The dilation response of brain surface arterioles during somatosensory activity most likely involves the intercellular conduction of membrane currents along intracerebral blood vessels. We have produced evidence against the involvement of a) a flow-mediated dilation mechanism, related to increases in blood shear rate, and b) the brain-to-CSF diffusion of vasoactive metabolites.

Ngai, A.C., and H.R. Winn. Estimation of shear and flow rates in pial arterioles during somatosensory stimulation. Am. J. Physiol. 270 (Heart Circ. Physiol. 39): H1712-H1717, 1996.Ngai, A.C., and H.R. Winn. Pial arteriole dilation during somatosensory stimulation is not mediated by an increase in CSF metabolites. Am. J. Physiol. Heart Circ. Physiol., 282: H902-H907, 2002.

We tested the hypothesis that a shear stress-dependent mechanism is involved in the dilation of pial arterioles during somatosensory stimulation. In rats implanted with cranial windows, we simultaneously measured the diameter and flow velocity of pial arterioles with video and dual-slit methods. Stimulation of the contralateral sciatic nerve evoked consistent dilator responses in pial arterioles without affecting blood pressure. The dilator responses consisted of peak and plateau components. Mean velocity increased by 16% at 5 s after stimulus onset. Wall shear rate and volume flow were calculated from diameter and velocity data by assuming a parabolic flow profile. There was no significant change in wall shear rate, whereas flow rate increased significantly during sciatic nerve stimulation. These findings suggest that a flow (shear stress)-mediated mechanism does not play an important role in the dilator response of pial arterioles to sciatic nerve stimulation.