A quantal method has been employed to determine the numbers of functioning motor units in small muscles of the hand. In healthy subjects below the age of 60 years the mean number of hypothenar motor units was 380 ± 79 and the mean number of thenar units innervated by the median nerve was 340 ± 87. In older subjects there was a progressive reduction in the numbers of functioning units in both muscle groups; in some of these subjects the population of motor units in the extensor digitorum brevis was also estimated for comparison. A study was also performed on patients diagnosed as having generalized denervating diseases or with lesions of peripheral nerves and cervical roots; in every instance the number of functioning motor units was markedly reduced.
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