Study design: Electrophysiologic recordings were obtained from low threshold primary afferent neurons innervating lumbar multifidus and longissimus muscles in the anesthetized cat.
Objective: The purpose of this study was to classify sensory nerve endings in lumbar paraspinal muscles and characterize their responses to biomechanical loads applied over a range of durations that encompass those occurring during spinal manipulation.
Summary of background data: Neural responses arising from the mechanical input during spinal manipulation are thought to contribute to this maneuver's therapeutic effects. Because manual therapies are distinguished to a large extent on the basis of the speed with which they are applied, it is important to understand how their rate of application affects the signaling properties of primary afferent neurons innervating paraspinal tissues. If alterations in sensory input do contribute to the mechanism of spinal manipulation's therapeutic effect, it seems reasonable to expect that these primary afferents would respond to spinal manipulation in some unique fashion.
Methods: Experiments were performed on 6 adult cats. A L4-L5 laminectomy was performed and the L6 dorsal roots exposed. The L6-L7 vertebrae and associated paraspinal tissues remained intact bilaterally, including lumbodorsal fascia, multifidus, longissimus, iliocostalis muscles, and deeper tissues. Forceps were clamped tightly onto the lateral surfaces of the L6 spinous process through a thin narrow, slit in the lumbodorsal fascia. Single unit afferent activity was recorded from fine filaments teased from the L6 dorsal root. Instantaneous discharge frequency was calculated. Afferents were classified based on von Frey threshold, conduction velocity, and responses to direct muscle stimulation and to succinylcholine injection. Spinal manipulative-like loads were applied to the L6 vertebra (posterior to anterior) using a programmable electronic feedback control system. Force-time profiles were half-sine waves with durations of 25, 50, 100, 200, 400, and 800 milliseconds delivered at constant magnitudes of 33%, 66%, or 100% body weight.
Results: The 6 afferents were classified as low threshold mechanoreceptors based on von Frey thresholds being less than 6 g. Five afferents were Group I or II muscle proprioceptors and one afferent was a Group III muscle mechanoreceptor. The receptive field for 2 of the 6 afferents was in the multifidus muscle and the receptive field of the remaining 4 afferents was in the longissimus muscle. In general, the mean instantaneous discharge frequency for all 6 afferents increased abruptly as the duration of the impulse approached 100 milliseconds. An increase in loading magnitude (33% vs. 66% vs. 100% body weight) did not appear to systematically affect the discharge from the 6 low threshold mechanoreceptors.
Conclusions: This preliminary report suggests that abrupt changes in neural discharge (instantaneous frequency) of low threshold muscle mechanoreceptors of the lumbar spine occur as the duration of a biomechanical load approaches that typically used during spinal manipulation. These changes could comprise part of the mechanism contributing to this intervention's physiologic effects. Further studies are warranted to better understand the signaling properties of a wider range of sensory receptors as well as determine the central effects of these high frequency discharges.