The exercise pressor reflex; its afferent mechanisms

 

Marc P. Kaufman

Heart and Vascular Institute, Penn State College of Medicine, Hershey, PA 17036

 

Static exercise increases mean arterial pressure, heart rate, and minute volume of ventilation, responses which function to maintain an adequate supply of oxygen and blood to metabolically active muscles. A reflex arising from contracting muscle is believed to play an important role in evoking the cardiovascular and ventilatory responses to exercise. The afferent arm of this reflex is comprised of group III and IV afferents, the endings of which are stimulated by both mechanical and metabolic stimuli arising in contracting muscles. Nevertheless, the discharge properties of the thin fiber muscle afferents comprising the sensory arm of the exercise pressor reflex arc appear to differ markedly. In general, group III afferents, which are thinly myelinated, appear to be mechanically sensitive, whereas group IV afferents, which are unmyelinated, appear to be metabolically sensitive. For example, many group III afferents, but only a few group IV afferents, respond to tendon stretch, which mechanically distorts receptive fields in the muscle. In contrast, few group III afferents, but many group IV afferents, respond to post-contraction circulatory occlusion, which traps metabolic by-products of contraction in exercising muscle. The nature of the receptor that is responsible for group III afferents responding to mechanical stimuli is not known, but it is a mechanogated channel that can be blocked with gadolinium. In addition, several receptors are likely to be responsible for group IV afferents responding to metabolic stimuli. These include Purinergic 2X receptors, Acid Sensitive Ion Channels, and Bradykinin 2 receptors. Also, cyclooxygenase products of arachidonic acid metabolism play a role in stimulating metabolically sensitive group IV afferents. Finally, many of the metabolites that stimulate group IV afferents can sensitize group III afferents to contraction.

 

Key words: group III afferents, group IV afferents, neural control of the circulation, autonomic nervous system