A comparative study of changes operated by sympathetic nervous system activation on spindle afferent discharge and on tonic vibration reflex in rabbit jaw muscles

The effect of sympathetic activation on the spindle afferent response to vibratory stimuli eliciting the tonic vibration reflex in jaw closing muscles was studied in precollicularly decerebrate rabbits. Stimulation of the cervical sympathetic trunk, at frequencies within the physiologic range, consistently induced a decrease in spindle response to muscle vibration, which was often preceded by a transient enhancement. Spindle discharge was usually correlated with the EMG activity in the masseter muscle and the tension reflexly developed by jaw muscles. The changes in spindle response to vibration were superimposed on variations of the basal discharge which exhibited different patterns in the studied units, increases in the firing rate being more frequently observed. These effects were mimicked by close arterial injection of the selective α₁-adrenoceptor agonist phenylephrine. Data presented here suggest that sympathetically-induced modifications of the tonic vibration reflex are due to changes exerted on muscle spindle afferent information.