Motor endplate position of rat gastrocnemius muscle.

In this study, the relative endplate position of fibers of rat gastrocnemius caput mediale (GM) muscle was determined by counting numbers of sarcomeres. Isolated fibers were teased from the proximal, intermediate, and distal regions of the muscle. Endplates of distal fibers were located on the proximal third of their lengths. Endplates of intermediate fibers were located at half fiber length, and for proximal fibers, a variable endplate position was obtained: in half of the muscles studied, endplates occurred around the proximal one-third and in the other half near the midpoint of the fiber. Endplate position relative to fiber length was thus found to be dependent on the region of the muscle. Changes in the orientation of endplate zone relative to the muscle belly is likely to take place with changes in muscle length, as shown by a planimetric muscle model. It is argued that architecture of pennate muscles may highly affect characteristics of motor unit potentials.