Human nerve potentials evoked by tactile stimuli. 1. Maximum conduction and properties of compound potentials

Sensory potentials were evoked by tactile stimuli to the dorso-lateral aspect of the foot and recorded via a near nerve electrode from the sural nerve. The averaged potential contained a burst of 6-8 components each 0.3-1.0 microV and a few later components. The averaged potential was reproducible and its time integral was used as a gauge of the size of the response. That the potential was a nerve action potential was indicated by (i) the diminution in amplitude with increasing distance of the electrode from the nerve, (ii) by the reversible abolition of the potential during local anaesthesia of the skin and the subcutaneous tissue and (iii) during anoxia. Fractionate local anaesthesia of the skin demonstrated that the response mainly originated from the activation of Pacinian corpuscles. The innervation area divided by the number of large myelinated fibres in the distal cutaneous branch of the sural nerve gave 10-20 mechanoreceptive units per cm2, one tenth of the density on the index finger. The shortest latency of the potential evoked by a tactile stimulus was 0.5-1.0 ms longer than the latency of the potential evoked by an electrical stimulus. The conduction velocity of the response evoked by a tactile stimulus was slower in the distal than in the proximal part of the nerve. This difference did not appear in the plot of potentials evoked by electrical stimuli probably because the electrical stimulus circumvents the distal portion of the nerve fibres.