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1. Vibration or stretch of the medial gastrocnemius muscle in the decerebrate cat each caused a significant increase in the tension of a synergist, the lateral gastrocenmius. 2. Simultaneous vibration and stretch of the medial gastrocnemius resulted in a substantial increase of lateral gastrocnemius tension which was greater that that produced by medial gastrocnemius vibration alone. The size of this force increase was proportional to the amplitude of medial gastrocnemius stretch, for the limited range of amplitudes examined. 3. Since the discharge of the medial gastrocnemius I a afferent fibres was held constant by vibration, the additional tension in lateral gastrocnemius provoked by medial gastrocnemius stretch must have resulted from the activation of an excitatory pathway separate from the I a afferent system. The secondary spindle afferent pathway was considered to be the most likely candidate. 4. The contributions of the Ia afferents and the additional stretch induced excitation to the stretch reflex were compared. The Ia potency was calculated from the ration of tonic vibration reflex force and the vibration frequency. The total Ia contribution to the stretch reflex, which was estimated from the product of this ratio and the primary ending stretch sensitivity, seemed modest, and was consistently smaller than the proposed secondary contribution. 5. The medial gastrocnemius nerve was subjected to anodal blockade at a strength sufficient to eliminate Ia transmission. Under these conditions, the lateral gastrocnemius excitation produced by medial gastrocnemius stretch or vibration was largely eliminated. When lateral gastrocnemius vibration was superimposed, the excitatory effect of medial gastrocnemius stretch was partly restored suggesting that some central facilitation by group Ia afferents may be necessary for group II pexcitatory effects to be manifested. 6. Although the additional excitatory actions of medial gastrocnemius stretch were examined exclusively in a synergist, it is suggested that similar effects are likely to occur in the homonymous stretch reflex.

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