During voluntary finger tapping, functional MRI shows a localized increase in blood flow within the contralateral motor cortex while systemic blood pressure and global arterial PCO2 remain unchanged. The increase in regional flow is mediated chiefly by:

• A Sympathetic vasoconstrictor withdrawal in the motor cortex territory
• B Local release of vasoactive metabolites such as CO2, H+, K+, adenosine, and nitric oxide by active neurons and astrocytes ✓
• C Rise in cerebral perfusion pressure limited to the activated region
• D Choroid plexus secretion of a vasodilating factor into adjacent tissue

Explanation

Regional cerebral blood flow is coupled to local synaptic and metabolic activity through locally produced vasodilators, including CO2, H+, potassium ions, adenosine, and nitric oxide released by active neurons, astrocytes, and endothelium. This metabolic coupling forms the basis of the BOLD signal. Systemic pressure and arterial PCO2 are unchanged here, ruling out options A and C, and the choroid plexus secretes CSF rather than vasodilators into cortex.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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