[Link between cerebral blood flow and brain metabolism: validity and limits]

A century ago, Roy and Sherrington suggested that there was a direct relationship between cerebral metabolism and brain blood flow. While this concept is now generally accepted, it is still the subject of considerable debate. There is undoubtedly a link between local metabolism and local blood flow, but the mechanisms involved remain unknown. Several factors, such as H+, K+, CO2, lactate, and recently adenosine, have been suggested as chemical modulators adapting the blood flow in the tissue to local metabolic demand. The demonstration of a functional innervation of the pial and intracerebral blood vessels has led to the hypothesis of autonomic and central nervous regulation of cerebral blood flow (CBF). Most neurotransmitters and neuromodulators, appear to be implicated in this regulatory mechanism. The endothelial cells also play a part in vasomotor effects, as they can produce relaxing (EDRF) and constricting (EDCF) factors in response to a variety of stimuli. Thus it is now no longer possible to consider metabolism as the only factor regulating cerebral blood flow; nervous, endothelial, myogenic and humoral factors must also be included.