During a finger-tapping task, functional MRI detects increased signal in the contralateral motor cortex seconds after activation. The physiological event linking neuronal activity to this regional increase in cerebral blood flow is primarily:

• A Retrograde propagation of action potentials along cortical arterioles
• B Sympathetic vasodilator fibers traveling with the middle cerebral artery
• C Systemic rise in arterial carbon dioxide from muscle activity
• D Release of vasoactive metabolites such as adenosine, potassium ions and hydrogen ions from active neurons and astrocytes ✓

Explanation

Neurovascular coupling matches regional blood flow to local metabolic demand: active neurons and astrocytes release adenosine, K+, H+, lactate and nitric oxide, all of which relax nearby arterioles, raising local flow well beyond the modest rise in oxygen consumption. This overdelivery of oxygenated blood creates the BOLD signal exploited by fMRI. Sympathetic nerves regulate large vessel tone and do not mediate focal activation responses.

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

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