Sympathetic regulation of cerebral blood flow in humans: a review.

Cerebral blood flow (CBF) is regulated by vasomotor, chemical, metabolic, and neurogenic mechanisms. Even though the innervation of cerebral arteries is quite extensively described and reviewed in the literature, its role in regulation of CBF in humans remains controversial. We believe that insufficient attention has so far been focused on the potential role of the innervation of the cerebral vasculature in cerebral autoregulation in humans. We have performed an extensive search and selection of available literature on electrical, chemical, and surgical manipulations of the sympathetic innervation of cerebral arteries, and the effects of circulation sympathetically active agents on CBF. Studies on (surgical) ganglion block show a role of sympathetic tone in preventing increases in CBF in humans, which are consistent with the view based on animal studies. Both direct innervation of the cerebral arteries from cervical ganglia and stimulation of adrenergic receptors by circulating sympathomimetics prevent sudden increases of CBF associated with hypertension and hypercapnia. We postulate that under normal physiological conditions neurogenic control has little influence on cerebral autoregulation as other methods of control (vasomotor, chemical, and metabolic) are dominant. In severely challenging circumstances, such as delayed cerebral ischaemia after subarachnoid haemorrhage, these methods might be overwhelmed, increasing the relative importance of neurogenic, sympathetic control of CBF. This insight might lead to future therapeutic possibilities.