Interaction between the muscle metaboreflex and central command – A clearer picture of cardiorespiratory control during exercise

Abstract

Neural control of the circulatory and ventilatory responses to exercise has been an intriguing area of investigation for well over 100 years. However, development of experimental protocols that clarify mechanisms and their relative importance has proved difficult because of the likelihood of redundancy and the complexity of central integration. In this issue of Experimental Physiology, Lam, Greenhough, Nazari, White, & Bruce (2019) met this challenge with an elegant and cleverly designed study in humans that improves our understanding of the contribution of metabolically sensitive neural feedback from active skeletal muscle to the cardiorespiratory response to dynamic exercise. Ventilation and heart rate responses to exercise occur rapidly to a level well matched to the workload. Descending neural signals from higher brain centres (i.e. central command) have a prominent role in increasing ventilation and heart rate in proportion to the effort exerted (Goodwin, McCloskey, &Mitchell, 1972). When examining the contribution of metabolically sensitive neural feedback from active skeletal muscle (i.e. the metaboreflex) in isolation after exercise, which effectively increases blood pressure, the ventilatory and heart rate responses generally trend towards resting values. Given this, the observation that ventilation and heart rate rapidly return towards resting levels during metaboreflex isolation (i.e. postexercise ischaemia) has led some to suggest that the muscle metaboreflex has little influence on both ventilation and heart rate. Countering evidence shows a contribution of the afferent feedback from contracting