ORGANIZATION OF SINGLE MOTOR UNITS IN FELINE SARTORIUS

1. We depleted single motor units in feline sartorius muscles of glyco gen by stimulating their motoneurons intracellularly. We mapped the in tramuscular distribution of depleted fibers by inspecting histological cross-sections throughout the length of sartorius. 2. We selected ten depleted motor units for detailed study and quantitative analysis. Ni ne motor units were located in the anterior head of sartorius. One was located in a muscle whose distal half appeared to have been damaged s ome time before the acute experiment. A single motor unit was located in the medial head of sartorius. 3. Five motor units were composed of fast-twitch glycolytic (FG) muscle fibers, two of fast-twitch oxidativ e glycolytic (FOG) muscle fibers, and three of slow-twitch oxidative ( SO) muscle fibers. Estimates of the numbers of depleted fibers in moto r units of anterior sartorius indicated that FG motor units were large r (mean 566 fibers) than FOG and SO motor units( SO mean 190, FOG mean 156 fibers). The SO motor unit in the damaged muscle had 550 fibers. One motor unit depleted in the medial head of sartorius had 270 fibers with FG profiles. 4. Muscle fibers belonging to each anterior motor u nit were never distributed throughout the whole cross-section of anter ior sartorius at any proximodistal level. Furthermore, fibers were dis tributed nonuniformly along the proximodistal axis of the muscle. In m ost muscles at least a few depleted fibers were found at all proximodi stal levels. However, in one normal muscle and the damaged muscle, dep leted fibers were confined to the proximal end. 5. The fibers in the m edial motor unit were confined to a strip that did not extend across t he whole cross-section of the muscle head. Fibers within this strip we re scattered quite evenly from origin to insertion. This medial FG mot or unit occupied a smaller territory and contained fewer fibers than a nterior motor units of the same histochemical type. 6. These results s how that sartorius motor units are not distributed uniformly in the me diolateral plane; those in anterior sartorius were distributed asymmet rically in the proximodistal axis as well. This finding has important functional implications for the way in which we model force developmen t and transmission in sartorius and other long muscles.