Primary afferent nerve fibers that contribute to muscle pain sensation in humans

Muscle pain and deep hyperalgesia are symptoms associated with a wide variety of painful syndromes. Electrophysiologic studies of the response properties of primary afferent fibers supplying skeletal muscle, as well as psychophysical measures of muscle pain sensation, have provided important information on the peripheral neural mechanisms underlying muscle pain sensation. It is believed that muscle pain is subserved by nociceptors with slowly conducting afferent fibers; however, several issues regarding the relationship between evoked activity of muscle afferent fibers and muscle pain sensation need to be resolved. These include identifying the adequate stimulus for exciting muscle nociceptors, determining whether the quality of sensation evoked by different classes of muscle nociceptors is heterogeneous, and determining mechanisms underlying poor localization of muscle pain. It is proposed that (1) the most relevant stimuli for exciting muscle nociceptors are mechanical and chemical stimuli; (2) only one quality of pain sensation, cramping pain, arises from skeletal muscle regardless of the type of nociceptors excited; and (3) temporal summation mechanisms account for diffuse, poorly localized muscle pain.