Evidence that neural mechanisms do not have important effects on cerebral blood flow.

CEREBRAL blood flow may be regulated by four major factors: chemical stimuli (blood gases and extracellular fluid pH), autoregulation, metabolic factors, and neural stimuli. The first three factors are clearly important in control of cerebral vessels. In contrast, we propose that, despite many years of intensive study, there is no convincing evidence that neural control plays an important role in regulation of cerebral blood flow. Before accepting this conclusion, we must address three major questions, which will be the theme of this review: First, why are cerebral vessels richly innervated if the innervation has no important function? In most vascular beds the density of adrenergic innervation correlates with responsiveness of vessels to neural stimuli. Because cerebral vessels are extensively innervated, one might expect important neural effects. If, however, the mechanism of neuromuscular transmission were unusual in cerebral vessels, the vessels might be relatively unresponsive to the neurotransmitter. There is good evidence that neuromuscular transmission is unusual and that cerebral vessels are relatively unresponsive to the adrenergic neurotransmitter, norepinephrine. Second, if in fact neural stimuli do not have significant effects, why have numerous studies indicated important effects of neural stimuli on cerebral blood vessels? We suggest that, in the studies that have indicated important effects of neural stimuli on cerebral vessels, the methods for measuring cerebral blood flow and the experimental protocols have had serious limitations. Third, could nerves have important cerebral vascular effects other than in regulation of blood flow? It is possible, for example, that nerves may affect capillary surface area without affecting blood flow significantly.