Figure 2-11. The median nerve is stimulated percutaneously (1) at the wrist and (2) in the antecubital fossa with the resultant compound muscle action potential recorded over the abductor pollicis brevis (arrow).The motor waveform is recorded as the voltage between the surface electrode and a reference electrode (Ref.) more distally. Sweep 1′ on the display depicts the stimulus artifact followed by the compound muscle action potential. The distal latency, A′, is the time from the stimulus artifact to the onset of the compound muscle action potential and corresponds to conduction over distance A. The same is true for sweep 2′, where stimulation is at site 2 and the time from the artifact to the response is B′. The maximum motor conduction velocity over segment C is calculated by dividing the distance between the two stimulating electrodes, C, by the time C′.
Figure 2-12. The principal pathologic alterations of CMAP. A. The normal CMAP, representing the summed discharges from a group of motor units activated by a supramaximal stimulus, measured over the muscle. B. With loss of motor axons, fewer motor units are activated and the CMAP has reduced amplitude. C. With demyelination of motor axons, the same number of motor units activate, but over a prolonged duration; thus the CMAP has reduced amplitude because there is temporal dispersion of the waveform.
Figure 2-13. SNAP recording. A. Electrical stimulation of the median nerve at the wrist with recording of sensory action potentials at two sites in the second digit. The responses are generated by antidromic propagation of action potentials from the site of stimulation. B. A SNAP recorded from G1. Sensory nerve conduction velocity can be calculated by dividing the distance between G1 and G2 by the difference in onset latencies from these two sites.
Figure 2-14. Late responses. A. The H reflex is elicited by stimulating a sensory nerve. The action potentials travel in an orthodromic fashion through the dorsal root into the spinal cord, where synapses occur with motor neurons. The motor axons innervate a muscle (the gastrocnemius) from which the late CMAP response is recorded. B. The F response is elicited by stimulating a motor nerve. Some of the action potentials that have traveled in an antidromic fashion though the anterior horn are volleyed back in an orthodromic fashion along the same motor neurons. The late CMAP response is recorded from the muscle innervated by these axons.
Figure 2-15. Repetitive stimulation of the hypothenar muscles. A. Patient with myasthenia gravis—typical pattern of decrement in first four responses followed by slight increment. At this rate of stimulation (3 per second), the decrement in response does not continue to zero. B. Patient with Lambert-Eaton syndrome and oat cell carcinoma—marked increase from low toward normal amplitude with rapid repetitive stimulation (20 per second). Horizontal calibration: 250 ms.
AB˜°C2 msecRef.ABC
Figure 2-16. The shaded areas on the muscle (A, B, and C) represent zones of the propagating action potential depicted by the dashed arrow. The correspondingly lettered portions of the triphasic muscle action potential displayed on the screen reflect the potential difference between the active (vertical arrow) and reference (Ref.) electrodes. Polarity in this and subsequent figures is negative upward as depicted.
Figure 2-17. Patterns of motor unit recruitment. A. Normal. With each increment of voluntary effort, more and larger units are brought into play until, with full effort at the extreme right, a complete “interference pattern” is seen in which single units are no longer recognizable. B. After denervation, only a single motor unit is recorded despite maximal effort. It is seen to fire repetitively. C. With myopathic diseases, a normal number of units are recruited on minimal effort, though the amplitude of the pattern is reduced. Calibration: 50 ms (horizontal) and 1 mV (vertical).