Extensibility of Skeletal Muscle Undergoing Reinnervation

¹ From the Department of Physiology, State University of Iowa, Iowa City, Iowa

Changes in the extensibility of rat gastrocnemius muscles resulting from crushing the sciatic nerve, and determined from static passive tension curves, were followed from 12 to 70 days after the lesion. Paralyzed muscle became progressively more resistant to stretch until the 21st day (experimentals averaging 235% above control level), after which it became less resistant to stretch and leveled off at 56 and 70 days (about 56% above control level). Attempts were made to correlate extensibility changes with other previously determined changes in gastrocnemii denervated by sciatic crushing. It is concluded that changes in extensibility are probably not due to: condensation of intracellular material due to decrease in muscle cell diameters; nor to changes in nonmuscle cell phase: muscle cell phase ratios; nor to changes in amount or arrangement of collagen fibers in the connective tissue. Loss of extensibility during atrophy and increased extensibility during reinnervation and recovery may be due either to changes in tendon extensibility or to changes in muscle cell extensibility, or to both. Changes in the muscle cell may be due to stiffening of muscle cell protoplasm by lowered metabolic activity, or (more probably) to lack of mechanical activity, allowing a ‘setting’ of the thixotropic muscle cell protoplasm. Recovery of normal extensibility through reinnervation may be due to an increased metabolic rate, or (more probably) to a mechanical stirring up of the protoplasm on resumption of muscular activity.