Neurovascular activation requires conduction through vessels.

In experiments on rats, a hind leg was transected except for the femoral nerve, artery and vein. The femoral nerve was stimulated by electrical pulses, with one electrode attached to the nerve in the amputation gap and another placed in the inferior caval vein. At each pulse of stimulation, contraction could be recorded in femoral muscles. Ligation of the femoral vessels suspended the contractions within a fraction of a second; contractions resumed when ligation ended. Propagation of neural stimuli to femoral muscles therefore required an intact electrical communication through associated vessels. This is possible because conducting blood plasma and its capillary junction with the interstitial fluid form an "external" closed circuit branch of low resistance with the neuromuscular unit. In testing the interstitial tissue fluid as an alternative to the vascular "outer" communication, a higher voltage of stimulation was required for muscle contractions.