Cerebral activation and bloodflow changes

Try closing your eyes and then opening them again. At the moment that you open your eyes, neurones in the occipital cortex that are specialized for the perception of visual stimuli will show a sudden and dramatic increase in their rate of discharge. There is a short delay (approximately 100 ms) between the stimulus and neural response owing to the propagation of electrical activity from the retina via the optic nerves and tracts to the visual cortex. Later, some 3 to 8 s after stimulus onset, there will be an accompanying change in the local blood supply to the stimulated area of cortex. Blood flow increases without a commensurate increase in oxygen uptake by the visual cortex, leading to a local increase in the ratio of oxygenated to deoxygenated forms of haemoglobin.

The linkage between neural activity and regional cerebral blood flow, sometimes called neurovascular coupling, has been known since Roy and Sherrington first reported 'changes in blood supply in accordance with local variations of functional activity' in 1894. However, the molecular mechanisms for neurovascular coupling are still uncertain. Neuronal activity causes a local accumulation of vasodilating metabolites, such as hydrogen ions, which might mediate a vascular response. There may also be a neurogenic mechanism whereby groups of neurones control their own perfusion by innervating the smooth muscular walls of the arterioles supplying them with blood.

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