Neural effects on cerebral vessels: alteration of pressure-flow relationship.

Cerebral vessels are well innervated by adrenergic and cholinergic fibers. Effects of sympathetic stimulation on cerebral blood flow (CBF) are controversial, but we have concluded that, during normotension and hypotension, sympathetic nerves have little effect on CBF. During acute hypertension, however, sympathetic nerves may have important effects of CBF. Sympathetic stimulation attenuates the passive increase in CBF that occurs during severe steady-state hypertension and after a sudden, modest increase in pressure. Disruption of the blood-brain barrier, which occurs during severe hypertension, also is minimized by sympathetic stimulation. Thus, the concept has emerged that, in contrast to minimal effects of sympathetic nerves under most conditions, sympathetic stimulation may have important protective effects during hypertension. The role of cholinergic nerves in regulation of CBF has not yet been clarified.