- Nerve fibres from the facial pit of rattlesnakes (Crotalus) usually show a continual non-rhythmic discharge in the absence of environmental change.
- The adequate stimulus for increasing this activity is a relative increase in the influx or a decrease in efflux of radiant energy in the middle and long infrared regions. Equally effective stimuli, which however inhibit the spontaneous discharge, are relative increases in efflux or decreases in influx.
- No response is obtained to sound vibration, a number of chemicals or heat-filtered light, but change in temperature by conduction from ambient media and mechanical deformation of the sensory membrane stimulate. It is concluded that these are minor or incidental in ordinary conditions.
- All the sensory nerve fibres from the pit appear to be warm receptor fibres. The mechanical sensibility resides in the same fibres.
- An object provides a stimulus if its temperature contrasts with that of the background, independent of the temperature of the snake or of the air, within limits. If the background is warm, regardless of snake or air temperature, an object 0.1°C cooler elicits a 'cold' response and vice versa.
- The normal sensibility is directional. Different receptor units have different, sharply defined cones of reception.
- Sensitivity of units by oscilloscopic end-point extends to less than 3 x 10-4 cal/cm2/sec or a dose of 5 x 10-10 cal in 0.1 sec on the area of terminal ramification of one fibre. By behavioural end-point (Noble & Schmidt, 1937) these figures would be 1.3 x 10-5 and 2 x 10-11. Calculated as change in temperature of the membrane, these two end-points give 0.02 and 0.001°C. Direct measurement of the change, necessary to elicit a response, in temperature of water flowing over the membrane gave values of 0.003-0.005°C.
- The initial burst of response to a sudden warm stimulus rises in frequency from 2.2 to 4.5 times higher for each tenfold increase in intensity but never exceeds about 200/sec. The useful range in intensity is usually not above 1: 500. Latency is from 10 to 150 msec, depending on intensity.
- Adaptation to weak stimuli occurs in several seconds and is complete. Stimuli 10 to 30 times threshold produce long-lasting discharge with an early and a late phase of adaptation. The latter may not begin for several minutes and may require many minutes to come to equilibrium, but is then almost or quite complete. Stimuli several hundred times threshold (several tenths of a degree C) elicit a complex sequence of phases: burst, silence, burst, depression and after minutes a plateau slightly above the original spontaneous level. The interrelationship of these and the effect of varying stimulus intensity suggest that high intensity involves several separate intracellular processes of both signs and different time-courses. High intensity intermittency in the form of stuttering bursts and of waxing and waning surges are described.
- After the abrupt end of a stimulus one of several sequences follows, involving after-discharge, off burst, postexcitatory depression or silence, supernormal or overshooting recovery and even later phases.
- The sensitivity to different steady-state temperatures or radiant fluxes is very low. Approximate values are given for the slowest rates of change which are physiologically equivalent to a square stimulus at different final intensities and at different durations of rise and similarly for the threshold rate of change. The relation is complex, but the physiologically just square stimulus is at least several tenths of a second in rising and the threshold slope may require several minutes. At any one final intensity, the ratio between these, i.e. the range of slope which can be discriminated, is much narrower than it is comparinig high and low final intensities. The receptor cannot properly be called a rate-of-change detector, but must be regarded as an intensity detector with adaptations of varying time-course. The rate of change determines not only sensitivity (Q10 about 4 and 10^30 at threshold and effectively square slopes respectively) but also useful range, resolution, maximum frequency and symmnetry of response/temperature curve.
- The non-rhythmicity and fluctuations in frequency, even when integrated over some seconds, apparently inhere in the unit receptor not in the environment or in synchronized changes in many units.
- It is concluded that we are dealing with a reception of the temperature of the tissue, not of specific wave-lengths as in the eye. We have failed to find any similar response in the homologous nerve in non-crotalids and in other nerves of crotalids.
