A METHOD FOR THE LONGITUDINAL-STUDY OF HUMAN THENAR MOTOR UNITS

Methods currently available to study the electrophysiological and cont ractile properties of single human motor units (MUs) preclude the same MU from being studied on repeated occasions. We have developed a meth od which allows single thenar MUs to be studied longitudinally. Sites were located along the median nerve where, using percutaneous stimulat ion, it was possible to selectively activate a single thenar motor axo n as determined by: (a) the presence of an ''all-ornothing'' surface-d etected motor unit action potential (S-MUAP); (b) the occasional prese nce of F-responses identical in size and shape to the direct S-MUAP; ( c) excitation thresholds widely separated from the next highest thresh old motor axon so as to allow high frequency stimulation. To ensure th at the same motor axon had been located and stimulated in successive s tudies, additional criteria included: (a) the motor axon was found at approximately the same site along the nerve; (b) the threshold for exc itation remained widely separate from the next highest threshold motor axon; (c) the shape of the S-MUAP was al most identical on repeated s tudies; (d) S-MUAP and F-response latencies were similar. This method provides, for the first time, the ability to track longitudinally the electrical and contractile properties of single MUs in health and duri ng the course and treatment of disease. (C) 1994 John Wiley & Sons, In c.