Functional properties of motor units in medial gastrocnemius muscles of rats: Remodelling in old age.

Abstract

The decline in skeletal muscle function in old age is associated with decreases in whole muscle maximum isometric force (F$\sb{\rm o}$), specific F$\sb{\rm o}$ (F$\sb{\rm o}$ per unit area), and number of motor units. The purposes of this study were to test whether: (1) the deficit in whole muscle F$\sb{\rm o}$ in old age can be explained by a loss in the number of motor units; (2) the innervation ratio of fast and slow motor units are altered in old age; and (3) a population of denervated fibers in muscles of old animals can explain the deficit in specific F$\sb{\rm o}$. In addition, the graded stimulation method applied clinically for estimation of the number of motor units was evaluated based on the ventral root dissection method. Unlike the graded stimulation method, the ventral root dissection method samples fast and slow motor units with equal probability, and therefore, provides the best estimate of the number of motor units. The properties of whole medial gastrocnemius muscles and motor units were studied in situ in adult (10-12 months) and old (24-26 months) F344 rats. The number of motor units in muscles of old rats was 20% smaller than the value for adult rats, but the mean F$\sb{\rm o}$ of motor units was not different. Compared with adult rats, the innervation ratios of fast and slow motor units were smaller by 15% and greater by 300%, respectively. The remodelling of motor units is consistent with the hypothesis that some of the muscle fibers denervated by the loss of fast motoneurons are reinnervated by axonal sprouting of slow motoneurons. Denervated fibers in muscles of old rats explained only 5% of the 20% deficit in specific force. In the absence of a major change in the extracellular components such as water content, connective tissue and fiber architecture, the deficit in specific F$\sb{\rm o}$ can only be explained by mechanisms that impair the ability of innervated single fibers to develop force. When compared with ventral root dissection method, the graded stimulation method, a less invasive procedure underestimated the number of motor units in muscles of adult and old rats by 28% and 36%, respectively. The degree of underestimation was not different in muscles of adult and old rats. Consequently, the graded stimulation method can still be used to predict the percentage decrease in the number of motor units that occurs in old age and likely with neurogenic diseases.