M. V. L. BENNETT
402
little spread to the main part of the cell. Stronger stimulation causes a
larger PSP, and a spike is initiated in the stalk that is earlier, larger, and
longer lasting than the spikes recorded across the faces of the cell. With a
stronger and longer lasting stimulus the spike is initiated directly in the
stalk (or posterior face) as indicated by the short latency of the response
(Fig. 28C). At the time that the PSP occurs in Fig. 28A there is a depression in the peak of the spike in the stalk. This response is the PSP that is
inverted because the potential during the spike exceeds the reversal
potential of the PSP (see del Castillo and Katz, 1956). If a cathodal
stimulus is applied external to the uninnervated face a large depolarization of this face can be produced without exciting it, demonstrating its
inexcitability (Fig. 28D). Stimulation of this polarity hyperpolarizes the
innervated face, and if a prior brief stimulus initiates a neurally evoked
response in the stalk (Fig. ZSC), this activity can fail to invade the main
part of the innervated face of the cell. In this event only a small potential
is observed across the innervated face even though the spike recorded in
the stalk remains large.
The role of stalks in electrocyte function remains obscure. It will be
considered again in Section 11, F.
One unidentified species of Hypopomus has been studied in which
external recording suggests that electrocytes in about the anterior twothirds of the body generate monophasic potentials and the posterior cells
generate diphasic potentials. This possibility requires exploration using
microelectrode techniques. No species of Hypopomus has been observed
to have a rostra1 accessory organ like those of Gymnotus, Steatogenys,
and G ymnorhamphicthys.
d. Steatogenys. Steatogenys is quite similar in appearance to
Hypopomus and is often confused with it by fish suppliers. The main
organ of Steatogenys appears morphologically identical to that of the
diphasically discharging Hypopomus (Fig. 29), and the form of discharge recorded near the tail or distant from the fish is similar (Fig. 30).
The frequency of resting discharge is more like that of Gymnotus, and
various stimuli cause moderate accelerations [although there is some
dispute as to this point (see Bullock, 1970)l.
As in Hypopomus innervation occurs on a short stalk from the
posterior face (Fig. 29). Both faces of the cells generate spikes and
morphologically the two faces are similar although there is a somewhat
greater proliferation of the uninnervated face ( Schwartz et al., 1971).
Recordings across the faces and stalk of an electrocyte of the main organ
are shown in Fig. 31. A brief stimulus is applied to the nerve supply and
evokes a threshold synaptic potential. This PSP is much larger in the
stalk than across either face of the cell. The impulse arises in the stalk
402
little spread to the main part of the cell. Stronger stimulation causes a
larger PSP, and a spike is initiated in the stalk that is earlier, larger, and
longer lasting than the spikes recorded across the faces of the cell. With a
stronger and longer lasting stimulus the spike is initiated directly in the
stalk (or posterior face) as indicated by the short latency of the response
(Fig. 28C). At the time that the PSP occurs in Fig. 28A there is a depression in the peak of the spike in the stalk. This response is the PSP that is
inverted because the potential during the spike exceeds the reversal
potential of the PSP (see del Castillo and Katz, 1956). If a cathodal
stimulus is applied external to the uninnervated face a large depolarization of this face can be produced without exciting it, demonstrating its
inexcitability (Fig. 28D). Stimulation of this polarity hyperpolarizes the
innervated face, and if a prior brief stimulus initiates a neurally evoked
response in the stalk (Fig. ZSC), this activity can fail to invade the main
part of the innervated face of the cell. In this event only a small potential
is observed across the innervated face even though the spike recorded in
the stalk remains large.
The role of stalks in electrocyte function remains obscure. It will be
considered again in Section 11, F.
One unidentified species of Hypopomus has been studied in which
external recording suggests that electrocytes in about the anterior twothirds of the body generate monophasic potentials and the posterior cells
generate diphasic potentials. This possibility requires exploration using
microelectrode techniques. No species of Hypopomus has been observed
to have a rostra1 accessory organ like those of Gymnotus, Steatogenys,
and G ymnorhamphicthys.
d. Steatogenys. Steatogenys is quite similar in appearance to
Hypopomus and is often confused with it by fish suppliers. The main
organ of Steatogenys appears morphologically identical to that of the
diphasically discharging Hypopomus (Fig. 29), and the form of discharge recorded near the tail or distant from the fish is similar (Fig. 30).
The frequency of resting discharge is more like that of Gymnotus, and
various stimuli cause moderate accelerations [although there is some
dispute as to this point (see Bullock, 1970)l.
As in Hypopomus innervation occurs on a short stalk from the
posterior face (Fig. 29). Both faces of the cells generate spikes and
morphologically the two faces are similar although there is a somewhat
greater proliferation of the uninnervated face ( Schwartz et al., 1971).
Recordings across the faces and stalk of an electrocyte of the main organ
are shown in Fig. 31. A brief stimulus is applied to the nerve supply and
evokes a threshold synaptic potential. This PSP is much larger in the
stalk than across either face of the cell. The impulse arises in the stalk