11. ELECXRORECEPTION
533
Ultrastructural studies of the large receptors are in progress and
should provide further evidence as to mode of synaptic transmission.
Available data indicate that small branches of the nerve form some receptor synapses typical of those where transmission is chemically mediated (Derbin and Szabo, 1968). Morphological evidence of electrically mediated transmission has not been found but could have
been missed. It should be recalled that morphological features associated
with chemically mediated transmission often occur at electrically transmitting synapses, even when there is little or no chemically mediated
component in the PSPs ( Bennett et al., 1967a,b). Transmission at least at
one other receptor synapse appears to be electrically mediated. At calyx
synapses of the vestibular system the cuplike shape of the nerve terminal
would greatly impede postsynaptic currents through chemically sensitive
membrane on the inner surface of the calyx, and on this basis alone electrically mediated transmission appears likely ( Bennett, 1964). Recently,
close appositions between receptor cell and afferent nerve have been
found that resemble electrotonic synapses at other sites (Spoendlin,
1966; see Figs. 71 and 72 in Chapter 10, this volume).
B. Phasic Receptors of Gymnotids
The phasic receptors of gymnotids generally contain 10 or more receptor cells in a single receptor cavity (Fig. 20). The cells protrude into
the cavity, and the lumenal surface is further increased by microvilli
(Fig. 22). All the cells are innervated by a single nerve fiber, and in some
species (Eigenmunnia and Sterrwpygus) a group of receptors lying close
together are also innervated by the same fiber.
The response of a phasic receptor in the gymnotid Gymnotus is shown
in Fig. 28. The first trace shows stimulating current, the second trace
shows the response recorded outside the receptor, and the third trace
shows the impulses in the innervating nerve. All but the largest stimulus
(Fig. 28H) evoke damped oscillations outside the receptor. These responses occur at onset and termination of both anodal and cathodal
stimuli. As the stimulus strength is increased, the responses to onset of
anodal and termination of cathodal stimuli gradedly increase in amplitude and finally become spikelike (except in Fig. BH, see below). These
are responses where the stimuli depolarize the inner face of the receptor
cells. The responses to termination of anodal and onset of cathodal stimuli
increase in amplitude as stimulus strength is increased, but they are progressively delayed. These are responses where the stimuli hyperpolarize
the inner face of the receptor cells and presumably are analogous to
533
Ultrastructural studies of the large receptors are in progress and
should provide further evidence as to mode of synaptic transmission.
Available data indicate that small branches of the nerve form some receptor synapses typical of those where transmission is chemically mediated (Derbin and Szabo, 1968). Morphological evidence of electrically mediated transmission has not been found but could have
been missed. It should be recalled that morphological features associated
with chemically mediated transmission often occur at electrically transmitting synapses, even when there is little or no chemically mediated
component in the PSPs ( Bennett et al., 1967a,b). Transmission at least at
one other receptor synapse appears to be electrically mediated. At calyx
synapses of the vestibular system the cuplike shape of the nerve terminal
would greatly impede postsynaptic currents through chemically sensitive
membrane on the inner surface of the calyx, and on this basis alone electrically mediated transmission appears likely ( Bennett, 1964). Recently,
close appositions between receptor cell and afferent nerve have been
found that resemble electrotonic synapses at other sites (Spoendlin,
1966; see Figs. 71 and 72 in Chapter 10, this volume).
B. Phasic Receptors of Gymnotids
The phasic receptors of gymnotids generally contain 10 or more receptor cells in a single receptor cavity (Fig. 20). The cells protrude into
the cavity, and the lumenal surface is further increased by microvilli
(Fig. 22). All the cells are innervated by a single nerve fiber, and in some
species (Eigenmunnia and Sterrwpygus) a group of receptors lying close
together are also innervated by the same fiber.
The response of a phasic receptor in the gymnotid Gymnotus is shown
in Fig. 28. The first trace shows stimulating current, the second trace
shows the response recorded outside the receptor, and the third trace
shows the impulses in the innervating nerve. All but the largest stimulus
(Fig. 28H) evoke damped oscillations outside the receptor. These responses occur at onset and termination of both anodal and cathodal
stimuli. As the stimulus strength is increased, the responses to onset of
anodal and termination of cathodal stimuli gradedly increase in amplitude and finally become spikelike (except in Fig. BH, see below). These
are responses where the stimuli depolarize the inner face of the receptor
cells. The responses to termination of anodal and onset of cathodal stimuli
increase in amplitude as stimulus strength is increased, but they are progressively delayed. These are responses where the stimuli hyperpolarize
the inner face of the receptor cells and presumably are analogous to