A histochemical analysis of identified compartments of cat lateral gastrocnemius muscle.

The lateral gastrocnemius muscle of cats can be divided into four discrete subvolumes or compartments which are supplied by the primary branches of its muscle nerve. The histochemical profile of each compartment was determined from the reaction for myosin ATPase after acid preincubation. Fibers were classified as fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG), or slow-twitch oxidative (SO). Each compartment in each cat examined was found to contain a relatively uniform distribution of different types of fibers. The most proximal LG compartment contains mainly type FG fibers, with relatively few type SO fibers; the most distal compartment, while still predominated by type FG fibers, contains a significantly larger proportion of type SO fibers. The histochemical profile of the intermediate compartments indicates that they contain fibers which lie intermediate in composition between these two. These results are consistent with the notion that LG compartments consist of aggregations of motor units, arranged such that muscle fibers comprising a single motor unit are contained within a single compartment.