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Effects on seventy-one single lumbar gamma-motoneurones, evoked by graded electrical stimulation of fibres running in the posterior articular nerve of the ipsilateral knee joint (p.a.n.), were studied by micro-electrode recording in twenty-one cats anaesthetized with alpha-chloralose. Sixty-seven of the gamma-cells were classified indirectly as dynamic (thirty-seven) or static (thirty) using the method of mesencephalic stimulation (cf. Appelberg, Hulliger, Johansson & Sojka, 1982). A high general responsiveness (i.e. number of cells with effect/number of cells tested) was found for the whole sample of gamma-cells (91.9% for dynamic and 93.3% for static cells). The thresholds for the effects were related to the stimulation intensity at which the early negative cord dorsum potential appeared (T). For all subpopulations of gamma-cells (dynamic and static, flexor and extensor cells) excitatory as well as inhibitory effects were observed at 0.9-1.1 T, probably corresponding to 1.1-1.4 times the threshold for evoking a compound action potential in p.a.n. (cf. Discussion). In addition, a considerable number of high-threshold effects were found. Some cells were influenced only from low-threshold joint afferents, some only from high-threshold joint afferents and some cells were influenced from both low- and high-threshold joint afferents. No statistically significant differences in thresholds were found between dynamic and static cells. Among flexor gamma-cells excitatory effects were found to predominate, while for extensor gamma-cells excitation and inhibition occurred with about equal frequency. The shortest latencies for excitatory effects on dynamic gamma-motoneurones were compatible with a trisynaptic pathway, while the routes for excitation of static units and for inhibition of both types of gamma-cells seemed to be longer. The possible functional significance of the findings is discussed. The findings seem to support the idea, as suggested by Freeman & Wyke (1967b), that the joint receptors may contribute to the 'co-ordination of muscle tone in posture and movement' via the gamma-loop. It is furthermore suggested that the latter mechanism may serve to regulate joint stiffness and joint stability.

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