Muscle contraction of short duration gives rise to prolonged enhanced activity in muscle spindle afferent fibers. This postcontraction sensory discharge is sufficient in intensity to frequency-modulate motor neurons demonstrated to be tightly coupled to stretch receptors of the activated muscle. To determine whether or not postcontraction activation of motor neurons is dependent on supraspinal pathways, further experiments were done on cats with low spinal (T12) lesions. Sixty-one motor neurons were isolated in ventral root filaments and categorized according to their stretch reflex response and discharge pattern. Units found to be facilitated by stretch were significantly increased (P < 0.01) in resting discharge following contraction. Phasic motor neurons responded with a short postcontractile burst lasting only a few seconds whereas tonically firing units showed more persistent responses. Intact supraspinal pathways are therefore not necessary to demonstrate postcontractile activation of presumably alpha motor neurons serving homonymous and synergistic muscles. The proposed significance of these responses would be to increase muscle stability against minor perturbations in length.
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