PROPERTIES OF SINGLE MOTOR UNITS IN MEDIAL GASTROCNEMIUS-MUSCLES OF ADULT AND OLD RATS

1. The purpose of this study was to determine the role of motor unit r emodelling in the deficit that develops in the maximum isometric tetan ic force (F-o) of whole medial gastrocnemius (MGN) muscles in old comp ared with adult rats. The F-o values and morphological data were deter mined for MGN muscles and eighty-two single motor units in muscles of adult (10-12 months) and sixty-two units in those of old (24-26 months ) F344 rats. During an unfused tetanus, fast and slow (S) motor units were identified by the presence and absence of sag, respectively. Fast -fatigable (FF) and fast-fatigue-resistant (FR) units were classified by fatigue indices less than or greater than 0 . 50, respectively. 2. For old rats, whole MGN muscle F-o was 29% less than the value of 11 . 2 N measured for adult rats. The deficit in whole muscle F-o of old r ats resulted from equivalent decreases in the number of motor units, 1 6% smaller than the adult value of ninety-seven, and in the mean motor unit F-o value, 14% less than the adult value of 117 mN. 3. With agei ng, little motor unit remodelling occurred in FR units, whereas the S and FF motor units demonstrated dramatic, but opposing, changes. For S units, the number of units remained constant, but the number of fibre s per motor unit increased 3-fold from 57 to 165. In contrast, the num ber of FF units decreased by 34% and the number of fibres per motor un it of the remaining units decreased to 86% of the adult value of 333. The age-related remodelling of motor units appeared to involve denerva tion of fast muscle fibres with reinnervation of denervated fibres by axonal sprouting from slow fibres.