Reflex autonomic responses evoked by group III and IV muscle afferents

Exercise is well known to increase mean arterial pressure, heart rate, and ventilation, effects caused, in part, by a reex arising from contracting skeletal muscles. This phenomenon has been named the exercise pressor reex (Mitchell, Kaufman, and Iwamoto 1983) and is thought to relay information to the central nervous system regarding the metabolic state and the mechanical activity of the exercising muscles (Hayes and Kaufman 2001; Kaufman and Forster 1996). The afferent arm of the exercise pressor reex arc is composed of thinly myelinated group III and non-myelinated group IV muscle afferents. Group III afferents primarily transmit information about mechanical stimuli arising in the exercising muscles, whereas the group IV afferents primarily transmit information about metabolic stimuli (Hayes and Kaufman 2001; Kaufman and Forster 1996). Group III and IV muscle afferents are also thought to be activated by nociceptive stimuli and are likely the sole source of pain from skeletal and cardiac muscle. The aim of this account is to correlate what is known about the afferent arm of the exercise pressor reex and to what extent it is evoked by nociceptive versus non-nociceptive stimulation of skeletal muscle.