The human cerebral circulation is anatomically unique in its degree of extracranial to intracranial anastamoses and its need to supply a highly developed and metabolically highly dependent cerebral neocortex. The physiology of the cerebral circulation is key to understanding its pathophysiology and thus the management of patients with cerebrovascular disease. It has been covered by a number of monographs (Edvinsson et al., 1993; Mackenzie et al., 1984; Mraovitch & Sercombe, 1996; Purves, 1972; Welch et al., 1997) and particular aspects are routinely and expertly reviewed in specialized journals, such as the Journal of Cerebral Blood Flow and Metabolism. The cerebral circulation has certain key physiological features that can be clinically meaningful:

1. (i) the relationship between brain blood flow and metabolism, vasoneuronal coupling;

2. (ii) the maintenance of constant flow in the face of variations in perfusion pressure, autoregulation;

3. (iii) the effect of key respiratory gases, oxygen and carbon dioxide, on cerebral blood flow (CBF); and

4. (iv) the direct effects of nerves on the cerebral circulation, neurovascular influences on CBF.

Vasoneuronal coupling is a well-established principle (Kuschinsky, 1989), as is the fact that the brain can regulate flow in the face of changes in perfusion pressure. The effect of changes in CO2 and O2 on CBF is also an old and established concept. Neural control or neurogenically mediated changes in cerebral blood flow are a relatively recently accepted concept, although the observation that nerves exist on the vessels dates to Thomas Willis in 1664. Some established concepts will be considered here as a backdrop to the clinical discussions that follow. The classical view of the cerebral circulation has been that blood flow and cerebral metabolism are tightly coupled under the influence of substances, such as H+, adenosine, and K+ that ensure a rapid and matched supply of blood when required without neural influence. As the role of the neural innervation and its neuromodulators emerges and their functions are clarified opportunities for intervention and understanding will arise and eventually inform the management of patients with cerebrovascular disorders.

Vasoneuronal coupling

The fact that cerebral blood flow tracks closely cerebral metabolic activity remains a tenet of cerebrovascular physiology. It is the basis for many modern functional neuroimaging methods (Frackowiak & Friston, 1994).