Abstract
A method is described by which a small muscle in the tail is used in the study of postural tonus in the decerebrate cat. With this as a nerve muscle preparation gradually increasing single shocks are applied to the motor-nerve; the response of the muscle is by contraction occurring in steplike groups with no intermediate gradation. Each group step represents, presumably, the activity of a single motor neurone and the muscle fibers innervated by it. Tonic contraction may be induced or increased in the tail muscle by various measures which are described, or it may occur spontaneously. By whatever means produced the increase in contraction height is never smooth and uniform but always by sharp steps. The steplike increments in tonic contraction height are approximately the same size as the steps in a nerve-muscle record from the same muscle. The inference is drawn that in producing an increase in postural tonus the central nervous system throws into action one motor neurone after another. The all-or-none contraction of all muscle fibers innervated by each neurone produces the steplike increase in contraction height. In certain records when the tonus is spontaneous and self-perpetuated, steps may be seen to drop out from the record and reappear again while the general level of contraction remains fairly constant. This is taken as evidence of rotation of activity of single neuromuscular units of contraction. This supports the theory that the apparent lack of fatigue in postural tonus depends on such rotational activity (Forbes, 1922). The fact that steps of an all-or-none character corresponding in size to nerve-muscle steps may be demonstrated both in the pinal flexion reflex and in postural tonic contraction is regarded as favoring the view that the same mechanism on the motor-side is operative in both cases.

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