RESUMEN
It has been debated whether muscle spindles have a role in myofascial pain or not. We present a number of arguments for the former hypothesis. It was hypothesized that firing of intrafusal muscle fibres, i.e. fusimotor activity can be observed as "end plate spikes" (EPSs) in electromyography (EMG). The EPSs may be found in local active spots of muscle, often associated with miniature end plate potentials (MEPPs). Insertion of EMG needle electrodes into an active spot is painful, indicating nociception in the muscle spindle. Myofascial syndrome patients have taut bands with active trigger points (TrPs) in painful muscles. End plate activity (EPSs and MEPPs) is a significantly more common finding in TrPs of myofascial pain than in control points of the muscle, indicating the presence of muscle spindles. However, some control sites may show EPSs of normal muscle spindles. Increased amount of inflammatory metabolites have been observed in active TrPs. Muscle spindle is a capsulated gel-filled container, where inflammatory and contraction metabolites may be heavily concentrated during sustained fusimotor activation. Thus the intrafusal chemosensitive pain mediating III- and IV-afferents are sensitized and activated. Intrafusal inflammation causes further reflex activation of the fusimotor and skeletofusimotor systems via sensitized III- and IV-afferents. The taut band itself may be a contracture (rigor) of local skeletofusimotor (beta) units caused by sustained reflex drive by the given muscle spindles. In EMG this may be seen as complex repetitive discharges. We conclude that TrPs of myofascial pain are related to painful muscle spindles in taut bands.