Effect of sympathetic nerve stimulation on cerebral blood flow and on large cerebral arteries of dogs.

This study was performed to determine whether sympathetic stimulation reduces or redistributes cerebral blood flow. Total and regional cerebral blood flows were measured with the use of microspheres. Left stellate and superior cervical ganglia were stimulated electrically in anesthetized dogs. Sympathetic stimulation did not alter blood flow to ipsilateral cerebrum, cerebellum, or brainstem. Responsiveness of cerebral vessels to vasoconstrictor stimuli was intact, since cerebral blood flow decreased during systemic hypocapnia. Sympathetic stimulation dilated the ipsilateral pupil and reduced flow to ipsilateral temporalis and genioglossus muscles, which indicates that sympathetic pathways to the head were activated. We tested the hypothesis that sympathetic stimulation might constrict large cerebral arteries while dilation of small vessels through autoregulation maintains flow constant. Sympathetic stimulation during systemic hypercapnia, which interferes with autoregulation, did not decrease ipsilateral cerebral blood flow. In additional studies responses of large arteries which supply the brain were evaluated by determining the gradient between common carotid artery pressure and vertebral artery wedge pressure. Serotonin constricted large arteries that supply the brain, but bilateral stimulation of superior cervical ganglia did not increase the resistance of these arteries. As a result of these studies, we conclude that (1) sympathetic stimulation does not reduce or redistribute cerebral blood flow, (2) sympathetic stimulation does not constrict large cerebral arteries significantly, and (3) serotonin constricts large arteries to the brain but dilation of small cerebral vessels tends to maintain cerebral blood flow constant.