528
M. V. L. BENNETT
cell remain constant, the voltag+time integral of the externally recorded
response will be zero, or in graphical terms the area under the positive
phase of the externally recorded impulse will equal the area under the
negative phase.
In summary, three properties resulting from the presence of a series
capacity have been described: (1) equivalence of onset and termination
of long-lasting stimuli of opposite polarity; (2) absence of excitability
change during maintained stimuli; and (3) absence of net current flow
during externally recorded responses.
A. Large Receptors of Mormyrids
These receptors contain 1-8 large receptor cells that are all innervated
by a single fiber (Harder, 1968; Szabo, 1965). Those containing the larger
numbers are the ones that are visually recognizable in vivo as belonging
to this class of receptor. A cross section through the base of one receptor
cell and tangential to another is shown in Fig. 19. Each receptor cell is
covered by microvilli on its lumenal surface and largely fills the cavity
that it occupies.
The response of a large receptor in the mormyrids is illustrated in
Fig. 25. The first trace shows activity in the single fiber that innervates
the receptor, the second trace shows the potential recorded outside the
receptor, and the third trace shows the stimulating current. Figure 25A
and E give the threshold responses to anodal and cathodal stimuli which
are long lasting in that lengthening them has no effect on the responses.
At the onset of the anodal stimulus, a brief all-or-none impulse is recorded outside the receptor. The response to a cathodal pulse is identical
but occurs following the termination of the stimulus at a threshold amplitude identical to that for an anodal stimulus. The externally recorded
impulse is always followed by an impulse in the innervating fiber.
In Fig. 25 the externally recorded response is about 5 mV in amplitude. When the preparation is immersed in water the spikes are 1-2 mV
in amplitude. When the skin is allowed to dry, the amplitude can approach 20 mV. The maximum potential is always obtained when the recording electrode is directly on the receptor.
The thresholds of the large receptors, that is, the stimulus strengths
required to evoke firing about 50% of the time, are usually 0.2-0.5 mV.
Often the receptors are spontaneously active, and with sufficient data
processing it is probable that they could be shown to be affected by much
smaller voltages. When the stimulus strength is increased, the response
changes little, except that it occurs in every sweep (Figs. 25B, C, F, and
M. V. L. BENNETT
cell remain constant, the voltag+time integral of the externally recorded
response will be zero, or in graphical terms the area under the positive
phase of the externally recorded impulse will equal the area under the
negative phase.
In summary, three properties resulting from the presence of a series
capacity have been described: (1) equivalence of onset and termination
of long-lasting stimuli of opposite polarity; (2) absence of excitability
change during maintained stimuli; and (3) absence of net current flow
during externally recorded responses.
A. Large Receptors of Mormyrids
These receptors contain 1-8 large receptor cells that are all innervated
by a single fiber (Harder, 1968; Szabo, 1965). Those containing the larger
numbers are the ones that are visually recognizable in vivo as belonging
to this class of receptor. A cross section through the base of one receptor
cell and tangential to another is shown in Fig. 19. Each receptor cell is
covered by microvilli on its lumenal surface and largely fills the cavity
that it occupies.
The response of a large receptor in the mormyrids is illustrated in
Fig. 25. The first trace shows activity in the single fiber that innervates
the receptor, the second trace shows the potential recorded outside the
receptor, and the third trace shows the stimulating current. Figure 25A
and E give the threshold responses to anodal and cathodal stimuli which
are long lasting in that lengthening them has no effect on the responses.
At the onset of the anodal stimulus, a brief all-or-none impulse is recorded outside the receptor. The response to a cathodal pulse is identical
but occurs following the termination of the stimulus at a threshold amplitude identical to that for an anodal stimulus. The externally recorded
impulse is always followed by an impulse in the innervating fiber.
In Fig. 25 the externally recorded response is about 5 mV in amplitude. When the preparation is immersed in water the spikes are 1-2 mV
in amplitude. When the skin is allowed to dry, the amplitude can approach 20 mV. The maximum potential is always obtained when the recording electrode is directly on the receptor.
The thresholds of the large receptors, that is, the stimulus strengths
required to evoke firing about 50% of the time, are usually 0.2-0.5 mV.
Often the receptors are spontaneously active, and with sufficient data
processing it is probable that they could be shown to be affected by much
smaller voltages. When the stimulus strength is increased, the response
changes little, except that it occurs in every sweep (Figs. 25B, C, F, and