Extraocular motor unit and whole-muscle contractile properties in the squirrel monkey. Summation of forces and fiber morphology.

In order to understand the neural control of movement, many investigations have examined the contractile properties of single motor units contracting in isolation, and a great majority of those studies have been done in the cat. Fewer studies, again primarily in the cat, have examined motor units acting in concert in both hind-limb and extraocular muscles. It has been shown, in general, that when individual motor unit forces are added together they do not always add linearly, which makes our understanding of motor control somewhat more complicated. In addition, complex neuronal firing patterns can yield unexpected force outputs or muscle positions whether those patterns occur naturally or are induced through motoneuron stimulation. The current investigation extends these findings of nonlinearity to the primate extraocular system. In studies of the squirrel monkey lateral rectus muscle and its motor units, we show that individual units lose an average of 45% of their force output when they fire in concert with a small number of other motor units. Also, when individual motor units are stimulated at a constant rate of 100 Hz, the force output is most often dramatically different if that constant 100-Hz stimulation is preceded by brief (25 ms), high-frequency stimulation burst or pulse, as occurs during saccades. The force at 100 Hz is usually significantly higher than when no pulse is delivered. However, we now show that an identical stimulation pattern applied to a number of motor units simultaneously does not always yield these force differences. These "nonlinearities" are addressed in terms of the complex muscle architecture that we show in the squirrel monkey lateral rectus muscle. Muscle fibers do not always run in parallel from tendon to tendon. Instead, they may branch or attach to each other laterally or end to end, serially.