Referral of muscle pain

Muscle pain has a tendency to be referred to other deep somatic tissues remote from the site of the original muscle lesion. Branching of primary afferent fibers does not seem to provide a satisfactory explanation for the referral. The convergence-projection theory of pain referral, in its original form, it is not applicable to muscle pain, since in dorsal horn neurons there is little convergence from deep tissues. Based on results from animal experiments a hypothetical explanation of muscle pain referral is proposed that adds two new components to the convergence-projection theory: convergent connections from deep tissues to dorsal horn neurons are not present from the beginning but are opened by nociceptive input from skeletal muscle, and referral to myotomes outside the lesion is due to spread of central sensitization to adjacent spinal segments.