Considerations concerning the neurobiological basis of muscle pain.

Nociceptors in skeletal muscle can be sensitized by substances that are released from pathologically altered tissue. In the sensitized state, nociceptors can be activated by low-intensity stimulation; this is probably one of the mechanisms producing deep tenderness. Dorsal horn cells processing input from muscle nociceptors often have multiple receptive fields and additional input from other deep tissues or skin. This may be one of the reasons for the diffuse and ill-localized nature of muscle pain. The degree of convergence from deep tissues and skin in neurones with muscle input can be increased by noxious stimulation of deep tissues. This mechanism might explain phenomena such as spread and referral of muscle pain. In the development of chronic muscle pain, vicious circles may be involved which operate locally in the damaged tissue or via spinal reflexes that alter the biochemical environment of the nociceptors in skeletal muscle.