11. ELECTRORECEPTION
515
at electroreceptors exceeds the stimulus in amplitude; that is, the synapse
amplifies the applied potential. Verification of this point will require
intracellular recording from afferent fibers, but the change in discharge
frequency of the nerve appears to require a PSP considerably larger than
the stimulus. Amplification would, of course, be further evidence for
chemically mediated transmission.
Two alternative explanations of the relationship between stimulus
strength and impulse frequency should be considered. It cannot be excluded by present data that stimuli act on a tonic release of an inhibitory
transmitter that alters spontaneous activity of the nerve. Another hypothesis is that the nerve is spontaneously active and that excitatory and inhibitory stimuli cause the release of excitatory and inhibitory transmitters.
This hypothesis also requires a linear relationship between secretion and
stimulus strength and is unlikely in view of the constancy of the slope of
the impulse frequency-stimulus relationship on either side of zero stimulus strength. Exclusion of these possibilities should be feasible where it
is possible to penetrate the nerve fibers close to the synapses (Section
111, D ) . The hypothesis of a single excitatory transmitter requires that
resistance decreases during the postsynaptic change in potential evoked
by an excitatory stimulus and that the resistance increases during the response evoked by an inhibitory stimulus.
C. Characteristics in Catfish
Electroreceptors in the catfish Amiurus ( IctuZurus) are the so-called
small pit organs (Mullinger, 1964; Roth, 1968, 1969). These receptors
are similar to tonic receptors of freshwater electric fish except that generally the ampullary canal is shorter. Similar receptors occur in the electric catfish (R. B. Szamier and M. V. L. Bennett, unpublished data).
The receptor synapse and relationships between receptor and supporting
cells are like those of the electric fish, but the skin lacks the multiple
layers of flattened epithelial cells that are found in the gymnotids and
mormyrids ( Mullinger, 1964; Wachtel and Szamier, 1969). An intriguing
variation is found in the transparent catfish Kryptopterus ( Wachtel and
Szamier, 1969). In this species a cluster of receptor cells are located in a
small beehive-shaped cavity that has an opening to the exterior on one
side. The receptor cells of these receptors, particularly of the ones on
the fins ( Fig. 5 ) , are readily visible in transillumination (Fig. 16).
Potentials can be recorded in the receptor cells of Kryptopterus by
passing a microelectrode in through the receptor opening (Bennett,
1971a,b). The receptor cells have a small internally negative resting po-
515
at electroreceptors exceeds the stimulus in amplitude; that is, the synapse
amplifies the applied potential. Verification of this point will require
intracellular recording from afferent fibers, but the change in discharge
frequency of the nerve appears to require a PSP considerably larger than
the stimulus. Amplification would, of course, be further evidence for
chemically mediated transmission.
Two alternative explanations of the relationship between stimulus
strength and impulse frequency should be considered. It cannot be excluded by present data that stimuli act on a tonic release of an inhibitory
transmitter that alters spontaneous activity of the nerve. Another hypothesis is that the nerve is spontaneously active and that excitatory and inhibitory stimuli cause the release of excitatory and inhibitory transmitters.
This hypothesis also requires a linear relationship between secretion and
stimulus strength and is unlikely in view of the constancy of the slope of
the impulse frequency-stimulus relationship on either side of zero stimulus strength. Exclusion of these possibilities should be feasible where it
is possible to penetrate the nerve fibers close to the synapses (Section
111, D ) . The hypothesis of a single excitatory transmitter requires that
resistance decreases during the postsynaptic change in potential evoked
by an excitatory stimulus and that the resistance increases during the response evoked by an inhibitory stimulus.
C. Characteristics in Catfish
Electroreceptors in the catfish Amiurus ( IctuZurus) are the so-called
small pit organs (Mullinger, 1964; Roth, 1968, 1969). These receptors
are similar to tonic receptors of freshwater electric fish except that generally the ampullary canal is shorter. Similar receptors occur in the electric catfish (R. B. Szamier and M. V. L. Bennett, unpublished data).
The receptor synapse and relationships between receptor and supporting
cells are like those of the electric fish, but the skin lacks the multiple
layers of flattened epithelial cells that are found in the gymnotids and
mormyrids ( Mullinger, 1964; Wachtel and Szamier, 1969). An intriguing
variation is found in the transparent catfish Kryptopterus ( Wachtel and
Szamier, 1969). In this species a cluster of receptor cells are located in a
small beehive-shaped cavity that has an opening to the exterior on one
side. The receptor cells of these receptors, particularly of the ones on
the fins ( Fig. 5 ) , are readily visible in transillumination (Fig. 16).
Potentials can be recorded in the receptor cells of Kryptopterus by
passing a microelectrode in through the receptor opening (Bennett,
1971a,b). The receptor cells have a small internally negative resting po-