Nervous control of the cerebrovascular system: doubts and facts.

Increased function of the central neurons results in increased neuronal metabolism and, as a consequence, increased concentration of metabolic end-products (H+, K+, adenosin) results in an increased cerebral blood flow (CBF). There is a general agreement among investigators that products of cerebral tissue metabolism as well as chemical stimuli are key factors that determine the rate of blood flow in the brain. CBF, however, may increase out of proportion to metabolic demands, may increase without significant change in local metabolism, and may increase much faster than the accumulation of the metabolic end-products. Therefore, the 100-year-old metabolic hypothesis of Roy and Sherrington, cannot fully explain the increases of CBF during increased functional activity of the central neurons. The tight coupling of neuronal activity and blood flow in the brain is demonstrated by a large amount of data. Therefore, the likelihood exists that neurogenic stimuli via perivascular nerve endings may act as rapid initiators, to induce a moment-to-moment dynamic adjustment of CBF to the metabolic demands, and further maintenance of these adjusted parameters is ensured by the metabolic and chemical factors. Perivascular nerve endings were identified in the outer smooth muscle layer of the cerebral arteries, arterioles and veins. Their axonterminals contain a large variety of neurotransmitters, often co-localised in synaptic vesicles. Stimulation of the nerves results in a release of transmitters into the narrow neuromuscular synaptic clefts in the cerebrovascular smooth muscle, close to specific receptor sites in the vessel wall. In spite of these facts, however, and in spite of the large number of new experimental evidences, the role of the nervous control of the cerebrovascular system is underestimated both in medical textbooks and in the common medical knowledge since decades. In the last 20 years major advances have been made that make it necessary to revise this false view. The purpose of this review is to facilitate this process at the end of this century, when the importance of the nervous control of the cerebral circulation has been fully appreciated among investigators.