Irreverisble differential block of A- and C-fibres following local nerve heating in the cat.

1. Local heating of a nerve was used to block conduction in the myelinated or A-fibres while preserving normal function in most of the non-myelinated or C-fibres. Compound action potentials of A- and C-fibres, and impulses in single C-fibres were recorded to measure the block. 2. Most experiments were done on the posterior tibial nerve of the cat's hind limb. Conducted heat was applied in successive periods of about 2 min each to 15 mm of the exposed nerve. 3. The range of temperatures used was between 45 and 52 degrees C. Below 46 degrees C no conduction block was obtained. With increasing temperature the cumulative time of heating required to produce a block in at least 99% of A-fibres decreased from around 110 min at 46.5 degrees C to 10 min at 51 degrees C. 4. In the posterior tibial nerve the C-fibre compound action potential was reduced to 0.66 of control (mean of twenty-two experiments) when the A-fibre action potential was less than 1% of control. The corresponding value of this selectivity coefficient was 0.20 in seven superficial peroneal and sural nerves. 5. Once a conduction block of A-fibres was reached the remaining C-fibres showed normal functioning, including conduction of repetitive impulses at up to 30 Hz evoked by electrical nerve and adequate skin stimulation. C-fibre conduction persisted for at least 33 h, but not longer than 3 days. 6. This pure C-fibre nerve might be useful for studying central nervous system effects of C-fibres.