10. ELECTRIC ORGANS
389
tion produced by the stimulus; the delay can be made much shorter
if a stronger stimulus is given. When the exploring electrode is advanced
out through the uninnervated face of the cell, the resting potential
disappears demonstrating that the resting potentials across the two faces
are equal (Fig, 1SC). The response amplitude however does not
change. As discussed in respect to electrocytes of Astroscopus this
finding indicates that the resistance of the uninnervated face is very
low. Direct measurements using current pulses confirm its low resistance, which is much lower than that of the innervated face. The
degree of surface elaboration seen cytologically correlates with the
different resistances.
Stimulation of the nerve supply evokes PSPs that can depolarize the
innervated membrane to the point where it generates a spike. Current
applied as in Fig. 18A-C but in the opposite direction hyperpolarizes
the innervated face and does not excite it. Such stimuli can excite nerve
fibers in the tissue that then produce PSPs that arise after a delay of
about 2 msec (Fig. 1 8 0 ) . If enough nerve fibers are stimulated the
PSP initiates a spike (Fig. 18E).
The ionic basis of the action potential has been well studied in electrocytes of the eel. The cells are depolarized by high potassium solutions
approximately as predicted by the Nernst relation, which indicates
that at rest the potassium permeability of the cell predominates and
the potential is largely determined by the intra- and extracellular concentrations of potassium (Higman et al., 1964). The inward current
responsible for the rising phase of the spike is sodium dependent
( Keynes and Martins-Ferreira, 1953). Furthermore, it is eliminated by
the pharmacological agent tetrodotoxin which is a specific blocking
agent for the increase in sodium permeability produced by depolarization (Nakamura et al., 1965).
Unlike the squid axon and many other tissues, the eel electrocytes
lack K activation or delayed rectification, but they do have anomalous
rectification in the innervated face (Nakamura et al., 1965). Actually,
it makes sense for the cells to have anomalous rectification and not to
have K+ activation. For maximum effectiveness as an electric organ, the
circuit for all the current carried inward by Na' should be completed by
current in the external environment, not by local currents in the innervated face. The time constant of the cells is sufficiently short that
the membranc rapidly returns to the resting potential without the
restoring effect of delayed rectification (as is also very nearly true of
myelinated nerve fibers, see Frankenhaeuser and Huxley, 1964). The
anomalous rectification decreases the conductance enough to result in
a severalfold decrease in eddy currents in the innervated face,
389
tion produced by the stimulus; the delay can be made much shorter
if a stronger stimulus is given. When the exploring electrode is advanced
out through the uninnervated face of the cell, the resting potential
disappears demonstrating that the resting potentials across the two faces
are equal (Fig, 1SC). The response amplitude however does not
change. As discussed in respect to electrocytes of Astroscopus this
finding indicates that the resistance of the uninnervated face is very
low. Direct measurements using current pulses confirm its low resistance, which is much lower than that of the innervated face. The
degree of surface elaboration seen cytologically correlates with the
different resistances.
Stimulation of the nerve supply evokes PSPs that can depolarize the
innervated membrane to the point where it generates a spike. Current
applied as in Fig. 18A-C but in the opposite direction hyperpolarizes
the innervated face and does not excite it. Such stimuli can excite nerve
fibers in the tissue that then produce PSPs that arise after a delay of
about 2 msec (Fig. 1 8 0 ) . If enough nerve fibers are stimulated the
PSP initiates a spike (Fig. 18E).
The ionic basis of the action potential has been well studied in electrocytes of the eel. The cells are depolarized by high potassium solutions
approximately as predicted by the Nernst relation, which indicates
that at rest the potassium permeability of the cell predominates and
the potential is largely determined by the intra- and extracellular concentrations of potassium (Higman et al., 1964). The inward current
responsible for the rising phase of the spike is sodium dependent
( Keynes and Martins-Ferreira, 1953). Furthermore, it is eliminated by
the pharmacological agent tetrodotoxin which is a specific blocking
agent for the increase in sodium permeability produced by depolarization (Nakamura et al., 1965).
Unlike the squid axon and many other tissues, the eel electrocytes
lack K activation or delayed rectification, but they do have anomalous
rectification in the innervated face (Nakamura et al., 1965). Actually,
it makes sense for the cells to have anomalous rectification and not to
have K+ activation. For maximum effectiveness as an electric organ, the
circuit for all the current carried inward by Na' should be completed by
current in the external environment, not by local currents in the innervated face. The time constant of the cells is sufficiently short that
the membranc rapidly returns to the resting potential without the
restoring effect of delayed rectification (as is also very nearly true of
myelinated nerve fibers, see Frankenhaeuser and Huxley, 1964). The
anomalous rectification decreases the conductance enough to result in
a severalfold decrease in eddy currents in the innervated face,