Nervous control of the cerebrovascular system: doubts and facts

Increased function of the central neurons results in increased neuronal met abolism and, as a consequence, increased concentration of,metabolic end-pro ducts (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 determin e the rate of blood flow in the brain. CBF, however, may increase out of pr oportion to metabolic demands, may increase without significant change in l ocal 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 incr eased functional activity of the central neurons. The tight coupling of neu ronal activity and blood flow in the brain is demonstrated by a large amoun t of data. Therefore, the likelihood exists that neurogenic stimuli via per ivascular nerve endings may act as rapid initiators, to induce a moment-to- moment dynamic adjustment of CBF to the metabolic demands, and further main tenance of these adjusted parameters is ensured by the metabolic and chemic al factors. Perivascular nerve endings were identified in the outer smooth muscle layer of the cerebral arteries, arterioles and veins. Their axonterm inals contain a large variety of neurotransmitters, often co-localised in s ynaptic vesicles. Stimulation of the nerves results in a release of transmi tters into the narrow neuromuscular synaptic clefts in the cerebrovascular smooth muscle, close to specific receptor sites in the vessel wall. In spit e of these facts, however, and in spite of the large number of new experime ntal evidences, the role of the nervous control of the cerebrovascular syst em is underestimated both in medical textbooks and in the common medical kn owledge since decades. In the last 20 years major advances have been made t hat 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 importa nce of the nervous control of the cerebral circulation has been fully appre ciated among investigators. (C) 1999 Elsevier Science Ltd. All rights reser ved.