10. ELECTRIC ORGANS
453
such as Gymnotus and most mormyrids, the opposed faces of the electrocytes are active sequentially to achieve the same effect. Other convergences in terms of pulse shape and patterns of organ discharge can
be found. Mormyrids are similar to the variable frequency gymnotids,
and Gymnarchus is similar to the constant frequency gymnotids. The
more striking differences are sure to be associated with differences in
the operation of the sensory part of the system, and some of the subtler
ones may be associated with species recognition.
The fine structure of electrocytes shows a few correlations with
their functional properties. Where one face is low resistance and inexcitable and the other face generates PSPs or spikes, the surface of
the low resistance face is greatly increased by projections or invaginations [ torpedinids, electric eel, monophasically discharging Hypopomus,
accessory organs of Steatogenys (Mathewson et al., 1961; Sheridan,
1965; Bloom and Barnett, 1966; Schwartz et al., 1971)l. The area of
the active face may also be increased but to a smaller extent. The relative areas correlate with the changes in resistances during activity
and are such that these resistances tend to become more nearly equal.
The probable significance of this feature in terms of increased efficiency
was noted in Section 11, B. In Gymnarchus one face acts as a series
capacity and this face is markedly increased in area; the morphological
relations are similar in Eigenmannia (Schwartz et al., 1971). In
Sternopygus both uninnervated and innervated faces are quite smooth,
which supports the hypothesis that the very large apparent capacity
is a result of shifts of ionic concentrations rather than a dielectric
capacity. Furthermore, the resistances during and between spikes are
about equal.
A number of electrocytes in which both faces generate spikes have
been studied at the fine structural level [Gymnotus, Steatogenys main
organ, Gnathonemus, and Malapterurus (Mathewson et al., 1961;
Schwartz et al., 1971)l. In each case there is some proliferation of both
faces, but the degree is always less than in the inexcitable faces of
monophasically responding cells. The proliferation in diphasically responding cells is somewhat greater in the uninnervated or nonstalk
face; that in all but Malapterurus is the higher threshold, later firing
face. The morphological difference may be an adaptation to the fact
that the earlier firing face must operate through the series resistance
of the later firing face at rest and that the latter should therefore have
a low resting impedance. In Gymnotus at least the time constant of
the later firing face is greater than that of the earlier firing face, and
a significant fraction of the early outward current through the later
firing face may be capacitative. The activity of the later firing face
453
such as Gymnotus and most mormyrids, the opposed faces of the electrocytes are active sequentially to achieve the same effect. Other convergences in terms of pulse shape and patterns of organ discharge can
be found. Mormyrids are similar to the variable frequency gymnotids,
and Gymnarchus is similar to the constant frequency gymnotids. The
more striking differences are sure to be associated with differences in
the operation of the sensory part of the system, and some of the subtler
ones may be associated with species recognition.
The fine structure of electrocytes shows a few correlations with
their functional properties. Where one face is low resistance and inexcitable and the other face generates PSPs or spikes, the surface of
the low resistance face is greatly increased by projections or invaginations [ torpedinids, electric eel, monophasically discharging Hypopomus,
accessory organs of Steatogenys (Mathewson et al., 1961; Sheridan,
1965; Bloom and Barnett, 1966; Schwartz et al., 1971)l. The area of
the active face may also be increased but to a smaller extent. The relative areas correlate with the changes in resistances during activity
and are such that these resistances tend to become more nearly equal.
The probable significance of this feature in terms of increased efficiency
was noted in Section 11, B. In Gymnarchus one face acts as a series
capacity and this face is markedly increased in area; the morphological
relations are similar in Eigenmannia (Schwartz et al., 1971). In
Sternopygus both uninnervated and innervated faces are quite smooth,
which supports the hypothesis that the very large apparent capacity
is a result of shifts of ionic concentrations rather than a dielectric
capacity. Furthermore, the resistances during and between spikes are
about equal.
A number of electrocytes in which both faces generate spikes have
been studied at the fine structural level [Gymnotus, Steatogenys main
organ, Gnathonemus, and Malapterurus (Mathewson et al., 1961;
Schwartz et al., 1971)l. In each case there is some proliferation of both
faces, but the degree is always less than in the inexcitable faces of
monophasically responding cells. The proliferation in diphasically responding cells is somewhat greater in the uninnervated or nonstalk
face; that in all but Malapterurus is the higher threshold, later firing
face. The morphological difference may be an adaptation to the fact
that the earlier firing face must operate through the series resistance
of the later firing face at rest and that the latter should therefore have
a low resting impedance. In Gymnotus at least the time constant of
the later firing face is greater than that of the earlier firing face, and
a significant fraction of the early outward current through the later
firing face may be capacitative. The activity of the later firing face