Objective: To evaluate the changes of motor unit number estimation (MUNE) and the amplitude of compound muscle action potential (CMAP) according to severity of nerve injury in rat.

Method: The sciatic nerve was compressed with two different severity (mild, severe) in 20 rats each. MUNE was performed in gastrocnemius of rat using the Poisson statistics. MUNE and amplitude of CMAP were compared according to severity of injury at two and four weeks after injury.

Results: The MUNE and amplitude of CMAP were significantly diminished at two and four weeks after nerve injury in severely injured rat. Both values were also diminished at two weeks after injury in mildly injured rat. The amplitude of CMAP was slightly increased at four weeks after mild injury despite of further decrement of MUNE.

Conclusion: The MUNE and amplitude of CMAP changed differently according to severity of nerve injury in rat, which might represent different denervation and reinnervation process.

Motor unit number estimation(MUNE) was performed in the thenar muscles of 22 hemiplegic patients without a peripheral nerve lesion using the statistical method. The studies were done bilaterally in the affected and unaffected sides. The distal latency and conduction velocity of median nerve in the affected side were not different from those in the unaffected side. But the amplitude and area of the compound muscle action potential in the affected side were smaller than those in the unaffected side. Motor unit numbers in the affected side decreased than those of the unaffected side, especially in the first 6 months after the onset of hemiplegia. And the motor unit numbers in the affected side decreased as the muscle strength decreased. Single motor unit potential(SMUP) area in the affected side increased after 1 year from the onset of hemiplegia. The estimation of the number of motor units in the thenar muscles of hemiplegic patients could be an indicator for predicting recovery of the muscle power in hemiplegia.

Thenar motor unit number estimation(MUNE) was performed in 49 normal subjects without known neuromuscular diseases using statistical method. The purposes of this study were to compare MUNE parameters between the dominant and non-dominant hands and to evaluate the changes according to age. Reproducibility was assessed in 20 subjects also. The results showed that thenar motor units number, maximum negative peak compound muscle action potential (CMAP) area and mean area of single motor unit potentials(SMUP) were not different between dominant and non-dominant sides, but maximum negative peak CMAP amplitude was larger in dominant than non-dominant side. With aging, the motor unit number, maximum negative peak CMAP amplitude and area decreased and the mean area of single motor unit potentials increased. There was a high reproducibility between the test and the retest values for motor unit number, maximum negative peak CMAP amplitude and area as well as mean area of single motor unit potentials.

The statistical estimate of the motor unit number is a reliable method and easily available in clinical settings. The results of this study are expected to be used as a baseline data for the future.