Properties of types of motor units in the medial gastrocnemius muscle of the cat

Abstract

Summary The mechanical properties of motor units from the medial gastrocnemius muscle of the cat have been studied following single shock and repetitive stimulation of motor axons. Motor units were classified into one of 3 types, FF, FR and S, according to the criteria proposed by Burkeet al.10. This classification proved effective for the majority of units, although a small group with properties intermediate between types could be recognised. An approximate relation between time-to-peak of twitch contractions and axonal conduction velocity could be demonstrated, particularly for types FR and S. A relation was also found between the tetanic tension developed by motor units and their axonal conduction velocity. It was suggested for types FR and S that the area of an axon (as calculated from the conduction velocity) might determine the degree of branching of the axon and therefore the number of muscle fibres innervated. A plot of axonal area against tetanic tension did reveal an approximately linear relation for motor units of type FR and S, but not for type FF. Prolonged stimulation produced characteristic changes in the ratio of tetanic to twitch tension for each type of motor unit.