M. V. L. BENNETT
550
catfish Plotosus have long canals like the receptors of marine elasmobranchs ( Gkrard, 1947). Conversely ampullae of Lorenzini of the
freshwater stingray have very short canals (R. B. Szamier and M. V. L.
Bennett, unpublished data). It seems likely that the difference in canal
lengths is an adaptation to the relative resistivities of fresh- and saltwater. The tissue of a marine fish is of higher resistivity than the water
surrounding it, and even if its surface resistance is low the tissue will not
“load’ the current pathway through the seawater surrounding the fish.
In contrast the tissue of a freshwater fish is of considerably lower resistivity than is the water in its environment. If there were not a high
surface resistance, the resistance of the environment would be large compared to the resistance of the fish and the voltage across the animal
would be reduced. Because of its high surface resistance the freshwater
electric fish presents less of a load to the environment. Actually, there
seems to be some impedance matching in freshwater electric fish, at
least as far as dc voltages are concerned. These fish do load their environment to some extent, as they should for maximum power dissipation
across the receptors.
B. Receptor Responses to Electric Organ Discharges
Many of the studies of afferent impulses evoked by organ discharge
have not involved identification of receptor type, although it is often possible to infer from the response what kind of receptor was being stimulated. Because tonic receptors have low sensitivity to higher frequency
(sinusoidal) stimuli, they do not respond to the electric organ discharge
in species in which the discharge has little low frequency dc component
(Bennett, 1970; Bullock and Chichibu, 1965; Roth, 1967; Suga, 1967a;
Szabo, 1970). It appears then that in most electric fish tonic receptors act
largely passively and detect low frequency signals of extrinsic origin.
What are probably tonic receptors in the eel do respond to discharges
of the weak electric organ (Hagiwara et al., 1965a); these pulses are
monophasic and therefore have a significant low frequency component.
The tonic receptors could probably also be activated by the electric organ
discharge in the gymnotid species of Hypopoinus that has a monophasic
discharge and in the mormyrid Mormyrw. The rostra1 accessory organs
of several gymnotids also emit monophasic pulses. There may be some
stimulation of tonic receptors by muscle action potentials and dc potentials such as those resulting from injuries of skin or muscle (Bennett,
1967; Bullock and Chichibu, 1965). In the dogfish, ampullae of Lorenzini
apparently respond to potentials associated with the animal’s own res-
550
catfish Plotosus have long canals like the receptors of marine elasmobranchs ( Gkrard, 1947). Conversely ampullae of Lorenzini of the
freshwater stingray have very short canals (R. B. Szamier and M. V. L.
Bennett, unpublished data). It seems likely that the difference in canal
lengths is an adaptation to the relative resistivities of fresh- and saltwater. The tissue of a marine fish is of higher resistivity than the water
surrounding it, and even if its surface resistance is low the tissue will not
“load’ the current pathway through the seawater surrounding the fish.
In contrast the tissue of a freshwater fish is of considerably lower resistivity than is the water in its environment. If there were not a high
surface resistance, the resistance of the environment would be large compared to the resistance of the fish and the voltage across the animal
would be reduced. Because of its high surface resistance the freshwater
electric fish presents less of a load to the environment. Actually, there
seems to be some impedance matching in freshwater electric fish, at
least as far as dc voltages are concerned. These fish do load their environment to some extent, as they should for maximum power dissipation
across the receptors.
B. Receptor Responses to Electric Organ Discharges
Many of the studies of afferent impulses evoked by organ discharge
have not involved identification of receptor type, although it is often possible to infer from the response what kind of receptor was being stimulated. Because tonic receptors have low sensitivity to higher frequency
(sinusoidal) stimuli, they do not respond to the electric organ discharge
in species in which the discharge has little low frequency dc component
(Bennett, 1970; Bullock and Chichibu, 1965; Roth, 1967; Suga, 1967a;
Szabo, 1970). It appears then that in most electric fish tonic receptors act
largely passively and detect low frequency signals of extrinsic origin.
What are probably tonic receptors in the eel do respond to discharges
of the weak electric organ (Hagiwara et al., 1965a); these pulses are
monophasic and therefore have a significant low frequency component.
The tonic receptors could probably also be activated by the electric organ
discharge in the gymnotid species of Hypopoinus that has a monophasic
discharge and in the mormyrid Mormyrw. The rostra1 accessory organs
of several gymnotids also emit monophasic pulses. There may be some
stimulation of tonic receptors by muscle action potentials and dc potentials such as those resulting from injuries of skin or muscle (Bennett,
1967; Bullock and Chichibu, 1965). In the dogfish, ampullae of Lorenzini
apparently respond to potentials associated with the animal’s own res-