Mass sympathetic discharge

The effect of the skeletal muscle pump is essential during exercise. Although a mass sympathetic discharge and venous vasoconstriction enhance VR, this mechanism alone is insufficient to increase VR and, therefore, CO to meet the metabolic demands of strenuous exercise. The skeletal muscle pump mobilizes the blood stored in these tissues and keeps it flowing toward the heart. As the number of muscles involved in the exercise increases, so does the magnitude of the increase in VR and CO. 

At the onset of exercise, signals from the cerebral cortex are transmitted to the vasomotor center in the medulla of the brainstem. This central command inhibits parasympathetic activity and also initiates the mass sympathetic discharge associated with exercise. Sympathetic activity (including release of catecholamines from the adrenal medulla) increases proportionally with the intensity of exercise. 

Proprioceptors originating in muscles and joints of the exercising limbs provide substantial input to the medullary respiratory center. In fact, even passive movement of the limbs causes an increase in ventilation. Therefore, the mechanical aspects of exercise also contribute to the ventilatory response. The increased metabolism associated with exercise increases body temperature, which further contributes to the increase in ventilation during exercise. (Not surprisingly, ventilation is also enhanced in response to a fever.) Exercise is associated with a mass sympathetic discharge. As a result, epinephrine release from the adrenal medulla is markedly increased. Epinephrine is believed to stimulate ventilation. 

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