[What is different about muscle pain?].

BACKGROUND: The bulk of available knowledge about pain mechanisms is derived from studies on cutaneous pain. However, deep somatic pain (from muscle, fascia, tendon, joint) is clinically of much greater importance. The existing subjective differences between muscle and skin pain (e.g. muscle pain is poorly localized and shows referral) suggest that muscle and skin pain do not share the same mechanisms.

AIMS OF THE STUDY: To answer the question if the nociceptive information from muscle has neuroanatomical connections and mechanisms that are distinct from those of cutaneous nociception.

MATERIALS AND METHODS: The results were obtained partly in animal experiments on anaesthetised rats, partly in studies with healthy subjects or fibromyalgia patients.

RESULTS: 1. At the spinal level, the excitatory effects of unmyelinated afferent fibres from muscle are subject to a strong segmental inhibition by myelinated afferent fibres, which is largely absent in the effects of cutaneous C fibres. 2. At the cortical level, experimental muscle pain excites other regions than does cutaneous pain. 3. At the level of descending pain-modulating pathways, interruption of the activity in these pathways leads to higher activity of nociceptive neurones caudal to the site of interruption. The activity was higher in neurones with input from deep nociceptors than in cells mediating cutaneous nociception.

CONCLUSIONS: The data demonstrate that at all central nervous levels the connections and processing of nociceptive information from muscle and skin are different. The findings regarding descending pain-modulating pathways suggest that a dysfunction of this system could lead to chronic deep pain as in fibromyalgia.