11. ELECTRORECEP’ITON
513
the equivalent circuit of Fig. 9 is greatly increased. Although the degree
of residual shunting by the skin and remaining water has not been satisfactorily evaluated, the current voltage relation measured under these
conditions is likely to have considerable contribution from the receptor
cells because the input resistances become large, as high as several
megohms. Furthermore, receptor cells of phasic receptors contribute importantly to the external potentials under these conditions (Section IV) .
The receptor cells of tonic receptors appear to behave very nearly like
linear electrical elements; that is, their membranes have fixed internal
potentials, resistances, and capacitances. They are, in this sense, electrically inexcitable, and as shown below they differ markedly in this respect from phasic receptor cells. Of course, electric stimuli do affect the
receptor cells by causing them to secrete transmitter.
There is evidence that at the squid giant synapse and the neuromuscular junction, an idlux of Ca2+ is a necessary step in transmitter release (Katz, 1969; Katz and Miledi, 1969a,b). Depolarization of the presynaptic terminal causes an increase in Ca2+ influx which causes further
depolarization. Under certain circumstances this activity can lead to an
all-or-none response. A small change in Ca2+ permeability, which would
cause only a small electrical nonlinearity, may well occur in the cells of
tonic receptors.
The short latency of responses to strong stimuli indicates that stimuli
act directly on the presynaptic secretory membrane or at least very close
to it. The minimum synaptic delay approaches 1 msec, which would leave
little time for diffusion of some activating substance from a sensitive site
to the secretory membrane, It is likely that the outer faces of the receptor
cells are of low resistance compared to the inner, secretory faces (as
has been directly shown in Kryptopterus, see Section 111,C; Bennett
1971a,b). While not required, this feature would maximize the sensitivity
of the receptors to the external fields, and as seen in Fig. 14 the receptors
can be very sensitive.
From the foregoing data one may draw an approximate equivalent
circuit of the tonic receptors in which the inner and outer faces of the
receptor cells can be represented as “lumped” resistances (Fig. 9). In this
circuit, membrane capacities are ignored, as are the resistances of the
canal lumen and receptor cell cytoplasm. If the inner faces of the receptor
cells are indeed of higher resistance than the outer faces, the inner faces
should have a longer time constant because their area is larger and
specific capacity of cell membranes is generally about the same. The
time constant of the inner faces may be one factor limiting the high frequency response of the receptors. As stimulus frequency increases the
voltage drop across these faces becomes smaller, and more of the applied
513
the equivalent circuit of Fig. 9 is greatly increased. Although the degree
of residual shunting by the skin and remaining water has not been satisfactorily evaluated, the current voltage relation measured under these
conditions is likely to have considerable contribution from the receptor
cells because the input resistances become large, as high as several
megohms. Furthermore, receptor cells of phasic receptors contribute importantly to the external potentials under these conditions (Section IV) .
The receptor cells of tonic receptors appear to behave very nearly like
linear electrical elements; that is, their membranes have fixed internal
potentials, resistances, and capacitances. They are, in this sense, electrically inexcitable, and as shown below they differ markedly in this respect from phasic receptor cells. Of course, electric stimuli do affect the
receptor cells by causing them to secrete transmitter.
There is evidence that at the squid giant synapse and the neuromuscular junction, an idlux of Ca2+ is a necessary step in transmitter release (Katz, 1969; Katz and Miledi, 1969a,b). Depolarization of the presynaptic terminal causes an increase in Ca2+ influx which causes further
depolarization. Under certain circumstances this activity can lead to an
all-or-none response. A small change in Ca2+ permeability, which would
cause only a small electrical nonlinearity, may well occur in the cells of
tonic receptors.
The short latency of responses to strong stimuli indicates that stimuli
act directly on the presynaptic secretory membrane or at least very close
to it. The minimum synaptic delay approaches 1 msec, which would leave
little time for diffusion of some activating substance from a sensitive site
to the secretory membrane, It is likely that the outer faces of the receptor
cells are of low resistance compared to the inner, secretory faces (as
has been directly shown in Kryptopterus, see Section 111,C; Bennett
1971a,b). While not required, this feature would maximize the sensitivity
of the receptors to the external fields, and as seen in Fig. 14 the receptors
can be very sensitive.
From the foregoing data one may draw an approximate equivalent
circuit of the tonic receptors in which the inner and outer faces of the
receptor cells can be represented as “lumped” resistances (Fig. 9). In this
circuit, membrane capacities are ignored, as are the resistances of the
canal lumen and receptor cell cytoplasm. If the inner faces of the receptor
cells are indeed of higher resistance than the outer faces, the inner faces
should have a longer time constant because their area is larger and
specific capacity of cell membranes is generally about the same. The
time constant of the inner faces may be one factor limiting the high frequency response of the receptors. As stimulus frequency increases the
voltage drop across these faces becomes smaller, and more of the applied