Central inhibition of depressor reflex at spinal level

Abstract

The classical concept that the central inhibition of the depressor reflex occurs entirely in the bulbar reticular formation has recently been challenged by several reports, which have indicated the existence of a descending sympathetic inhibitory pathway2,4, 7,9. In the present study direct evidence about this spinal inhibition will be presented. In 15 experiments performed in cats, surgical intervention was carried out under Nembutal 38 mg/kg, i.p. The left carotid blood pressure was continuously monitored by sphygmomanometry. The vagal nerve of either side was dissected and sectioned for stimulation purposes (Fig. 1Aa). The cervical cord was first hemi-sectioned at the C1level on the left side, and then mid-sectioned caudally by a 10 mm long incision. A pair of insulated fine steel electrodes were then placed into this 'isolated' lateral column (Fig. lAb). The left splanchnic nerve was dissected manually with fine watch forceps into bundles cut distally for the recording of sympathetic preganglionic efferent discharges. The recording electrodes consisted of a pair of platinum wires (diameter 27 pm; Fig. 1Ae). During observation the animal was paralyzed by tubocurarine at a dose of 1 mg/kg i.v. and maintained by artificial ventilation. Once single unit discharges were found on the screen of the CRO, they could be identified as the vasomotor neuron discharges by verifying whether they ceased to discharge under vagal stimulation or during the pressor response induced by i.v. adrenaline administration. The identified vasomotor unit discharges were then recorded spontaneously. Repetitive electrical stimulation of the lateral column (0.3 msec, 10 Hz, 5-12 V, for several sec) invariably elicited at the spinal level sustained accelerated discharges of the vasomotor neuron together with a remarkable pressor response. During this response, the application of vagal stimulation (0.1 msec, 50 Hz, 5-10 V, 3 sec) could either effectively inhibit these enhanced unit discharges (9 cases) or greatly reduce them (4 cases), always accom-