Blood-nerve barrier permeability to ions and nonelectrolytes is very low, indicating limited exchange between plasma and nerve endoneurium. The nerve barrier, unlike the brain barrier, does not have regulatory transport systems for potassium and calcium. Nerve concentrations of these ions can change during chronic alterations in plasma concentrations. Glucose transport across the blood-nerve barrier from plasma to nerve endoneurium is mediated by a facilitated system that demonstrates stereospecificity and saturation, and allows matching of transport and metabolic demand. Permeabilities of both nerve capillaries and perineurium increase during the first few weeks of Wallerian degeneration. Nerve capillary permeabilitY eventually returns to normal, but perineurial permeability remains elevated, suggesting that nerve fibers are required to maintain the blood-nerve barrier integrity. Blood flow in the rat sciatic nerve, as measured with laser Doppler flowmetry, increases during hypercapnia, indicating that the vasa nervorum is sensitive to local metabolic conditions. The localization of alkaline phosphatase to caveolae of frog perineurium and rat endoneurial capillaries indicates that these structures may function as microdomains of enzymatic activity.
