5. TEMPERATURE RECEPTORS
125
IV. ELECTROPHYSIOLOGY
A. Teleosts
It is likely that a number of unreported attempts have been made to
identify impulse activity from the thermoreceptors in teleost skin, but so
far none has proved successful (Hoagland, 1935, reported such a negative
result). One possible reason for this is that surviving preparations cannot
normally be isolated from the body for electrophysiology in the way that
they can be in elasmobranchs or amphibians. In most teleosts the sense
organs must be left intact and the fish maintained under artificial
respiration during the experiment.
But there may also be a much more serious theoretical reason, namely,
that there are no specific thermoreceptors at all, i.e., no nerve fibers
mediating responses to temperature or temperature change alone, but
only mechanoreceptors whose responses to touch are sensitively altered
according to skin temperature and its changes (Spath, 1967). In the redeye, Leuciscus rutilus, immobilized with curare and under artificial gill
ventilation, Spath found no impulse discharge in response to thermal
stimuli alone, in cutaneous nerves, but the phasic impulse responses to
standardized single or repeated mechanical stimuli were digerent at
different temperatures, and they varied following sudden temperature
changes. Figures 1-3 illustrate his results.
An example of the response to a standard mechanical stimulus and the
way in which it can be modified is shown in Fig. 1. The nature of the
temperature effect in this experiment will be described later. The simplest
situation in which temperature affects the response to mechanical stimulation is illustrated by Fig. 2, where the standard stimulus is applied at
a number of steady temperatures. The curves show that there is an
“optimum” ( i.e., the response consists of the greatest number of impulses)
at a temperature in the middle of the range covered by the experiment,
in this instance between about 20” and 22°C. The optimum temperature
depends to some extent on the temperature to which the fish had previously been adapted (15°C for 4 weeks in this example). The optimum
for fish adapted to 5°C was at 18”C, but for fish adapted to 25°C it fell
above the range tested, i.e., the response increased all the way to 30°C.
On changing the temperature suddenly by means of the water flowing over the stimulus point, the responses do not merely alter at once
to the level characteristic of the new temperature. There is a specific
effect of change. Mechanical stimuli applied within a few seconds of a rise
of temperature produce a smaller response than at the previous steady
125
IV. ELECTROPHYSIOLOGY
A. Teleosts
It is likely that a number of unreported attempts have been made to
identify impulse activity from the thermoreceptors in teleost skin, but so
far none has proved successful (Hoagland, 1935, reported such a negative
result). One possible reason for this is that surviving preparations cannot
normally be isolated from the body for electrophysiology in the way that
they can be in elasmobranchs or amphibians. In most teleosts the sense
organs must be left intact and the fish maintained under artificial
respiration during the experiment.
But there may also be a much more serious theoretical reason, namely,
that there are no specific thermoreceptors at all, i.e., no nerve fibers
mediating responses to temperature or temperature change alone, but
only mechanoreceptors whose responses to touch are sensitively altered
according to skin temperature and its changes (Spath, 1967). In the redeye, Leuciscus rutilus, immobilized with curare and under artificial gill
ventilation, Spath found no impulse discharge in response to thermal
stimuli alone, in cutaneous nerves, but the phasic impulse responses to
standardized single or repeated mechanical stimuli were digerent at
different temperatures, and they varied following sudden temperature
changes. Figures 1-3 illustrate his results.
An example of the response to a standard mechanical stimulus and the
way in which it can be modified is shown in Fig. 1. The nature of the
temperature effect in this experiment will be described later. The simplest
situation in which temperature affects the response to mechanical stimulation is illustrated by Fig. 2, where the standard stimulus is applied at
a number of steady temperatures. The curves show that there is an
“optimum” ( i.e., the response consists of the greatest number of impulses)
at a temperature in the middle of the range covered by the experiment,
in this instance between about 20” and 22°C. The optimum temperature
depends to some extent on the temperature to which the fish had previously been adapted (15°C for 4 weeks in this example). The optimum
for fish adapted to 5°C was at 18”C, but for fish adapted to 25°C it fell
above the range tested, i.e., the response increased all the way to 30°C.
On changing the temperature suddenly by means of the water flowing over the stimulus point, the responses do not merely alter at once
to the level characteristic of the new temperature. There is a specific
effect of change. Mechanical stimuli applied within a few seconds of a rise
of temperature produce a smaller response than at the previous steady