Motor Units: Physiological/Histochemical Profiles, Neural Connectivity and Functional Specializations

This brief review considers muscles as ensembles of motor units, the viewpoint usually taken by motor systems physiologists. The morphological, histochemical and mechanical properties of muscle units are discussed in relation to the intrinsic properties of the motoneurons that innervate them, and in connection with the organization of synaptic inputs that play a significant role in determining functional usage. These factors, from synaptic organization to muscle fiber physiology and biochemistry, are all precisely interrelated. The overall design of the soleus (SOL) and medial gastrocnemius (MG) motor unit populations in the cat hindlimb seems ideally suited to the functional roles played by these contrasting muscles. As more information accumulates about these populations, and about others with different functional roles, we should have increasingly clear ideas about the fundamental question of why different muscles look and act as they do in various animal species.