Action Potentials from Single Motor Units in Voluntary Contraction

In 1929 Adrian and Bronk demonstrated a more refined method than had hitherto been used for obtaining action potentials from muscle, not only in the laboratory animal, but also in voluntary contraction in man. These .workers devised very small concentric electrodes which detect activity of the single motor units in a much restricted area. Such electrodes have since been used extensively in this laboratory and elsewhere in animal work, and they have also been used in man by McKinley and Berkwits (1933) in determinations of “tonus,” and by Hathaway (1932) in a study of reaction times, but so far as the writer is aware no further systematic investigation of voluntary contraction by this means has appeared. The present series of observations has been carried out on eight biceps and five triceps muscles in eight young normal individuals, four men and four women. METHOD. The electrodes are made by inserting a fine insulated copper wire of 36 p diameter into a 25 gauge steel hypodermic needle. A “bakel&e” lacquer subsequently hardened by baking, holds the wire firmly and secures the insulation, and finally both the steel shaft and the copper point, shaved off to suit the bevel of the needle, are silver plated. The device may be sterilized by autoclaving. The butt of the needle is made to fit a small holder in such a way that the central wire leads to the grid of a three-stage condenser-coupled ampliCe+ and the shaft is connected to ground. A Cambridge string galvanometer and an electrocardiograph camera were used to make permanent records at any desired moment, while the investigator listened to the discharge through ear phones from the amplifier. The subject sat with the wrist hanging in a sling attached to a spring balance, in such a position that the forearm was at rest approximately at right angles to the arm. Resting position on the scale varied with the weight of the forearm and hand of the individual. Contraction in the biceps was obtained by lifting the forearm from resting position to zero position on the scale, at which point the forearm was being held, still ap