Human motor units in health and disease.

Recent advances in clinical neurophysiology have made it possible to non-invasively stimulate single motor axons and determine the physiological characteristics of the associated motor units. Some motor units lend themselves to longitudinal studies of their electrical and contractile characteristics. The former include the conduction velocities of their motor axons and the sizes and shapes of their motor unit action potentials and the latter such contractile characteristics of the motor unit as their contractile speeds, twitch and tetanic tensions, and resistance to fatigue. The feasibility of serially examining the same motor unit has made it possible to study the responses of single motor units to conditioning as well as changes in the responses of single motor units to diseases such as amyotrophic lateral sclerosis. The non-invasive character of these approaches offers an attractive means of studying the responses of single human cells, in these cases motor neurons, in health and disease.