11. ELECTRORECEPTION
567
troreceptors are capable of operating. The cochlear microphonic would
not cause much electrical interference between cells since the external
current from a single receptor is divided into the low resistance of the
external path around the cells and the parallel paths provided by all the
other cells. Intracellular recording from receptor cells of lateral line receptors of an amphibian indicates that a quite small microphonic, as low
as 100 pV across the hair cell inner membrane, can be associated with a
stimulus well suprathreshold for excitation of the postsynaptic fiber
(Harris et al., 1970). This amplitude of potential would be adequate to
excite tonic electroreceptors of freshwater fish.
Thus, it appears that at least the most sensitive mechanoreceptors of
the acoustico-lateralis system are of similar sensitivity to electroreceptors
in terms of the potentials generated across their secretory faces. (An unanswered question is the mechanism whereby high frequency sinusoidal
microphonics still cause release of transmitter. Possibly the secretory
membrane of tonic electroreceptors would respond to high frequencies,
but the potential across it at these frequencies is limited by its large time
constant. )
The sensitive input-output relationship of electroreceptor synapses
may be found in some neurons as well as receptor cells. In short axon
cells such as the bipolar cells of the retina (Dowling and Boycott, 1965)
and at reciprocal synapses such as those of the olfactory bulb (Rall et d,
1966), the propagated action potential probably is not required to transmit potentials between receptive and output parts of the cells. Slow and
small PSPs generated at one site could cause release of transmitter by
secretory membrane without intervention of large regenerative responses.
In this connection it is interesting that the bipolar cell synapses have presynaptic ribbons like those found otherwise only in receptor cells.
The experimental advantage of the tonic electroreceptors of freshwater fish is that they are specialized to detect low frequency voltages
across the skin. The specializations that channel current through the receptor cells and the electrical linearity of the cells allow the experimenter
considerable control over the presynaptic potential and has made possible
the determination of the input-output relationship. The presynaptic POtential cannot be well controlled at phasic receptors, which do however
demonstrate the possibility of electrical excitability and regenerative responses in receptor cells. A series capacity is an elegant way to achieve
accommodation, but no other receptors are likely to have it. The only
other cells where a membrane is known to act as a series capacity are
electrocytes of several electric organs.
The mechanisms of central analysis in electrosensory systems are
largely unknown and may not turn out to be easier to analyze than other
567
troreceptors are capable of operating. The cochlear microphonic would
not cause much electrical interference between cells since the external
current from a single receptor is divided into the low resistance of the
external path around the cells and the parallel paths provided by all the
other cells. Intracellular recording from receptor cells of lateral line receptors of an amphibian indicates that a quite small microphonic, as low
as 100 pV across the hair cell inner membrane, can be associated with a
stimulus well suprathreshold for excitation of the postsynaptic fiber
(Harris et al., 1970). This amplitude of potential would be adequate to
excite tonic electroreceptors of freshwater fish.
Thus, it appears that at least the most sensitive mechanoreceptors of
the acoustico-lateralis system are of similar sensitivity to electroreceptors
in terms of the potentials generated across their secretory faces. (An unanswered question is the mechanism whereby high frequency sinusoidal
microphonics still cause release of transmitter. Possibly the secretory
membrane of tonic electroreceptors would respond to high frequencies,
but the potential across it at these frequencies is limited by its large time
constant. )
The sensitive input-output relationship of electroreceptor synapses
may be found in some neurons as well as receptor cells. In short axon
cells such as the bipolar cells of the retina (Dowling and Boycott, 1965)
and at reciprocal synapses such as those of the olfactory bulb (Rall et d,
1966), the propagated action potential probably is not required to transmit potentials between receptive and output parts of the cells. Slow and
small PSPs generated at one site could cause release of transmitter by
secretory membrane without intervention of large regenerative responses.
In this connection it is interesting that the bipolar cell synapses have presynaptic ribbons like those found otherwise only in receptor cells.
The experimental advantage of the tonic electroreceptors of freshwater fish is that they are specialized to detect low frequency voltages
across the skin. The specializations that channel current through the receptor cells and the electrical linearity of the cells allow the experimenter
considerable control over the presynaptic potential and has made possible
the determination of the input-output relationship. The presynaptic POtential cannot be well controlled at phasic receptors, which do however
demonstrate the possibility of electrical excitability and regenerative responses in receptor cells. A series capacity is an elegant way to achieve
accommodation, but no other receptors are likely to have it. The only
other cells where a membrane is known to act as a series capacity are
electrocytes of several electric organs.
The mechanisms of central analysis in electrosensory systems are
largely unknown and may not turn out to be easier to analyze than other