENNETT
392
drum-shaped cells, the flat faces of which are oriented anteroposteriorly
(Fig. 19). The cells are about a millimeter in diameter and 300p thick
in a fish 20 cm long. The number of columns is reduced anteriorly, and
there may be several additional columns caudally. In each column there
is one electrocyte per segment and there are about 90 segments in the
animal. Each column of cells is enclosed in a connective tissue tube that
is divided into chambers by loose septa between the cells. The cells of
all but the most dorsal column are innervated by a number of fibers on
their posterior faces. Aside from innervation the two faces are very
similar. They are quite smooth with relatively few inpocketings and
canaliculi (Schwartz et al., 1971). The cells of the most dorsal column
have their main innervation on the anterior face, but they have a few
fibers ending on their posterior faces as well (Szabo, 1961d). As will be
shown below, these cells behave physiologically like the more ventral
cells but are oriented in the opposite direction. No obvious function of
the posterior innervation was observed in the early physiological study of
these cells (Bennett and Grundfest, 1959), but since the presence of the
posterior innervation was not rwognized at that time the question could
well be reinvestigated.
The single electrocytes of Gymnotus generate external potentials
that are diphasic (Fig. 20) in contrast to the monophasic discharges of
the eel. This form of potential is produced because both faces generate
spikes. The lower threshold face is the posterior, innervated face in all
but the dorsal column of cells in which it is the anterior face. When the
nerve supply is activated or when current is applied by an intracellular
electrode, the lower threshold, innervated face fires first. Current flows
inward through this face and outward through the opposite, uninnervated face which becomes excited, but with some delay with respect to
firing of the innervated face. By this time the spike of the lower threshold
(innervated) face is decreasing, and current flows in the reverse direction along the axis of the cell. This pattern of activity is indicated by the
external recordings shown in Fig. 20. The monopolarly recorded external
potentials are of opposite sign outside the two faces. External to the
innervated face, the potential is negative during PSPs, and when a spike
arises, goes rapidly more negative. During the later part of the monopolarly recorded intracellular spike, the potential external to the innervated face reverses to go positive, indicating that the uninnervated face
has a larger potential across it than the innervated face. The potential
outside the uninnervated face has the same shape and about the same
amplitude as that outside the innervated face but is opposite in sign.
External to the edges of the cell the potentials are very small. This
feature and the opposite polarity of potentials external to the two faces
392
drum-shaped cells, the flat faces of which are oriented anteroposteriorly
(Fig. 19). The cells are about a millimeter in diameter and 300p thick
in a fish 20 cm long. The number of columns is reduced anteriorly, and
there may be several additional columns caudally. In each column there
is one electrocyte per segment and there are about 90 segments in the
animal. Each column of cells is enclosed in a connective tissue tube that
is divided into chambers by loose septa between the cells. The cells of
all but the most dorsal column are innervated by a number of fibers on
their posterior faces. Aside from innervation the two faces are very
similar. They are quite smooth with relatively few inpocketings and
canaliculi (Schwartz et al., 1971). The cells of the most dorsal column
have their main innervation on the anterior face, but they have a few
fibers ending on their posterior faces as well (Szabo, 1961d). As will be
shown below, these cells behave physiologically like the more ventral
cells but are oriented in the opposite direction. No obvious function of
the posterior innervation was observed in the early physiological study of
these cells (Bennett and Grundfest, 1959), but since the presence of the
posterior innervation was not rwognized at that time the question could
well be reinvestigated.
The single electrocytes of Gymnotus generate external potentials
that are diphasic (Fig. 20) in contrast to the monophasic discharges of
the eel. This form of potential is produced because both faces generate
spikes. The lower threshold face is the posterior, innervated face in all
but the dorsal column of cells in which it is the anterior face. When the
nerve supply is activated or when current is applied by an intracellular
electrode, the lower threshold, innervated face fires first. Current flows
inward through this face and outward through the opposite, uninnervated face which becomes excited, but with some delay with respect to
firing of the innervated face. By this time the spike of the lower threshold
(innervated) face is decreasing, and current flows in the reverse direction along the axis of the cell. This pattern of activity is indicated by the
external recordings shown in Fig. 20. The monopolarly recorded external
potentials are of opposite sign outside the two faces. External to the
innervated face, the potential is negative during PSPs, and when a spike
arises, goes rapidly more negative. During the later part of the monopolarly recorded intracellular spike, the potential external to the innervated face reverses to go positive, indicating that the uninnervated face
has a larger potential across it than the innervated face. The potential
outside the uninnervated face has the same shape and about the same
amplitude as that outside the innervated face but is opposite in sign.
External to the edges of the cell the potentials are very small. This
feature and the opposite polarity of potentials external to the two faces