536
M. V. L. BENNETT
other experiments similar results are obtained with oscillations following
much smaller and briefer stimuli (Fig. 32). All-or-none impulses can be
evoked at the end of the unresponsive state induced by strong cathodal
stimuli, and these responses show little net current flow. It can be concluded that these phasic receptors also exhibit the properties indicating
the presence of a series capacity. As in the mormyrids the morphological
relationships suggest that the capacity is in the outer faces of the receptor
cells. The polarity of the externally recorded responses is consistent with
a conventional polarity of responsiveness in the inner faces. Although the
tendency to oscillate is unusually great, similar graded subthreshold
oscillations are predicted by the Hodgkin-Huxley equations (Mauro et al.,
1970).
In gymnotid phasic receptors the response in the nerve is poorly
correlated with the response recorded outside the receptor. There may
be oscillations outside the receptor with no nerve discharge, and the nerve
discharge can outlast the externally recorded oscillations. Also, when the
external resistance is increased by removing overlying water, the small
and brief nerve impulse can be recorded external to the receptor in the
same way as at tonic receptors (Fig. 15; Bennett, 1967). These data by
themselves suggest that the externally recorded oscillations are generated
by the receptor cells and that transmission to the nerve is chemically
mediated. Confirmation of chemically mediated transmission was obtained by experiments similar to those of Fig. 15 where it was found that
strong cathodal stimuli do not block responses initiated by brief anodal
stimuli and that there is a synaptic delay. Furthermore, the external response at phasic receptors can be obtained at least one month following
denervation which is after externally recorded nerve impulses have disappeared at both phasic and tonic receptors (Bennett, 1967). Finally,
tetrodotoxin blocks the nerve responses, but not the externally recorded
oscillations ( Zipser, 1971).
C. Medium Receptors of Morrnyrids
The medium receptors are morphologically more complex than other
electroreceptors. There is an outer and an inner receptor cavity connected
by a small channel (Fig. 21). The outer cavity is, as in other phasic receptors, covered over by loose epithelial tissue and the outer layer of
epidermis but not by the layers of flattened epidermal cells which are
closely apposed to the wall of the outer receptor cavity. Receptor cells
occur in both cavities. In the outer cavity the cells are embedded in its
wall; in the inner cavity the cells protrude into the cavity and are covered
by microvilli on their lumenal surfaces. Three nerve fibers innervate each
M. V. L. BENNETT
other experiments similar results are obtained with oscillations following
much smaller and briefer stimuli (Fig. 32). All-or-none impulses can be
evoked at the end of the unresponsive state induced by strong cathodal
stimuli, and these responses show little net current flow. It can be concluded that these phasic receptors also exhibit the properties indicating
the presence of a series capacity. As in the mormyrids the morphological
relationships suggest that the capacity is in the outer faces of the receptor
cells. The polarity of the externally recorded responses is consistent with
a conventional polarity of responsiveness in the inner faces. Although the
tendency to oscillate is unusually great, similar graded subthreshold
oscillations are predicted by the Hodgkin-Huxley equations (Mauro et al.,
1970).
In gymnotid phasic receptors the response in the nerve is poorly
correlated with the response recorded outside the receptor. There may
be oscillations outside the receptor with no nerve discharge, and the nerve
discharge can outlast the externally recorded oscillations. Also, when the
external resistance is increased by removing overlying water, the small
and brief nerve impulse can be recorded external to the receptor in the
same way as at tonic receptors (Fig. 15; Bennett, 1967). These data by
themselves suggest that the externally recorded oscillations are generated
by the receptor cells and that transmission to the nerve is chemically
mediated. Confirmation of chemically mediated transmission was obtained by experiments similar to those of Fig. 15 where it was found that
strong cathodal stimuli do not block responses initiated by brief anodal
stimuli and that there is a synaptic delay. Furthermore, the external response at phasic receptors can be obtained at least one month following
denervation which is after externally recorded nerve impulses have disappeared at both phasic and tonic receptors (Bennett, 1967). Finally,
tetrodotoxin blocks the nerve responses, but not the externally recorded
oscillations ( Zipser, 1971).
C. Medium Receptors of Morrnyrids
The medium receptors are morphologically more complex than other
electroreceptors. There is an outer and an inner receptor cavity connected
by a small channel (Fig. 21). The outer cavity is, as in other phasic receptors, covered over by loose epithelial tissue and the outer layer of
epidermis but not by the layers of flattened epidermal cells which are
closely apposed to the wall of the outer receptor cavity. Receptor cells
occur in both cavities. In the outer cavity the cells are embedded in its
wall; in the inner cavity the cells protrude into the cavity and are covered
by microvilli on their lumenal surfaces. Three nerve fibers innervate each