REFLEX SYMPATHETIC RESPONSES PRODUCED BY ACTIVATION OF VIBRATIONAL RECEPTORS

The experiments were performed on paralyzed (Flaxedil) cats under artificial respiration. The animals have been received a single dose of nembutal (30 mg/kg I. P.) in order to perform the surgical procedures. The reflex responses of the greater splanchnic nerve trunk, evoked by vibrational stimulation of the limbs were analyzed and compared with those elicited by electrical stimulation of afferent nerves.The vibrational stimulation applied to the wrist of foot produced in the afferent nerves (radial, medianus, peroneus superficialis) was a long lasting discharge, while the corresponding response recorded in the splanchnic nerve was brief (FIG. 2). This sympathetic reflex response appeared exclusively when the vibration was applied to the limbs ipsilateral to the splanchnic nerve recorded.The latency elapsing between the peak of the “first” response in the radial nerve and the peak of the “first” spike of the corresponding splanchnic response was of the order of 8-13 msec. The amplitude of the sympathetic reflex response increased when the strength of the stimuli augmented. For a given strength the repetitive vibrational stimuli (from 5-15/sec) produced a marked decrease in the sympathetic discharges.When the radial nerve was electrically stimulated (suprathreshold) through the same pair of electrodes from which the radial nerve responses to the vibrational stimulation had been previously recorded, a splanchnic response similar to that obtained with the vibrational stimulation was observed. The excitability recovery curve to the vibrational stimuli and to the electrical stimulation was similar.The splanchnic response that was produced either by vibrational stimuli applied to the limbs or by electrical stimulation of the afferent limb nerve fibers (with an intensity less than maximal) was called “first” splanchnic response because there was another efferent response (“second”) which appeared 40-100 msec after the stimulation. The “second” response was only produced when the strength of the stimuli was substantially higher than that needed for evoking the “first” splanchnic response. The “first” response had an ipsilateral and segmental character (confined to several spinal cord segments). The “second” response could be produced by the stimulation of either ipsi or contralateral afferents. In the generation of the “second” response supraspinal structures were involved since it never appeared in spinal animals.It is our feeling that the activation of mechanoreceptors of vibrational type (Paccinian corpuscles) produced preferentially the “first” splanchnic reflex response.