Properties of Motor Units in a Homogeneous Red Muscle (soleus) of the Cat1

THE INFORMATION WHICH IS AVAILABLE on the properties of motor units in skeletal muscles is chiefly of an indirect nature, having been inferred from observations on whole muscles. Although a muscle is no more than an assembly of motor units, the collective properties of the whole do not convey much information about its parts. The innervation ratios, contraction times, and tensions of motor units, reported by Eccles and Sherrington (5) and others, are average values calculated from data on whole muscles. Inevitably, these averages conceal the normal range of values for individual units. AS this study will illustrate, the properties of motor units vary over a wide range even within a homogeneous muscle. The distribution of these properties confers on the muscle functional characteristics it would not otherwise possess. The soleus and m. gastrocnemius muscles of the cat were selected for study because they seemed to be ideally suited for comparison. Although their origins are slightly different, they insert on a common tendon and both serve to extend the ankle. They differ considerably, however, in their gross and microscopic appearance and their functional capacities. M. gastrocnemius, which is considered in the paper following this one, is a typical pale muscle, consisting of three types of muscle fibers (10). In histological studies carried out in this laboratory it has been found that the soleus, which is the object of this first study, is a homogeneous muscle, consisting wholly of one type of fiber. As will become apparent, the properties of soleus motoneurons clearly reflect its more uniform structure. In order to investigate single motor units, it was necessary to activate them one at a time, leaving the rest of the muscle completely quiescent. This was done by isolating and stimulating filaments of ventral roots which contained a single alpha fiber to the soleus muscle. Data were obtained on the following characteristics of 97 soleus units in the cat: conduction velocity of the axon, contraction time, twitch tension, and maximum tetanic tensions at several frequencies of stimulation.