11. ELECTRORECEPTION
503
circuits of Fig. 8. A receptor can be represented by a single arm of a
circuit where the skin is a distributed system around it with elements
T,, ri, and T, representing resistances of external medium, internal tissue,
and the skin, respectively. The “access resistance” from the indifferent,
distant electrode to the inside surface of the receptor is small compared
to the resistance through the receptor. Thus, a locally applied stimulus
results in a potential at the inside surface that is negligible with respect
to the potential produced at the receptor opening.
In the next part of this chapter, Sections I11 and IV, the responses of
the receptors to locally applied stimuli are described. In these experiments
it is generally convenient to immobilize the fish and block its electric
organ discharge by curarization.
111. TONIC ELECTRORECEPTORS
A. Characteristics in Gymnotids and Mormyricls
The morphology of a typical tonic receptor in the mormyrids and
gymnotids is diagrammed in the upper part of Fig. 9, and light micrographs of tonic receptors from a mormyrid and a gymnotid are shown
in Figs. 10 and 11.
In these receptors there is a canal that opens to the exterior and
usually enlarges at its base to form an ampulla. A number of receptor
cells are embedded in the wall of the ampulla. Only a small part of their
circumference is exposed to the lumen, although the surface in this
region may be increased by microvilli (Barets and Szabo, 1965; Lissmann
and Mullinger, 1968; Szamier and Wachtel, 1969, 1970). The receptor
cells synapse with the single afferent fiber on their inner surface and are
largely surrounded by what are presumably supporting cells. The
synapses are characterized by large numbers of presynaptic vesicles and
by a presynaptic ribbon or dense body (Fig. 12). Similar features are
found at other receptor synapses of the acoustico-lateralis and visual
systems of vertebrates ( Wersall et al., 1965; Dowling and Boycott, 1965).
Over much or all of the body the epidermis contains a specialized
zone that consists of many layers of flattened cells ( F in Figs. 10 and 11).
Presumably, this layer is responsible for the very superficial resistive
barrier that can be demonstrated in the skin (Section V, A). This layer
is closely apposed to cells of the wall of the receptor canal. There are
zonular tight junctions between cells of the canal walls and the supporting cells around the receptor cells. There are also tight junctions between
the supporting and receptor cells (Fig. 13; Szamier and Wachtel, 1969,
503
circuits of Fig. 8. A receptor can be represented by a single arm of a
circuit where the skin is a distributed system around it with elements
T,, ri, and T, representing resistances of external medium, internal tissue,
and the skin, respectively. The “access resistance” from the indifferent,
distant electrode to the inside surface of the receptor is small compared
to the resistance through the receptor. Thus, a locally applied stimulus
results in a potential at the inside surface that is negligible with respect
to the potential produced at the receptor opening.
In the next part of this chapter, Sections I11 and IV, the responses of
the receptors to locally applied stimuli are described. In these experiments
it is generally convenient to immobilize the fish and block its electric
organ discharge by curarization.
111. TONIC ELECTRORECEPTORS
A. Characteristics in Gymnotids and Mormyricls
The morphology of a typical tonic receptor in the mormyrids and
gymnotids is diagrammed in the upper part of Fig. 9, and light micrographs of tonic receptors from a mormyrid and a gymnotid are shown
in Figs. 10 and 11.
In these receptors there is a canal that opens to the exterior and
usually enlarges at its base to form an ampulla. A number of receptor
cells are embedded in the wall of the ampulla. Only a small part of their
circumference is exposed to the lumen, although the surface in this
region may be increased by microvilli (Barets and Szabo, 1965; Lissmann
and Mullinger, 1968; Szamier and Wachtel, 1969, 1970). The receptor
cells synapse with the single afferent fiber on their inner surface and are
largely surrounded by what are presumably supporting cells. The
synapses are characterized by large numbers of presynaptic vesicles and
by a presynaptic ribbon or dense body (Fig. 12). Similar features are
found at other receptor synapses of the acoustico-lateralis and visual
systems of vertebrates ( Wersall et al., 1965; Dowling and Boycott, 1965).
Over much or all of the body the epidermis contains a specialized
zone that consists of many layers of flattened cells ( F in Figs. 10 and 11).
Presumably, this layer is responsible for the very superficial resistive
barrier that can be demonstrated in the skin (Section V, A). This layer
is closely apposed to cells of the wall of the receptor canal. There are
zonular tight junctions between cells of the canal walls and the supporting cells around the receptor cells. There are also tight junctions between
the supporting and receptor cells (Fig. 13; Szamier and Wachtel, 1969,