R. W. MURRAY
130
does not preclude them from mediating responses to both warming and
cooling. The idea of either increase or decrease in a resting frequency
forming the basis of sensation has been implicit in sensory physiological
theory since Lowenstein and Sand's demonstration of the resting discharge from semicircular canals ( Lowenstein and Sand, 1936), although
strict experimental proof is rare ( e.g., Lowenstein, 1937).
As for the question of sensitivity, here again Spath's system would
seem to be just adequate, He records a 10% change in the impulse response
to standard mechanical stimulation on changing the temperature by about
1.5"C. Clearly this 10% is well below threshold for a single receptor, owing
to the small number of impulses involved in a single response (see Fig.
l ) , but it works out as a possible, although optimistic, threshold when
the results of a number of units are summed, as in his published results.
It is known experimentally that spatial summation occurs in fishes
(Bardach, 1956), the threshold falling from about 10°C for point
stimulation, to 2°C with a stimulus area of 2 mm2, and to less than 0.1"C
for the whole fish. Spath's stimulator had an area of about 1 mm2; thus,
his results are close to those obtained behaviorally, when an allowance is
made for some temporal summation and for the possibility that his recording did not show the activity of all the fibers which were in fact stimulated. The greater sensitivity obtained by summation results from the
statistical advantages to be gained from using a larger sample.
B. Elasmobranchs
Cutaneous nerves are known with sensitivities to either warming or
cooling ( Murray, 1961). However, Murray's experimental method,
while demonstrating the existence of bimodal touch-temperature units
analogous to those of teleosts, did not allow the possible existence of pure
thermoreceptors to be explored fully. The method involved recording
single unit activity by means of a wire electrode resting on the surface
of the damp skin (in Raja sp.); thus, the touch of the electrode which
was, of course, necessary to ensure electrical continuity in the recording
circuit, provided a continuous, controlled mechanical stimulus. Details
of three individual units were given. In the first, where there was complete adaptation to the mechanical stimulus, rapid radiant warming (at
lS°C/sec) elicited impulse activity, with a threshold 2"-3°C above room
temperature. This unit could have been either bimodal, i.e., hot-touch,
or possibly a specific warm unit, lying within range of the recording
electrode. The second was a cold-touch unit, responding to warming with
a decrease in the nonadapting response to the presence of the electrode.
The third, the most sensitive, was also a cold-touch unit, for the phasic
130
does not preclude them from mediating responses to both warming and
cooling. The idea of either increase or decrease in a resting frequency
forming the basis of sensation has been implicit in sensory physiological
theory since Lowenstein and Sand's demonstration of the resting discharge from semicircular canals ( Lowenstein and Sand, 1936), although
strict experimental proof is rare ( e.g., Lowenstein, 1937).
As for the question of sensitivity, here again Spath's system would
seem to be just adequate, He records a 10% change in the impulse response
to standard mechanical stimulation on changing the temperature by about
1.5"C. Clearly this 10% is well below threshold for a single receptor, owing
to the small number of impulses involved in a single response (see Fig.
l ) , but it works out as a possible, although optimistic, threshold when
the results of a number of units are summed, as in his published results.
It is known experimentally that spatial summation occurs in fishes
(Bardach, 1956), the threshold falling from about 10°C for point
stimulation, to 2°C with a stimulus area of 2 mm2, and to less than 0.1"C
for the whole fish. Spath's stimulator had an area of about 1 mm2; thus,
his results are close to those obtained behaviorally, when an allowance is
made for some temporal summation and for the possibility that his recording did not show the activity of all the fibers which were in fact stimulated. The greater sensitivity obtained by summation results from the
statistical advantages to be gained from using a larger sample.
B. Elasmobranchs
Cutaneous nerves are known with sensitivities to either warming or
cooling ( Murray, 1961). However, Murray's experimental method,
while demonstrating the existence of bimodal touch-temperature units
analogous to those of teleosts, did not allow the possible existence of pure
thermoreceptors to be explored fully. The method involved recording
single unit activity by means of a wire electrode resting on the surface
of the damp skin (in Raja sp.); thus, the touch of the electrode which
was, of course, necessary to ensure electrical continuity in the recording
circuit, provided a continuous, controlled mechanical stimulus. Details
of three individual units were given. In the first, where there was complete adaptation to the mechanical stimulus, rapid radiant warming (at
lS°C/sec) elicited impulse activity, with a threshold 2"-3°C above room
temperature. This unit could have been either bimodal, i.e., hot-touch,
or possibly a specific warm unit, lying within range of the recording
electrode. The second was a cold-touch unit, responding to warming with
a decrease in the nonadapting response to the presence of the electrode.
The third, the most sensitive, was also a cold-touch unit, for the phasic