Changes in effective diffusivity for oxygen during neural activation and deactivation estimated from capillary diameter measured by two-photon laser microscope.

The relation between cerebral blood flow (CBF) and cerebral oxygen extraction fraction (OEF) can be expressed using the effective diffusivity for oxygen in the capillary bed (D) as OEF = 1 - exp(-D/CBF). The D value is proportional to the microvessel blood volume. In this study, changes in D during neural activation and deactivation were estimated from changes in capillary and arteriole diameter measured by two-photon microscopy in awake mice. Capillary and arteriole vessel diameter in the somatosensory cortex and cerebellum were measured under neural activation (sensory stimulation) and neural deactivation [crossed cerebellar diaschisis (CCD)], respectively. Percentage changes in D during sensory stimulation and CCD were 10.3 ± 7.3 and -17.5 ± 5.3 % for capillary diameter of <6 μm, respectively. These values were closest to the percentage changes in D calculated from previously reported human positron emission tomography data. This may indicate that thinner capillaries might play the greatest role in oxygen transport from blood to brain tissue.