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.