It is an old observation that cold blocks nervous conduction and that nerve fibres subserving different modalities differ in their susceptibilities. Commonly it is believed that susceptibility to cold block is related to the diameter of the individual nerve fibre. The evidence for this belief is, however, fragmentary and conflicting. Comparing C fibres with A fibres, Lundberg (1952) found that the former were most resistant to cold block, but in Sinclair & Hinshaw's (1951) experiments the fibres most resistant to cold were not C fibres but certain fibres of the A group. A similar divergence of results is also apparent in experiments to assess the differential susceptibilities of different sized fibres within the large (myelinated) fibre group itself. In some experiments the largest of these fibres have proved the most sensitive to cold block (Torrance & Whitteridge, 1947;Whitteridge, 1948), while in others (Lundberg, 1952;Dodt, 1953) they have proved least sensitive. The work reported here describes the effects of localized cooling on several nerves in the cat.
METHODSMost experiments were made on cats under chloralose anaesthesia (80-100 mg/kg intravenously after ethyl chloride and ether). Some cats were anaesthetized with Nembutal (40 mg/kg intraperitoneally, supplemented as necessary).Nerves and spinal roots prepared for study in situ were immersed in liquid paraffin held in a bath formed by suitable arrangement of the skin flaps. In some experiments this paraffin bath was warmed by an immersion heater. Sometimes it was saturated with 5 % CO2 in 02. In other experiments the nerve studied was dissected free of the animal and lightly stretched across silver electrodes in a paraffin bath saturated with 5 % CO2 in 02. The nerves were stimulated at one or more points by square pulses delivered through silver electrodes. Action potential records were taken through silver electrodes and displayed on a double-beam cathode-ray oscillograph. Cold was applied to a length of the nerve between the point of recording and the point of stimulation by the thermode mllustrated in Fig. 1. Several interchangeable heads were used with this thermode to allow lengths from 2 to 20 mm to be cooled, and to accommodate nerves of different diameters.
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