11. ELECTRORECEPTION
565
as a series capacity distorts it or as a resting potential changes its dc level.
The sides of the receptor cell may have no active influence on the
(electrical) stimulus as in tonic receptors or they may alter it by responding as in most phasic receptors. The response may be regenerative, either
graded or all-or-none, or it may be only degenerative, as produced by
delayed rectification. The stimulus thus transformed (not really transduced) is transmitted electrotonically to excite the nerve, or it causes the
presynaptic membrane to increase or decrease the release of transmitter.
There may or may not be sufficient resting secretion to cause tonic
activity in the nerve.
The operation of other receptor cells is probably analogous. There is
considerable evidence that the outer faces of mechanoreceptor cells
generate a potential by changing their resistance and/or potential in
response to a deformation (cf. Davis, 1965; Furukawa and Ishii, 1967),
and carotid body receptors may operatc similarly (Eyzaguirre et al.,
1965, 1970). This potential change is (electronically) conducted to the
secretory face of the receptor cell and acts on the presynaptic membrane
to cause release of transmitter ( Furukawa and Ishii, 1967). In one kind of
vestibular receptor, morphological evidence indicates that the receptor
potential is directly transmitted to the postsynaptic nerve by low resistance pathways ( Spoendlin, 1966). There are little data except in electroreceptors indicating whether receptor responses are modified by electrically excitable membrane in the sides of receptor cells. The occurrence
of such membrane in one kind of receptor cell suggests that it will be
found in others as well. Of course, axons of primary sensory neurons
must conduct action potentials, if the axon is long compared to its space
constant.
What have been termed the outer face, sides, and presynaptic membrane may not be spatially segregated. Depending on how the stimulus
can reach the cell and the mode of innervation, the three idealized
regions could be intermixed with each other to a greater or lesser extent.
In most electroreceptors the sides and presynaptic membrane are probably comingled in the inner face. Nonethcless, one can distinguish three
separate functions and it is probable that the membranes mediating them
are distinguishable, at least on the molecular level. The restriction of
function to particular regions is only required where membranes of different function must be oriented differently with respect to the cell's
environment. Generally, exteroceptors will have an outer face that is
indeed directed outward, but sides and presynaptic membrane can be
intermixed.
Because the receptor cell is so short, it can be considered analogous
to a presynaptic terminal. In this respect it is interesting that the elec-
565
as a series capacity distorts it or as a resting potential changes its dc level.
The sides of the receptor cell may have no active influence on the
(electrical) stimulus as in tonic receptors or they may alter it by responding as in most phasic receptors. The response may be regenerative, either
graded or all-or-none, or it may be only degenerative, as produced by
delayed rectification. The stimulus thus transformed (not really transduced) is transmitted electrotonically to excite the nerve, or it causes the
presynaptic membrane to increase or decrease the release of transmitter.
There may or may not be sufficient resting secretion to cause tonic
activity in the nerve.
The operation of other receptor cells is probably analogous. There is
considerable evidence that the outer faces of mechanoreceptor cells
generate a potential by changing their resistance and/or potential in
response to a deformation (cf. Davis, 1965; Furukawa and Ishii, 1967),
and carotid body receptors may operatc similarly (Eyzaguirre et al.,
1965, 1970). This potential change is (electronically) conducted to the
secretory face of the receptor cell and acts on the presynaptic membrane
to cause release of transmitter ( Furukawa and Ishii, 1967). In one kind of
vestibular receptor, morphological evidence indicates that the receptor
potential is directly transmitted to the postsynaptic nerve by low resistance pathways ( Spoendlin, 1966). There are little data except in electroreceptors indicating whether receptor responses are modified by electrically excitable membrane in the sides of receptor cells. The occurrence
of such membrane in one kind of receptor cell suggests that it will be
found in others as well. Of course, axons of primary sensory neurons
must conduct action potentials, if the axon is long compared to its space
constant.
What have been termed the outer face, sides, and presynaptic membrane may not be spatially segregated. Depending on how the stimulus
can reach the cell and the mode of innervation, the three idealized
regions could be intermixed with each other to a greater or lesser extent.
In most electroreceptors the sides and presynaptic membrane are probably comingled in the inner face. Nonethcless, one can distinguish three
separate functions and it is probable that the membranes mediating them
are distinguishable, at least on the molecular level. The restriction of
function to particular regions is only required where membranes of different function must be oriented differently with respect to the cell's
environment. Generally, exteroceptors will have an outer face that is
indeed directed outward, but sides and presynaptic membrane can be
intermixed.
Because the receptor cell is so short, it can be considered analogous
to a presynaptic terminal. In this respect it is interesting that the elec-