Reflex control of the circulation by afferents from skeletal muscle.

Functionally, the pressor reflex, rather than the depressor reflex, appears to be the normal physiologic response to exercise. Available data indicate that chemoreceptors, probably in the form of free nerve endings, are stimulated by local factors, e.g., hypoxia or hyperkalemia. Impulses from these receptors are mediated by group IV afferent nerves to the central nervous system. Then, through efferent sympathetic and parasympathetic nerves, systemic blood pressure, cardiac output, heart rate, and myocardial contractility all increase. The baroreceptors tend to modify the reflex increase in heart rate. The potential significance of local control of the systemic circulation by neural reflexes from skeletal muscle is apparent when one considers that during exercise local control allows both rapid and precise control of the circulation from the metabolically active region of the body. However, the exact role of neural reflexes from skeletal muscle during voluntary exercise still remains to be defined. Clearly, more studies are necessary to elucidate fully their function in the overall control of the circulation in conscious animals and man.