Discharge characteristics of sympathetic efferents to the knee joint of the cat

Abstract

Sympathetic postganglionic neurons to the knee joint of the cat were studied to characterize the nerve supply and response to somatic stimulation. In halothane anesthetized cats, the sympathetic postganglionic units from a branch of medial articular nerve (MAN) were dissected. The other branch of MAN was left intact. Most of the central filaments of MAN showed spontaneous discharge. The frequency of the spontaneous discharge of single units ranged from 0.2 to 2.9 impulses per second. Cardiovascular rhythmic modulation was observed in most of the filaments tested. Phenylephrine-induced baroreceptor stimulation caused inhibition of the discharges. Repetitive stimulation of the lumbar sympathetic trunk or the peripheral cut end of MAN led to a decrease in the local temperature inside the joint. The frequency threshold for decreasing the temperature of the joint was approximately 1 Hz. Maximum effects were obtained with 5 Hz stimulation. Histograms of MAN sympathetic efferent fibre activity following electrical stimulation of afferent nerve fibres in the MAN exhibited two response periods. The first, of about 280 ms latency, was elicited by myelinated fibre excitation (the A-reflex) and the second, with a latency of approximately 700 ms, was evoked by unmyelinated fibre excitation (the C-reflex). Electrical stimulation of radial afferent nerve produced similar A- and C-reflex discharges in sympathetic fibres of MAN. Passive movement of the knee joint within its normal working range (flexion, extension or outward rotation) had very little effect on sympathetic efferent nerve activity in MAN, whereas noxious outward rotation of the joint produced a reflex increase in activity to about 140% of the prestimulus control level. In some cats efferent units to the knee were analyzed after inducing an experimental inflammation in the joint. The characteristics of the efferent activity were qualitatively similar to those of normal joints. This finding is discussed in regard to different results in other sympathetic subsystems.