Neural control of muscle blood flow during exercise

1 Hypertension Division, Department of Internal Medicine, The University of Texas Southwestern Medical Center, Dallas, Texas 75390; and 2 The John B. Pierce Laboratory and Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06519 Activation of skeletal muscle fibers by somatic nerves results in vasodilation and functional hyperemia. Sympathetic nerve activity is integral to vasoconstriction and the maintenance of arterial blood pressure. Thus the interaction between somatic and sympathetic neuroeffector pathways underlies blood flow control to skeletal muscle during exercise. Muscle blood flow increases in proportion to the intensity of activity despite concomitant increases in sympathetic neural discharge to the active muscles, indicating a reduced responsiveness to sympathetic activation. However, increased sympathetic nerve activity can restrict blood flow to active muscles to maintain arterial blood pressure. In this brief review, we highlight recent advances in our understanding of the neural control of the circulation in exercising muscle by focusing on two main topics: 1 ) the role of motor unit recruitment and muscle fiber activation in generating vasodilator signals and 2 ) the nature of interaction between sympathetic vasoconstriction and functional vasodilation that occurs throughout the resistance network. Understanding how these control systems interact to govern muscle blood flow during exercise leads to a clear set of specific aims for future research. muscle contraction; sympathetic nerves; motor nerves; vasoconstriction; vasodilation Address for reprint requests and other correspondence: G. D. Thomas, Hypertension Division, Univ. of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-8586 (E-mail: gail.thomas{at}utsouthwestern.edu ).