530
M. V. L. BENNETT
further investigation; they may be of physiological significance because
the electric organ can provide stimuli in this range.
Several lines of evidence suggest that the receptor cells generate the
externally recorded impulse and that it is not generated by the afferent
fiber. First, denervation of the skin leads to degeneration of the receptors
in a few days to weeks. The externally recorded impulses and receptor
cells disappear at about the same time and somewhat more slowly than
the nerve fibers, although there is considerable overlap in time and time
of block of nerve action potentials is unknown (Roth and Szabo, 1969).*
The second line of evidence for a receptor cell-nerve synapse is that
there is a point of low safety factor for transmission from the receptor to
the innervating nerve (Bennett, 1967). If a pair of stimuli are given, the
second nerve response can fail at a time when there is still a full-sized
response outside the receptor. The point of failure is close to the receptor
because a stronger testing stimulus can evoke a second nerve response
after a shorter interval. The most reasonable place for the point of failure
to be is at the junction between receptor cell and nerve fiber. A third
line of evidence is pharmacological. Tetrodotoxin blocks nerve activity
without affecting the externally recorded impulses (Zipser, 1971). A further but less satisfactory argument for receptor cell potentials at the
large receptors is comparative; receptor cells of other phasic receptors
generate responses that can be recorded external to the receptor opening.
The data in Fig. 25 illustrate the first property described as resulting
from a series capacity. The second property, absence of excitability change
during maintained stimuli, is illustrated in Fig. 26. A brief stimulus evokes
an externally recorded response and nerve impulse at the same threshold
in the presence or absence of a maintained stimulus, whether the maintained stimulus is anodal or cathodal. The third property, absence of net
current flow during an externally recorded response, is shown in Fig. 27.
The impulse recorded outside the receptor is diphasic, and it can be seen
particularly clearly in spontaneous discharges that the area under the
positive phase is very nearly equal to that under the negative phase.
It can be concluded that the large receptors do have a capacity in
series with their responsive membranes. The polarity of the responses is
consistent with generation of a conventional impulse by the inner faces
of the receptor cells. Furthermore, the morphology suggests that the capacity is located in the outer face of the receptor cells. Calculations indicate that the series capacity of the receptor is compatible with the area
* Earlier I stated that the receptor cells survive 1-2 months after denervation
(Bennett, 1965, 1967). This is an error because I did not know that small branches
from the main posterior lateral line nerve innervate some receptors in the dorsal
group (Fig. 2 ) , and I had sectioned only the dorsal branch of the nerve.
M. V. L. BENNETT
further investigation; they may be of physiological significance because
the electric organ can provide stimuli in this range.
Several lines of evidence suggest that the receptor cells generate the
externally recorded impulse and that it is not generated by the afferent
fiber. First, denervation of the skin leads to degeneration of the receptors
in a few days to weeks. The externally recorded impulses and receptor
cells disappear at about the same time and somewhat more slowly than
the nerve fibers, although there is considerable overlap in time and time
of block of nerve action potentials is unknown (Roth and Szabo, 1969).*
The second line of evidence for a receptor cell-nerve synapse is that
there is a point of low safety factor for transmission from the receptor to
the innervating nerve (Bennett, 1967). If a pair of stimuli are given, the
second nerve response can fail at a time when there is still a full-sized
response outside the receptor. The point of failure is close to the receptor
because a stronger testing stimulus can evoke a second nerve response
after a shorter interval. The most reasonable place for the point of failure
to be is at the junction between receptor cell and nerve fiber. A third
line of evidence is pharmacological. Tetrodotoxin blocks nerve activity
without affecting the externally recorded impulses (Zipser, 1971). A further but less satisfactory argument for receptor cell potentials at the
large receptors is comparative; receptor cells of other phasic receptors
generate responses that can be recorded external to the receptor opening.
The data in Fig. 25 illustrate the first property described as resulting
from a series capacity. The second property, absence of excitability change
during maintained stimuli, is illustrated in Fig. 26. A brief stimulus evokes
an externally recorded response and nerve impulse at the same threshold
in the presence or absence of a maintained stimulus, whether the maintained stimulus is anodal or cathodal. The third property, absence of net
current flow during an externally recorded response, is shown in Fig. 27.
The impulse recorded outside the receptor is diphasic, and it can be seen
particularly clearly in spontaneous discharges that the area under the
positive phase is very nearly equal to that under the negative phase.
It can be concluded that the large receptors do have a capacity in
series with their responsive membranes. The polarity of the responses is
consistent with generation of a conventional impulse by the inner faces
of the receptor cells. Furthermore, the morphology suggests that the capacity is located in the outer face of the receptor cells. Calculations indicate that the series capacity of the receptor is compatible with the area
* Earlier I stated that the receptor cells survive 1-2 months after denervation
(Bennett, 1965, 1967). This is an error because I did not know that small branches
from the main posterior lateral line nerve innervate some receptors in the dorsal
group (Fig. 2 ) , and I had sectioned only the dorsal branch of the nerve.