Work of the muscle receptors during controlled locomotion

1. (1) We have recorded the pulsation of single afferents from muscle spindles and Golgi receptors of the muscles acting on the talocrural joint during controlled locomotion. The spindles of the muscle extensors and the Golgi receptors generate pulses with the highest frequency in the phase of the step when the muscle to which they belong contracts. The spindles of the muscle flexors are also active in the phase of the step when the flexors contract but the frequency of their pulsation is high in the phase of the step when the flexors passively stretch. 2. (2) The more enrgetically the cat runs the greater the activity of the spindle on contraction of the muscle with which it is connected and the smaller the pulses it generates during passive stretching of this muscle. Activation of the spindle usually is not ahead of excitation of the corresponding muscle but continues for several tens of msec after arrest of the electrical activity of this muscle. 3. (3) The character of impulsation of the spindles described during controlled locomotion may be explained by the cyclic modulation of their sensitivity through fusimotor (including without doubt the γ-efferent) influences. The organization of the afferent flow of the spindles is such that it must condition the facilitation of the α-motor neurones in the phase of the step when the corresponding muscles must contract and impede their autogenic activation in the unscheduled phase of the step.