454
M. V. L. BENNETT
occurs during the phase of increased conductance owing to delayed
rectification (potassium activation) in the earlier firing face, and the
area of this face need not be so great. One would not be surprised to
find that delayed rectification was more pronounced in the earlier firing
face, but this possibility has not been investigated. Delayed rectification
is more marked in Gymnotus cells than in those of Steatogenys, which
may account for the smaller degree of proliferation of both surfaces
in Gymnotus as compared to Steatogenys.
Although some nerve-electrocyte junctions are continuously active
at very high rates, no morphologically distinctive features have been
found (Schwartz et al., 1971). All investigated do not appear significantly different from neuromuscular junctions.
It is a bit disappointing that current electron microscopic techniques
have not revealed any further correlations with function. One cannot
yet distinguish between a high resistance inexcitable membrane and a
low resistance excitable membrane. This failure probably reflects the
small fraction of membrane surface actually involved in ionic movements ( cf. Hille, 1970). Perhaps freeze-cleaving techniques, which
allow one to examine relatively large areas of membrane e n face, will
provide sufficient resolution. After all, it is to be expected that ionic
channels, although themselves small, have associated with them protein
molecules well within the range of resolution of the electron microscope.
An important kind of adaptation in electric fish is the arrangement
of connective tissue. In many electric organs, connective tissue appears
to provide an insulating sheath that channels current flow along the
axis of the organ. An example from Gymnotus is given in Fig. 69. In
Torpedo the resistance of the skin on the dorsal and ventral surfaces
of the organ is lower than that over the rest of the body (Bennett et al.,
1961). This difference tends to maximize current in the external medium
and minimize current flowing through the animal’s body.
An old but frequently recurring question is why strongly electric
fish do not shock themselves. Their resistance to their own discharges
as well as those of other fish is no doubt contributed to by the heavy
fat and connective tissue layers that surround much of the nervous
system. For example, the electromotor axons of Malapterurus are rather
small, but they are surrounded by a sheath so that their apparent
diameter is about 1 mm (in a fish about 15 cm long). This sheath
becomes attenuated as the fiber branches, but it nevertheless extends
beyond the synaptic terminals and part way down the electrocyte stalks
(Mathewson et al., 1961). One would not be surprised if the hearts
of strongly electric fish were less easily driven into fibrillation by shock
or if the electroconvulsive threshold were lower for their neural tissue,
but no data have been obtained.
Another answer to the question of why electric fish do not shock
M. V. L. BENNETT
occurs during the phase of increased conductance owing to delayed
rectification (potassium activation) in the earlier firing face, and the
area of this face need not be so great. One would not be surprised to
find that delayed rectification was more pronounced in the earlier firing
face, but this possibility has not been investigated. Delayed rectification
is more marked in Gymnotus cells than in those of Steatogenys, which
may account for the smaller degree of proliferation of both surfaces
in Gymnotus as compared to Steatogenys.
Although some nerve-electrocyte junctions are continuously active
at very high rates, no morphologically distinctive features have been
found (Schwartz et al., 1971). All investigated do not appear significantly different from neuromuscular junctions.
It is a bit disappointing that current electron microscopic techniques
have not revealed any further correlations with function. One cannot
yet distinguish between a high resistance inexcitable membrane and a
low resistance excitable membrane. This failure probably reflects the
small fraction of membrane surface actually involved in ionic movements ( cf. Hille, 1970). Perhaps freeze-cleaving techniques, which
allow one to examine relatively large areas of membrane e n face, will
provide sufficient resolution. After all, it is to be expected that ionic
channels, although themselves small, have associated with them protein
molecules well within the range of resolution of the electron microscope.
An important kind of adaptation in electric fish is the arrangement
of connective tissue. In many electric organs, connective tissue appears
to provide an insulating sheath that channels current flow along the
axis of the organ. An example from Gymnotus is given in Fig. 69. In
Torpedo the resistance of the skin on the dorsal and ventral surfaces
of the organ is lower than that over the rest of the body (Bennett et al.,
1961). This difference tends to maximize current in the external medium
and minimize current flowing through the animal’s body.
An old but frequently recurring question is why strongly electric
fish do not shock themselves. Their resistance to their own discharges
as well as those of other fish is no doubt contributed to by the heavy
fat and connective tissue layers that surround much of the nervous
system. For example, the electromotor axons of Malapterurus are rather
small, but they are surrounded by a sheath so that their apparent
diameter is about 1 mm (in a fish about 15 cm long). This sheath
becomes attenuated as the fiber branches, but it nevertheless extends
beyond the synaptic terminals and part way down the electrocyte stalks
(Mathewson et al., 1961). One would not be surprised if the hearts
of strongly electric fish were less easily driven into fibrillation by shock
or if the electroconvulsive threshold were lower for their neural tissue,
but no data have been obtained.
Another answer to the question of why electric fish do not shock