Do vasomotor nerves significantly regulate cerebral blood flow?

Abstract

The topics considered are: areas of uncertainty concerning the neural pathways; results of physiological experiments in animals to determine whether and by how much reflex pathways affect cerebral blood vessels; and various methods for measurement of cerebral blood flow as they affect the results. The discrepant experimental results which have been the immediate cause of controversy about the role of cerebral vasomotor nerves are not due to fundamental flaws in the methods. Although failure to control some experimental variables may contribute to the confusion, this is unlikely to explain all the discrepancies. The controversy is due in part to an imperfect understanding of the neural pathways themselves, a point particularly relevant to the dilator pathway whose origin and course is obscure. Species differences may be very important. The principal methods used, the results of which are at issue, may in fact be measuring different aspects of cerebral blood flow in space and time. The components of the cerebral circulation may be responding quite differently to nerve stimulation. Until these questions are resolved, it would be premature to dismiss a functional role for the extrinsic neural pathways.