A Tip for Differentiating Between Sensory and Muscle Action Potentials
The four potentials were recorded by changing the subject's hand position. The first trace (A) represents the potential recorded in the anatomically relaxed position; the second trace (B) represents the potential recorded with his fifth digit abducted; the third trace (C) represents the potential recorded with his fifth digit flexed at the metacarpophalangeal joint; and the fourth trace (D) represents the potential recorded with his index, middle, and ring digits flexed at their metacarpophalangeal joints.
The strength of the stimuli was supramaximal to evoke the early component of the potentials and was the same throughout the four trials.
Note that the configuration of an early element with an onset latency of approximately 2 milliseconds was stable in every trace; however, the configuration of the later element, with a longer latency, varied depending on the subject's hand position.
The tendon is used for reference in recording compound muscle action potentials because it is presumably electrically inactive.1 However, this is not always true. As Kincaid et al.2 have reported, tendon sites are not electrically inactive. In particular, the area from the hypothenar to the fifth digits would be electrically active after ulnar stimulation.
It is also known that the configuration of compound muscle action potential changes with muscle length, in other words, hand position.3 Conversely, sensory potentials are not affected.
Hence, the early component of the waveform, which has a stable configuration despite the hand position, is a sensory nerve action potential, whereas the later component of the potential, which is transfigured depending on the hand position, originates from the ulnar innervated muscles (Fig. 1).
Motor axons have a threshold similar to that of large myelinated sensory axons.4 Thus, differentiation between sensory and volume-conducted muscle potentials is often difficult in antidromic sensory nerve conduction studies, particularly in diseased states. When electromyographers encounter an ambiguous potential, especially antidromic ulnar sensory studies, they should record the potentials as they change with hand position of subjects. Then, they would be able to identify the stable elements of the potentials regardless of the hand position as sensory nerve action potentials.