Discharge variability of motor units in an intrinsic muscle of transplanted hand.

The study analyzed the discharge characteristics of the motor units in an intrinsic muscle of a transplanted hand. Multichannel electromyographic (EMG) recordings were obtained in 11 experimental sessions over 16 mo starting from day 205 after a hand was transplanted in a 35-yr-old man who had lost his right hand 22 yr earlier. The action potentials discharged by single motor units were identified from the surface EMG signals of the abductor digiti minimi muscle in the transplanted hand as the individual performed 60-s maximal and linearly increasing (ramp) contractions. Discharge rate decreased from 27.1 +/- 8.4 pulses per second (pps) at the start of the maximal contractions to 17.2 +/- 2.9 pps at the end (P < 0.001) and increased from 17.4 +/- 4.3 to 22.1 +/- 5.0 pps during the ramp contractions (P < 0.05). The SD of the interspike interval (ISI) nearly related to the mean ISI with a similar regression slope for the maximal (0.49 +/- 0.09) and ramp contractions (0.43 +/- 0.10). The coefficient of variation for ISI was higher than values in able-bodied persons and did not change during either the maximal (36.8 +/- 10.8%) or the ramp contractions (35.9 +/- 7.4%). High-frequency bursts of activity with <20 ms between two and six action potentials occurred during both maximal and ramp contractions. In conclusion, motor neurons that reinnervated a muscle in a transplanted hand discharged action potentials with a high degree of variability that suggested greater synaptic noise during the voluntary contractions.