The R.C.A.M.C. Electromyograph Mark III is an apparatus developed under the auspices of the National Research Council of Canada during the war for the study of casualties with peripheral nerve injuries. It was used principally to study the electrical activity of muscles as a sensitive test of their nerve supply and general condition with regards to atrophy, etc. It is now being used not only in the diagnosis and to follow the recovery of nerve injuries, but for the study of muscle activity in various neuromuscular diseases, including poliomyelitis.The electromyograph consists of (a) a high gain, low noise level, wide range calibrated, balanced, push–pull pre-amplifier, with built-in calibrator, (b) a cathode ray oscilloscope unit with photographic attachment, and incorporating a nerve and muscle stimulator of variable intensity and duration, and (c) a mobile loud speaker cabinet including the speaker power amplifier, battery, and battery charger.Action potentials from single muscle fibres, or groups of fibres known as the motor unit, are picked up by means of a small finely pointed needle electrode, insulated except at the very tip, thrust through the skin into the muscle. A reference electrode is placed on the skin next to the point of insertion of the needle and a distant grounded electrode is placed on the subject to aid in the elimination of stray electrical interference. Action potentials are then amplified through four stages of condenser coupled amplification. They are then of sufficient magnitude to be clearly observed or photographed on the calibrated screen of the cathode ray oscilloscope, which makes possible accurate measurement of each potential wave, both in voltage and duration. The sound of the muscle action potentials may be simultaneously heard in the loud speaker. With training, diagnosis of muscle conditions may be greatly aided by certain characteristic sounds produced by different kinds of muscle activity.For example, a denervated muscle gives rise, from single muscle fibres, to continuous low voltage random 'spikes' that produce in the loud speaker a 'crackling' sound recognized as fibrillation. At rest the normal muscle is electrically silent. It gives rise during contraction to action potentials that are much higher and of longer duration. They produce in the loud speaker a knocking sound recognized as a motor unit discharge. During recovery from a nerve injury, and in certain degenerative conditions, the motor unit becomes poorly synchronized or disintegrated producing very complex wave forms that produce a distinctive 'chugging' sound.The electrical stimulator, which is synchronized with the trace of the oscilloscope, makes it possible to test the responsiveness of single muscles to different forms of electrical current as well as to record the electrical activity of a given muscle in response to stimulation of its motor nerve at a distance. By the use of the same equipment, the nerve impulse can also be followed in its course down the nerve fibre.
Potentiometric measurement of membrane action potentials in frog muscle fibres
