Low-frequency filter was 0.1 Hz, high-frequency filter was 2 kHz, sweep speed was 1 ms/division, duration of pulse was 0.1 ms, rate of pulse was 1/s, and the machines used were a Grass S8 stimulator and a dual-beam Tektronix 565 oscilloscope. Sensitivity was not specified.

This study was performed in the supine position.

Following the orthodromic method [1], signals were recorded on the course of the median nerve (Fig. 1): on the palm (distal to the carpal tunnel, R1) and at the wrist (proximal to the carpal tunnel, R2, R3). Interelectrode distances were as follows: the cathode to the electrode distal to the carpal tunnel (R1), mean 7.81 cm (range 6.5–10.5 cm); the cathode to the electrode proximal to the carpal tunnel (R2), mean 12.15 cm (range 9.95–14.6 cm); across the carpal tunnel (R1–R2), mean 3.8 cm (range 2.75–4.7 cm); and between electrodes proximal to the carpal tunnel (R2–R3), mean 1.94 cm (range 1.85–2.15 cm). Sensory nerve action potentials (SNAPs) were simultaneously recorded from R1 (reference R2), R2 (reference R3), and R3 (reference R4). A large ground (G) electrode was placed between the stimulating and recording electrodes.

The median nerve was stimulated distally to digit III (middle finger) using pipe cleaners soaked in an electrolyte jelly. The pipe cleaners were wound around the proximal (stimulating cathode) and middle (anode) phalanges. Stimulus duration was kept constant at 0.1 ms, and stimulus voltage, which varied from 60 to 110 V, was adjusted to produce a just supramaximal response.

Onset latency (ms) was measured from the stimulus artifact to the onset of the negative deflection of the SNAP. Conduction time was measured as the time from the onset of the negative deflection of the sensory evoked potential at the proximal electrode to a corresponding point on the evoked potential at the distal electrode. Conduction velocity was expressed as meter per second (m/s) and calculated by dividing the interelectrode distance by the conduction time. Negative peak amplitude (μV) was determined from the onset to the peak of the negative peak of the SNAPs. In each recording, 20 responses were photographed, and measurements were made on ten times enlarged superimposed line drawings. Skin temperature adjacent to one recording electrode was continuously monitored and kept between 32 and 33 °C. The authors recorded SNAPs on the palm and at the wrist in 30 healthy (Table 1) subjects (21 men and 9 women, age range 18–38 years).