Background: Recent electrophysiological studies revealed that endplate noise (EPN) could be specifically recorded from a myofascial trigger point (MTrP) region. EPN has been considered as the focal graded potentials due to excessive acetylcholine release in neuromuscular junction. A recent histological study has demonstrated a free nerve ending at the vicinity of the site, from where EPN could be recorded in an MTrP region. However, the sensory (afferent) and the motor (efferent) connections between an MTrP and the spinal cord still has never been fully studied.
Aims: The aim of this study was to delineate both motor and sensory connections between an MTrP and the spinal cord by applying the stain with horseradish peroxidase (HRP).
Methods: Nine Wistar rats were studied. When the rat was anesthetized, its biceps femoris muscles were exposed for localizing the myofascial trigger spot (MTrS, equivalent to MTrP in human). In one side, a monopolar Teflon-coated, hollow-needle electrode was used for searching EPN in an MTrS region, and then HRP was injected via this hollow-needle electrode into the site where EPN was recorded. HRP was also injected into a normal (non-taut band, non-MTrS) site in the contralateral side to obtain the control data. Two days after HRP injection, the rats were sacrificed and their spinal cords and dorsal root ganglia (DRG) were sectioned for the identification of the sites where neurons were labeled with HRP.
Results: The HRP-labeled neurons were found in the ventral horn of the spinal cord and in the DRG over L3, L4, and L5, while most were found in the L5 level. The mean numbers of HRP-labeled neurons in the EPN side looked smaller than that in the control side, but the difference did not reach statistically significant level (P>0.05). The mean values of the diameters of the HRP-labeled neurons in the DRG were not significantly different between the EPN side and the control side (P>0.05). However, HRP-neurons in the ventral horn of the spinal cord in the EPN side showed mild tendency to be smaller than that in the control side.
Conclusions: The spinal cord connections of an MTrS are basically similar to that for a normal tissue region. The motor neurons related to MTrS tended to be smaller in their diameters. The findings in this study further supported the previously proposed hypotheses for the pathogenesis of an MTrP.