A dual effect of sympathetic nerve stimulation on jaw muscle spindles

In anaesthetized and paralyzed rabbits, electrical stimulation of the cervical sympathetic nerve at physiological frequencies induces in jaw muscle spindle afferents a short-latency decrease or suppression of discharge. This effect is very stereotyped in pattern and is attributed to direct sympathetic innervation of spindles. It is mediated by preganglionic S1-S2 sympathetic fiber groups. A longer-latency facilitatory effect follows, probably vasomotor in origin and mediated by S3-S4 groups. Both responses are eliminated by administration of alpha-adrenergic blocking agents. The latencies, patterns, thresholds, durations and reproducibility of these responses have been studied and the mechanisms possibly involved are discussed.