REFLEX ACTIVATION OF THE CARDIOVASCULAR SYSTEM DURING MUSCULAR CONTRACTION

摘要

The cardiovascular response to exercise is characterized by increases in blood pressure, heart rate and cardiac output and a redistribution of regional blood flow. A major mechanism underlying these responses is the exercise presser reflex. The reflexoriginates in skeletal muscle and is initiated and maintained by local chemical or mechanical stimulation of thinly myelinated group III and unmyelinated group IV afferent fibers. The corresponding hemodynamic changes are provoked by the autonomie nervous system and include increases in sympathetic nerve activity and decreases in cardiac vagal tone. The ventrolateral medulla is a major central nervous system structure involved in mediating sympathetic outflow via activation of the Cl cell group. The neurons of this cell group project to the intermediolateral cell columns of the spinal cord where they can excite cell bodies of preganglionic sympathetic neurons. Modulation of the afferent arm of exercise pressor reflex arc may occur in the spinal dorsalhorn. The dorsal horn is the first synapse of this reflex and both excitatory and inhibitory neurotransmitters that affect sensory nerve transmission are released from the terminal nerve endings of group III and IV afférents. Vasopressin and oxytocin containing neurons located in the paraventricular nucleus of the hypothalamus also terminate in the spinal dorsal horn. These peptides appear to exert inhibitory effects on the exercise pressor response. Lastly, activation of group III and IV afférents during contraction may inhibit the efferent cardiac vagal aspect of the baroreflex resulting in a resetting of this reflex response to higher arterial pressures.