M. V. L. BENNETT
352
that some electrocytes will be found to have electrogenic pumping
[movement of ions that involves net current flow and is linked to
reactions such as adenosine triphosphate ( ATP) hydrolysis (see Hodgkin, 1964; Albers, 1967)l. When the membranes on opposite faces of
the generating cells are at the same potential that at rest is the resting
potential, no current flows (Fig. 2A). When the membranes are at different potentials, current flows in a circuit that involves the two membranes, the cell cytoplasm, and the external medium (Fig. 2B). Figure
2C represents current flow around an electrocyte when the two faces
are at different, but uniform, potentials ( dotted lines). These currents
are associated with potential changes in the external medium, and surfaces at equal potentials ( equipotential surfaces) are also diagrammed
in Fig. 2C (solid lines). The current flow and field that an electric fish
sets up around itself is essentially like that of Fig. 2C but larger since
many cells in series and parallel are active at the same time. If an object
such as a hand is present in the external medium, a potential difference
is present across it and current flows through it. The current from a
strongly electric fish is of sufficient magnitude to excite nerves, muscles,
or receptors in the hand. An object distorts the electric field if it is of
different conductivity than the medium, and the distortions, if large
enough, can be detected by a fish's electrosensory system (see Chapter
11, this volume).
The relatively large size of external potentials generated by electric
organs (Table I ) as compared to other excitable tissues is not a result
of larger membrane potentials although they may be slightly larger in a
few instances. Rather the large outputs are a result of ( a ) arrangement
of a cell's membranes in such a way as to maximize current outside the
cell; ( b ) synchronous activity of many cells arranged in series and
parallel; ( c ) to some degree, lower membrane resistances; and ( d ) accessory structures tending to channel current flow. The different kinds of
adaptation will be discussed in connection with the various types of
organ.
The types and patterns of electric organ discharge can be divided
into several categories. The strongly electric organs all produce essentially monophasic pulses. They are all active intermittently, as indeed
they must be, for the power outputs are so large that the fish can maintain them for only short periods. The organ is normally silent and is
generally, if not exclusively, discharged in response to appropriate external stimuli. These stimuli may be tactile, chemical, electric, or perhaps
visual. Responses are single pulses or trains of pulses usually of fairly
constant size ( Fig. 3A,A').
Generally, weakly electric organs of freshwater fish continually emit
pulses of rather constant size. The pulses may be monophasic, diphasic,
352
that some electrocytes will be found to have electrogenic pumping
[movement of ions that involves net current flow and is linked to
reactions such as adenosine triphosphate ( ATP) hydrolysis (see Hodgkin, 1964; Albers, 1967)l. When the membranes on opposite faces of
the generating cells are at the same potential that at rest is the resting
potential, no current flows (Fig. 2A). When the membranes are at different potentials, current flows in a circuit that involves the two membranes, the cell cytoplasm, and the external medium (Fig. 2B). Figure
2C represents current flow around an electrocyte when the two faces
are at different, but uniform, potentials ( dotted lines). These currents
are associated with potential changes in the external medium, and surfaces at equal potentials ( equipotential surfaces) are also diagrammed
in Fig. 2C (solid lines). The current flow and field that an electric fish
sets up around itself is essentially like that of Fig. 2C but larger since
many cells in series and parallel are active at the same time. If an object
such as a hand is present in the external medium, a potential difference
is present across it and current flows through it. The current from a
strongly electric fish is of sufficient magnitude to excite nerves, muscles,
or receptors in the hand. An object distorts the electric field if it is of
different conductivity than the medium, and the distortions, if large
enough, can be detected by a fish's electrosensory system (see Chapter
11, this volume).
The relatively large size of external potentials generated by electric
organs (Table I ) as compared to other excitable tissues is not a result
of larger membrane potentials although they may be slightly larger in a
few instances. Rather the large outputs are a result of ( a ) arrangement
of a cell's membranes in such a way as to maximize current outside the
cell; ( b ) synchronous activity of many cells arranged in series and
parallel; ( c ) to some degree, lower membrane resistances; and ( d ) accessory structures tending to channel current flow. The different kinds of
adaptation will be discussed in connection with the various types of
organ.
The types and patterns of electric organ discharge can be divided
into several categories. The strongly electric organs all produce essentially monophasic pulses. They are all active intermittently, as indeed
they must be, for the power outputs are so large that the fish can maintain them for only short periods. The organ is normally silent and is
generally, if not exclusively, discharged in response to appropriate external stimuli. These stimuli may be tactile, chemical, electric, or perhaps
visual. Responses are single pulses or trains of pulses usually of fairly
constant size ( Fig. 3A,A').
Generally, weakly electric organs of freshwater fish continually emit
pulses of rather constant size. The pulses may be monophasic, diphasic,