Compound sensory action potentials evoked by tactile and by electrical stimulation in normal median and sural nerves

The compound sensory action potential evoked by electrical stimulation provides a measure of the number and physiological properties of myelinated fibers in the nerve but does not allow evaluation of the most distal part of the sensory nerve. This study compares the compound sensory action potential, evoked by electrical and tactile stimuli, and recorded through needle electrodes placed close to the median and sural nerves of 22 normal males aged 16-51 years. The tactile probe, with a slight preindentation, delivered an indentation of the skin of 200 microns at a rate of 400 microns/ms at the tip of digit III and the dorsolateral side of the foot. The responses were recorded from the median nerve at wrist and elbow and from the sural nerve at the lateral malleolus and midcalf. The amplitudes of the responses averaged 0.5 microV and 0.7 microV in the sural and the median nerves (P < 0.02), respectively, which was only 5-10% of the amplitude evoked by electrical stimulation. The mean maximal conduction velocity determined by tactile stimulation was 54 m/s in the sural nerve compared with 65 m/s in the median nerve and similar to that calculated after electrical stimulation. In the median nerve the sensory conduction velocity was 8% faster than the motor conduction velocity. These findings indicated that only a fraction of the fibers in the nerve were activated by the probe and that the response was conducted along large myelinated sensory fibers. The latency of the tactile response was longer than that of the electrically evoked response due to the receptor delay and conduction along thin distal fiber portions. The delay at the mechanoreceptors was about 1 ms in the sural and 0.65 ms in the median nerve (P < 0.01).