M. V. L. BENNETT
406
additional electric organ behind each operculum. The postopercular
organ is for part of its length a double column of cells; all the cells are
innervated on their posterior faces. These accessory organs are innervated by spinal nerves that run rostrally from the most anterior level of
the main organ (Fig. 32). Electrocytes of the accessory organs have
several stalks instead of the single one found in cells of the main organ
( Fig. 33). They are difficult to penetrate with microelectrodes because
of the connective tissue surrounding the cells. However the isolated
organs can be studied, and it can be concluded that they generate
monophasic action potentials as do the cells of the eel and the monophasically discharging Hypopornus, and they have a similar marked
proliferation of the uninnervated face ( Schwartz et al., 1971).
When a column is stimulated by an axial current depolarizing the
innervated faces, a monophasic spikelike potential is produced with a
very short latency (about 0.1 msec in Fig. 34A). This response is positive
with respect to the uninnervated face, and because of its short latency it
can be identified as a directly excited spike. This potential is followed at
a latency of about 1 msec by a smaller potential of similar sign that
evidently represents PSPs and perhaps spike activity in the stalks. If a
brief axial stimulus is given that hyperpolarizes the innervated face and
depolarizes the uninnervated face, there is no short latency response
indicating that the uninnervated face is inexcitable (Fig. 34C). This
mode of stimulation does excite the nerve fibers; the delayed response is
still present, but it becomes a full-sized spike since the innervated faces
are not refractory as in Fig. 34A. The neural origin of the delayed
response is confirmed by the effects of curare, a drug which blocks
transmission at cholinergic synapses like those of nerve-electrocyte junctions. Following curare treatment the delayed response is absent for both
polarities of stimulation, although the directly excited response persists
unchanged (Fig. 34B,D).
The accessory organs fire about 1 msec before the main organ (Fig.
35). The submental electrocytes are innervated on their anterior faces,
and the potential external to the anterior end of the organ is largely
negative. The postopercular electrocytes are innervated on their posterior
faces, and negativity is recorded external to the posterior end of this
organ. At the same time the potential in the gill opening and in the
mouth is positive. These electrocytes act to make the interior of the head
positive. If the accessory organs are removed, the discharge in the head
region becomes the simple diphasic potential of main organ (Fig. 30).
There is another much smaller species of Steatogenys (5-7 cm)
which has submental electrocytes that are all innervated on a single stalk
from their posterior faces. This species lacks a separate group of postopercular electrocytes. These cells also have a monophasic discharge and
406
additional electric organ behind each operculum. The postopercular
organ is for part of its length a double column of cells; all the cells are
innervated on their posterior faces. These accessory organs are innervated by spinal nerves that run rostrally from the most anterior level of
the main organ (Fig. 32). Electrocytes of the accessory organs have
several stalks instead of the single one found in cells of the main organ
( Fig. 33). They are difficult to penetrate with microelectrodes because
of the connective tissue surrounding the cells. However the isolated
organs can be studied, and it can be concluded that they generate
monophasic action potentials as do the cells of the eel and the monophasically discharging Hypopornus, and they have a similar marked
proliferation of the uninnervated face ( Schwartz et al., 1971).
When a column is stimulated by an axial current depolarizing the
innervated faces, a monophasic spikelike potential is produced with a
very short latency (about 0.1 msec in Fig. 34A). This response is positive
with respect to the uninnervated face, and because of its short latency it
can be identified as a directly excited spike. This potential is followed at
a latency of about 1 msec by a smaller potential of similar sign that
evidently represents PSPs and perhaps spike activity in the stalks. If a
brief axial stimulus is given that hyperpolarizes the innervated face and
depolarizes the uninnervated face, there is no short latency response
indicating that the uninnervated face is inexcitable (Fig. 34C). This
mode of stimulation does excite the nerve fibers; the delayed response is
still present, but it becomes a full-sized spike since the innervated faces
are not refractory as in Fig. 34A. The neural origin of the delayed
response is confirmed by the effects of curare, a drug which blocks
transmission at cholinergic synapses like those of nerve-electrocyte junctions. Following curare treatment the delayed response is absent for both
polarities of stimulation, although the directly excited response persists
unchanged (Fig. 34B,D).
The accessory organs fire about 1 msec before the main organ (Fig.
35). The submental electrocytes are innervated on their anterior faces,
and the potential external to the anterior end of the organ is largely
negative. The postopercular electrocytes are innervated on their posterior
faces, and negativity is recorded external to the posterior end of this
organ. At the same time the potential in the gill opening and in the
mouth is positive. These electrocytes act to make the interior of the head
positive. If the accessory organs are removed, the discharge in the head
region becomes the simple diphasic potential of main organ (Fig. 30).
There is another much smaller species of Steatogenys (5-7 cm)
which has submental electrocytes that are all innervated on a single stalk
from their posterior faces. This species lacks a separate group of postopercular electrocytes. These cells also have a monophasic discharge and