Muscle pain: mechanisms and clinical significance.

INTRODUCTION Muscle pain is common, but the understanding of its causes is still patchy. This article addresses the mechanisms of some important types of muscle pain. METHODS Selective literature review, predominantly of data derived from neuroanatomical and electrophysiological experiments on anesthetized rats. RESULTS Muscle pain is evoked by specialized nerve endings (nociceptors). Important stimuli for muscle pain are adenosintriphosphate (ATP) and a low tissue pH. Excitation of muscle nociceptors leads to hyperexcitability of spinal sensory neurones (central sensitization). Low frequency activity in muscle nociceptors is sufficient to induce central sensitization. DISCUSSION Central sensitization leads to increased excitation in the spinal cord and to referral of muscle pain. The motoneurones of a painful muscle are centrally inhibited. Muscular spasm is mostly secondary to a painful lesion in another muscle or joint. The pain of fibromyalgia is assumed to relate to a dysfunction of central nociceptive processing. Psychosocial factors also contribute to pain.