M. V. L. BENNETT
524
ficial epidermis (Barets and Szabo, 1964; Derbin and Szabo, 1968; Lissmann and Mullinger, 1968; Szabo and Wersall, 1970; Szamier and
Wachtel, 1970; Wachtel and Szamier, 1966). The layers of flattened cells
that presumably increase the skin resistance do not cover the receptor,
but they are closely apposed to the cells forming the receptor wall
which is roughly cylindrical with ordinary epidermis at the top and the
receptor cells at the bottom. Presumably there is little electrical resistance between the receptor lumen and exterior, but evidence is lacking
beyond the morphological relations and receptor sensitivity to applied
fields. As in the tonic receptors, zonular tight junctions are present between cells abutting on the receptor lumen including the receptor cells
(Fig. 22; Lissmann and Mullinger, 1968; Szamier and Wachtel, 1970).
Presumably these junctions increase current crossing the receptor cells
by sealing off the intercellular clefts. The receptor walls of phasic receptors tend to have many more layers than those of tonic receptors. This
feature should reduce the capacity of the wall, and perhaps it is related
to the greater sensitivity of phasic receptors to high frequencies.
In gymnotids the phasic receptor cells protrude' into the receptor
cavity and only their bases are attached. The largest part of the surface
is exposed in the cavity and this portion is greatly increased by a microvillous border ( Figs. 19 and 22). Similar characteristics are present in two
out of the three kinds of phasic receptor cells in mormyrids. At the base
of the receptor cells are their synapses with the innervating nerve fiber.
As at tonic receptors, there are presynaptic vesicles and ribbons or dense
bodies (Fig. 23; Derbin and Szabo, 1968). There is evidence that transmission is chemically mediated except in one case in which it appears to
be electrically mediated.
The equivalent circuit of a phasic receptor is similar to that of a tonic
receptor in respect to the resistances of the skin, external medium, and
internal tissue. However, the outer face of the receptor cells behaves as
a series capacity while the inner face has electrically excitable as well as
secretory membrane (Fig. 9). The inner face may generate graded
responses or even spikes.
The presence of a series capacity explains several characteristics of
phasic receptors, or to put it more logically, the demonstration of these
characteristics provides evidence for the existence of the series capacity.
The effects of a series capacity can be described with respect to Fig. 24
and the equivalent circuit of Fig. 9. If a voltage step is applied outside
the receptor, the potential across the inner face of the receptor cells is
quite distorted. Initially, there is no voltage drop across the capacity and
a large part of the applied voltage appears across the inner face (Figs.
24A and B). Gradually, however, the capacity of the outer face is charged
524
ficial epidermis (Barets and Szabo, 1964; Derbin and Szabo, 1968; Lissmann and Mullinger, 1968; Szabo and Wersall, 1970; Szamier and
Wachtel, 1970; Wachtel and Szamier, 1966). The layers of flattened cells
that presumably increase the skin resistance do not cover the receptor,
but they are closely apposed to the cells forming the receptor wall
which is roughly cylindrical with ordinary epidermis at the top and the
receptor cells at the bottom. Presumably there is little electrical resistance between the receptor lumen and exterior, but evidence is lacking
beyond the morphological relations and receptor sensitivity to applied
fields. As in the tonic receptors, zonular tight junctions are present between cells abutting on the receptor lumen including the receptor cells
(Fig. 22; Lissmann and Mullinger, 1968; Szamier and Wachtel, 1970).
Presumably these junctions increase current crossing the receptor cells
by sealing off the intercellular clefts. The receptor walls of phasic receptors tend to have many more layers than those of tonic receptors. This
feature should reduce the capacity of the wall, and perhaps it is related
to the greater sensitivity of phasic receptors to high frequencies.
In gymnotids the phasic receptor cells protrude' into the receptor
cavity and only their bases are attached. The largest part of the surface
is exposed in the cavity and this portion is greatly increased by a microvillous border ( Figs. 19 and 22). Similar characteristics are present in two
out of the three kinds of phasic receptor cells in mormyrids. At the base
of the receptor cells are their synapses with the innervating nerve fiber.
As at tonic receptors, there are presynaptic vesicles and ribbons or dense
bodies (Fig. 23; Derbin and Szabo, 1968). There is evidence that transmission is chemically mediated except in one case in which it appears to
be electrically mediated.
The equivalent circuit of a phasic receptor is similar to that of a tonic
receptor in respect to the resistances of the skin, external medium, and
internal tissue. However, the outer face of the receptor cells behaves as
a series capacity while the inner face has electrically excitable as well as
secretory membrane (Fig. 9). The inner face may generate graded
responses or even spikes.
The presence of a series capacity explains several characteristics of
phasic receptors, or to put it more logically, the demonstration of these
characteristics provides evidence for the existence of the series capacity.
The effects of a series capacity can be described with respect to Fig. 24
and the equivalent circuit of Fig. 9. If a voltage step is applied outside
the receptor, the potential across the inner face of the receptor cells is
quite distorted. Initially, there is no voltage drop across the capacity and
a large part of the applied voltage appears across the inner face (Figs.
24A and B). Gradually, however, the capacity of the outer face is charged