E11 Neurovascular coupling

Topic

Neurovascular coupling links local neuronal activity in brain tissue—specifically, the local firing rate of its neurons—to regional changes in three properties of the tissue: cerebral blood flow, blood volume, and tissue oxygenation. This neurovascular coupling forms the physiological basis for functional neuroimaging techniques such as functional magnetic resonance imaging (fMRI).

The "balloon model" describes this neurovascular coupling in brain tissue across three successive stages. In the first stage, a vasodilatory signal from the brain tissue responds to local neuronal activity and decays over time once that activity ceases. In the second stage, the blood flow entering the brain tissue integrates this vasodilatory signal over time: the more intense and sustained the vasodilatory signal, the greater the volume of incoming blood that accumulates in the tissue.

In the third stage, the incoming blood fills a venous compartment; both the normalized volume and the normalized deoxyhemoglobin content of this compartment evolve based on that incoming blood flow and the tissue's oxygen extraction fraction. Specifically, higher incoming blood flow leads to a larger normalized venous compartment volume, while a higher oxygen extraction fraction results in a greater accumulation of normalized deoxyhemoglobin within that compartment.

The cerebral metabolic rate of oxygen in the tissue is related to both the incoming blood flow and the oxygen extraction fraction: the higher the incoming blood flow and the oxygen extraction fraction, the higher the cerebral metabolic rate of oxygen. Finally, the blood-oxygen-level-dependent (BOLD) signal measured by fMRI is approximated as a linear combination of changes in the normalized volume and normalized deoxyhemoglobin content of the tissue's venous compartment. Ultimately, it is this very brain tissue—ranging from the local neuronal activity that triggers the vasodilatory signal to the changes in the venous compartment—that produces the blood-oxygen-level-dependent signal indirectly detected by functional magnetic resonance imaging.

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