M. V. L. BENNETT
544
W sec. The threshold currents are similar for the two kinds of receptor, about 1@l0 A. The threshold voltage is about 100-fold greater for
the mormyrid receptors, but the duration of stimulus required is at least
100-fold shorter (Bennett, 1965; Murray, 1967). The mormyrid receptors
do not appear to be limited by electrical noise of the Johnson type under
normal conditions ( Bennett, 1965), and the ampullae of Lorenzini should
be even farther from this limitation. The mormyrid large receptors in particular are remarkable in that they are poised very close to threshold,
generally without excessive spontaneous activity. Other nerve cells presumably can be held as close to threshold for short times, but subthreshold responses and accommodation would be expected to follow within
a few milliseconds.
V. RECEPTOR FUNCTION IN ELECTRORECEPTION
A. Accessory Structures and Receptor Responses to Potential
Gradients in the Environment
As the middle ear and lens are important to their sense organs, so are
accessory structures important to electroreceptors. It has already been
suggested that current is channeled through the receptor cells by the
flattened cells in the skin and receptor canal and by the circumferential
tight junctions between receptor and supporting cells. The morphological
relationships as well as our general knowledge of synaptic transmission
indicate that the receptors should be sensitive to the potential across the
skin and insensitive to the gradients along it. Experimental verification
of this point is shown in Fig. 34. In this experiment one recording electrode is placed external to a tonic receptor and a microelectrode is pushed
through the skin to record the potential beneath the receptor. When
stimuli are applied externally fairly close to the opening, the potential
inside the fish is small and the positioning of the internal electrode is
not very critical. When a stimulus is applied directly over the receptor,
the sensitivity is greatest; the minimum current is required to generate
a given potential across the skin and to evoke a given response in the
nerve ( Fig. 34D ) . When the stimulating electrode is moved 2 mm away
either rostrally (Fig. 34A), dorsally (Fig. 34B), ventrally (Fig. 34C), or
caudally, a considerably larger current is required to produce the same
potential across the skin, but the corresponding neural response is very
nearly the same. (There is some slowing of the time course of the
potential in Figs. 34A-C compared to Fig. 34D because of skin capacity
and separation of the electrodes.)
544
W sec. The threshold currents are similar for the two kinds of receptor, about 1@l0 A. The threshold voltage is about 100-fold greater for
the mormyrid receptors, but the duration of stimulus required is at least
100-fold shorter (Bennett, 1965; Murray, 1967). The mormyrid receptors
do not appear to be limited by electrical noise of the Johnson type under
normal conditions ( Bennett, 1965), and the ampullae of Lorenzini should
be even farther from this limitation. The mormyrid large receptors in particular are remarkable in that they are poised very close to threshold,
generally without excessive spontaneous activity. Other nerve cells presumably can be held as close to threshold for short times, but subthreshold responses and accommodation would be expected to follow within
a few milliseconds.
V. RECEPTOR FUNCTION IN ELECTRORECEPTION
A. Accessory Structures and Receptor Responses to Potential
Gradients in the Environment
As the middle ear and lens are important to their sense organs, so are
accessory structures important to electroreceptors. It has already been
suggested that current is channeled through the receptor cells by the
flattened cells in the skin and receptor canal and by the circumferential
tight junctions between receptor and supporting cells. The morphological
relationships as well as our general knowledge of synaptic transmission
indicate that the receptors should be sensitive to the potential across the
skin and insensitive to the gradients along it. Experimental verification
of this point is shown in Fig. 34. In this experiment one recording electrode is placed external to a tonic receptor and a microelectrode is pushed
through the skin to record the potential beneath the receptor. When
stimuli are applied externally fairly close to the opening, the potential
inside the fish is small and the positioning of the internal electrode is
not very critical. When a stimulus is applied directly over the receptor,
the sensitivity is greatest; the minimum current is required to generate
a given potential across the skin and to evoke a given response in the
nerve ( Fig. 34D ) . When the stimulating electrode is moved 2 mm away
either rostrally (Fig. 34A), dorsally (Fig. 34B), ventrally (Fig. 34C), or
caudally, a considerably larger current is required to produce the same
potential across the skin, but the corresponding neural response is very
nearly the same. (There is some slowing of the time course of the
potential in Figs. 34A-C compared to Fig. 34D because of skin capacity
and separation of the electrodes.)