- In an evaluation of the biological significance of the pit organ, the high specialization of physiological properties and anatomy are pointed out. Not only is the sensitivity thereby increased, in particular to small doses of radiant energy (e.g. from a mouse distant many cm, against common backgrounds) but a directional analysis is possible. The properties, however, suggest a relatively non-discriminating, largely presence-or-absence-signalling sense organ. Presumably crotalids should have an advantage hunting warm-blooded or cool (moist) prey at night or under ground and in locating warm or cool regions of the substratum.
- This organ presents in exaggerated form the general problem of the central nervous system in recognizing valid signals of environmental change on a background of fluctuating non-rhythmic spontaneity. The central analysing mechanism must sacrifice either sensitivity or temporal resolution or compromise between them, but it can improve its recognition of signals materially if it can perform three integrations with different time-constants and derive a value from them for the unit receptor signal. Additionally, by sacrificing resolution of shadows and hence directionality, it can sum signals in different afferent fibres to improve signal-to-noise ratio.
- Comparison with the other physiologically studied temperature receptors indicates that this one is far more sensitive to small steps but far less sensitive to maintained temperature. The snake's warm receptors are strikingly different from the others in respect to the block, slow adaptation and complex sequence of phases following strong stimuli.
- A simple electronic device is described for automatically and continuously plotting on the cathode-ray oscilloscope the intervals between successive nerve impulses in a single unit as a function of time.
The range of the action spectrum
It has been possible to obtain some information on the effective wave-lengths though not to plot quantitativelythe response to different wave-lengths with equal energy doses or the doses required for equal response. The reasons for the difficulty of such quantitative determination will be clear in the sections below on fluctuation of background frequency and of response frequency under given conditions. With a rock salt infra-red monochromator and Nernst glower as source, strong response was obtained in the range 1.5-4.0 µm peak wave-length, while no response occurred at 1.0, doubtful responses at 5-7, and none at 10.0 µm. The source emits strongly at 2.0 µm and falls off steeply on either side so that the stimulus available at 10 µm was weak, but the absence of response at 10 µm is probably significant as the lower limit.
By the use of heat filters which pass almost all the visible light (slightly greenish to the eye) and cut off more efficiently above 1.5µm, especially the Schott-Jena BG21, quite strong beams of light are rendered completely ineffective, for example, a direct, focused beam from a B and L spherical microscope lamp at full iris with a 100 W bulb. A zirconium arc of 100 W with a glass condensing lens or stronger incandescent sources will still deliver some stimulus through such filters, indicating that although visible wave-lengths are probably quite ineffective the near infrared (0.7-1.0 µm) is weakly effective. Inserting the Schott-Jena RG 7 (opaque to visible rays, but passing nearly all infrared) does not reduce the effectiveness of such beams noticeably, but a slight response when the filter is removed indicates that some visible or near infra.red is stimulating.
Naked sources such as the Nernst glower or a match (with peaks at about 2-2.3 µm) are rendered completely ineffective by a filter of a few mm of glass, or of water in glass, which cut off energy longer than 1.5-2.6 µm so that these longer wave-lengths must be part of the action spectrum. Glass-enclosed sources such as incandescent lamps fitted with condensing lenses have already suffered virtually complete removal of energy longer than 2.6 µm, but are still useful stimulators when sufficiently intense owing to radiation from the glass.
By the use of filters, wave-lengths longer than 3 or 4 µm are almost impossible to deliver without contamination of energy at 2 to 3µm. Our monochromator has too little energy in the long infra-red region for us to expect stimulation. But the efficacy of such sources as the hand, small mammals and black bodies at 1° C above a neutral temperature is good evidence that wave-lengths at least as long as 15 µm must be effective, for these low temperature emitters have their peak in this region. It will be shown below that the calculated threshold flux from a rat is the same as or lower than that from a Nernst glower, whose peak of emission is at 2µm.
It is reasonable to conclude that the action spectrum does not extend below 1.5 µm, at least not with a sensitivity above a small fraction of that at 2 µm, and that it extends at least out to 15 µm, where sensitivity is as great as at 2 µm.
Added by: Sarina Last edited by: Sarina