Schematic representation of the hypothetical mechanisms controlling changes in blood flow volume in the superior mesenteric artery. The supraspinal reflex is elicited by stimulation of the limbs, which influence the excitation of vagal nerves or inhibition of sympathetic nerves. An excitation or inhibition is differentiated by the quality of the stimulation. The blood flow volume is influenced by the sympathetic or vagal tone. : excitation; : inhibition.