Observations on the sensory nature of the intramuscular nerve action potential.

The nature and the origin of the small pre-potential wave which can be recorded immediately before the main muscle action potential on indirect stimulation was studied in the median nerve of 12 normal subjects. It is considered to be a sensory antidromic response recorded from the large afferent fibres which innervate the thumb because it was recorded in all 12 subjects, and the threshold was always below the motor threshold. An antidromic response was recorded with stimulating electrodes at the thumb and recording electrodes at the palm. Both antidromic and orthodromic responses were recorded with stimulating electrodes at the palm. The amplitude of the pre-potential was higher at the recording point closest to the sensory fibres for the thumb and progressively decreased with distance in the other points.