M. V. L. BENNETT
380
transmembrane potentials are slowly falling as they are for small PSPs.
Moreover, hyperpolarization increases the amplitudes of the potentials
because the PSP conductance change is unaffected while the driving
force, the difference between the membrane potential and the PSP
reversal potential, is augmented (see Bennett, 1961). If a large PSP
is evoked after a depolarizing pulse that activates the delayed rectification, the external potential is enlarged, the voltage drop across the
external response has only a single peak that occurs at nearly the same
time as the peak of the transmembrane potentials (Fig. 1 5 0 ) . In addition, the external potential is enlarged, the voltage drop across the
uninnervated face is reduced, and the PSP across the innervated face
is also reduced because of the greater electrical load placed on it. A
PSP that turns on the conductance increase causes the same changes
in subsequent responses as does directly applied depolarization.
The utilization of the delayed rectification to reduce the resistance
of the innervated face is an interesting adaptation. It allows the fish to
maintain a high resting resistance but to achieve a large external current
during activity. A possible disadvantage to this mechanism is that the
increase in conductance is delayed, a property that would not appear
to be very significant in an organ discharge that involves fused and
presumably repetitive responses. The earliest PSPs would cause the
conductance increase which would remain activated during the later
PSPs. The double peak of the initial external responses of single cells
is not seen in organ discharges although there may be an inflection on
the rising phases (Fig. 12A). The difference is undoubtedly a question
of synchronization because if electric stimuli are used to evoke synchronous PSPs in lengths of organ the external responses have the same
shape as the responses of single cells.
D. Freshwater Electric Fish
1. GYMNOTIDS
The gymnotids are a diverse group of fish living in fresh waters of
tropical South America. They are often divided into six families (Table
I ) , but the relationship between the families is obvious from the
similarities in body shape and in many other characteristics as well
as from the possession of electric organs (Fig. 1). This group includes
the electric eel, probably the best known electric fish, and a moderate
number of other species that have only weakly electric organs. Because
of their characteristic elongate shape, the weakly electric gymnotids
380
transmembrane potentials are slowly falling as they are for small PSPs.
Moreover, hyperpolarization increases the amplitudes of the potentials
because the PSP conductance change is unaffected while the driving
force, the difference between the membrane potential and the PSP
reversal potential, is augmented (see Bennett, 1961). If a large PSP
is evoked after a depolarizing pulse that activates the delayed rectification, the external potential is enlarged, the voltage drop across the
external response has only a single peak that occurs at nearly the same
time as the peak of the transmembrane potentials (Fig. 1 5 0 ) . In addition, the external potential is enlarged, the voltage drop across the
uninnervated face is reduced, and the PSP across the innervated face
is also reduced because of the greater electrical load placed on it. A
PSP that turns on the conductance increase causes the same changes
in subsequent responses as does directly applied depolarization.
The utilization of the delayed rectification to reduce the resistance
of the innervated face is an interesting adaptation. It allows the fish to
maintain a high resting resistance but to achieve a large external current
during activity. A possible disadvantage to this mechanism is that the
increase in conductance is delayed, a property that would not appear
to be very significant in an organ discharge that involves fused and
presumably repetitive responses. The earliest PSPs would cause the
conductance increase which would remain activated during the later
PSPs. The double peak of the initial external responses of single cells
is not seen in organ discharges although there may be an inflection on
the rising phases (Fig. 12A). The difference is undoubtedly a question
of synchronization because if electric stimuli are used to evoke synchronous PSPs in lengths of organ the external responses have the same
shape as the responses of single cells.
D. Freshwater Electric Fish
1. GYMNOTIDS
The gymnotids are a diverse group of fish living in fresh waters of
tropical South America. They are often divided into six families (Table
I ) , but the relationship between the families is obvious from the
similarities in body shape and in many other characteristics as well
as from the possession of electric organs (Fig. 1). This group includes
the electric eel, probably the best known electric fish, and a moderate
number of other species that have only weakly electric organs. Because
of their characteristic elongate shape, the weakly electric gymnotids