Myofascial pain dyndrom - Pathophysiology

Myofascial trigger point (MTrP) is a hyperirritable spot in a taut band of skeletal muscle. Based on previous human and animal studies, there are multiple "MTrP loci" in an MTrP region. The basic MTrP locus contains two subunits: a sensory component and a motor component. The sensory component is the site to elicit referred pain (ReP) and local twitch response (LTR) when it is mechanically stimulated; it is defined as an LTR locus. An LTR locus is a nociceptors (free nerve ending) based on a histological study. The motor component is the site where endplate noise (EPN) can be recorded; it is defined as an EPN locus. An EPN locus is a dysfunctional endplate with excessive leakage of acetylcholine. Formation of a taut band is due to local contracture of muscle fibers (contraction knot) in the endphate zone as a consequence of excessive acetylcholine leakage from the dysfunctional endplates in the MTrP region. The amount of MTrP loci is proportionate to the irritability of an MTrP. Two important clinical characteristics of MTrPs are ReP and LTR. Both ReP and LTRs are mediated through spinal cord mechanisms, demonstrated in both human and animal studies. The pathogenesis of MTrPs appears to be related to the integration in the spinal cord (formation of "MTrP circuits") in response to the disturbance of the nerve endings and abnormal contractile mechanism at multiple dysfunctional endplates. The "MTrP circuit" is the major center to control the MTrP phenomena. The MTrP circuit may be modified by a strong peripheral stimulus (high pressure massage or needling) so that the sensitivity is reduced, and thus, to block any painful impulse into the higher centers.