The exercise pressor reflex; its afferent mechanisms
Marc P. Kaufman
Heart and
Vascular Institute,
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