Changes in rat plantaris motor unit profiles with advanced age

The physiological characteristics of single motor units in rat plantaris muscles were determined in situ, for young adult (3 months) and very old (30–34 months) Fischer 344 rats. Old muscles generated 43% less tetanic force (P₀) per gram. Motor units classified as “slow”, using criteria of fatigue resistance and “sag” during unfused tetani, had a mean P₀ which was 255% of that in young muscles, while fast motor units were similar in P₀ in the two groups. Estimates were made of motor unit numbers using whole muscle and mean motor unit P₀ values. The typical young plantaris contained 48 units, of which 5–6 were slow, while old plantaris contained 29 units, of which 11 were slow. In spite of this large increase in slow motor unit presence (increased mean motor unit P₀, plus increased number) in old muscles, a comparatively modest (72%) increase occurred in the muscle cross-section occupied by histochemically demonstrated slow fibres. During senescence, there occurs a loss in muscle tetanic force capability which is accompanied by a loss of motor units and a reorganization of the remaining motor unit profile. An increase in slow motor unit number and size with advancing age can evidently occur without concomitant histochemical changes. Motor units do not “dedifferentiate”, but maintain their physiological distinctiveness into very old age.