1. The discharge of spindle afferents from medial gastrocnemius and soleus muscles was recorded in the decerebrated cat preparation, under isometric conditions and during ramp and hold stretches. Motor output was varied systematically by manual stimulation of the contralateral hindlimb. Twenty-six of 34 afferents showed response patterns consistent with enhancement of dynamic and/or static fusimotor input with increasing muscle force. 2. To establish whether force-related fusimotor effects were mediated at least partly by β-input, β-innervation to these same spindles was sought, using a ventral root stimulation protocol. Twenty-three of the 34 afferents were shown to receive β-innervation, which was most often static in type. 3. For two measures of fusimotor action, the slope of the afferent dynamic rate-length relation and the discharge rate measured during the last portion of ramp stretch, significant increases in the measure, which paralleled increases in muscle force, made it statistically more likely that the afferent received β-innervation. Our measures did not successfully predict the type of β-input (β-static or β-dynamic). 4. Procaine block of γ-fibers produced substantial reductions in fusimotor effect in seven spindle afferents (although modest residual fusimotor effects were detectable for 3/7 afferents). The severity of these reductions indicates that β-action probably requires concurrent γ-input to the spindle in order to be effective. In support of this possibility, the fusimotor effects of electrical stimulation of single β-fibers were greatly reduced for five out of six afferents during procaine block of γ-fibers, compared with the β-effects recorded when modest levels of spontaneous γ-activity were present. 5. We conclude that β-innervation to muscle spindles of triceps surae is common and that this innervation exerts significant fusimotor effects. It appears likely that β-motoneurons are able to produce both static and dynamic effects above extrafusal threshold, but that the actions require on-going γ-activity in order to be effective.
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