Conduction in unmyelinated fibres in experimental neuropathy

Conduction velocity of autonomic unmyelinated fibres has been measured in the cervical sympathetic trunk of normal rats, and in rats intoxicated by acrylamide or by isoniazid. The mean maximal conduction velocity in nerves from normal rats is 2·0 m/sec. There is no significant reduction in velocity of the unmyelinated fibres in nerves from intoxicated rats, although histological studies of the sural nerve confirmed severe degeneration of myelinated fibres in the same animals. It is shown that the amplitude of the compound nerve action potential is proportional to the resistance between the recording electrodes. If this is taken into account, there is no reduction in the amplitude of the monophasic action potential of unmyelinated fibres recorded from the cervical sympathetic trunk of intoxicated rats. The amplitude of the A component of the sural nerve compound action potential is markedly reduced in rats intoxicated by acrylamide or by isoniazid, but there is no significant reduction in the amplitude of the C component in the same nerve. It is concluded that in the rat an insignificant number of unmyelinated fibres of autonomic or dorsal root origin are affected in the neuropathy produced by acrylamide or isoniazid. The relevance of these findings to human neuropathies is discussed.