PubMed · 13496368
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G MIGNAULT. 1957. Anesthesia.. https://pubmed.ncbi.nlm.nih.gov/13496368/
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Transcranial magnetic stimulation (TMS) is increasingly utilized in clinical neurology and neuroscience. However, detailed knowledge of the impact and specificity of the effects of TMS on brain activity remains unresolved. We have used 14C-labeled deoxyglucose (14C-2DG) mapping during repetitive TMS (rTMS) of the posterior and inferior parietal cortex in anesthetized cats to study, with exquisite spatial resolution, the local and distant effects of rTMS on brain activity. High-frequency rTMS decreases metabolic activity at the primary site of stimulation with respect to homologue areas in the unstimulated hemisphere. In addition, rTMS induces specific distant effects on cortical and subcortical regions known to receive substantial efferent projections from the stimulated cortex. The magnitude of this distal impact is correlated with the strength of the anatomical projections. Thus, in the anesthetized animal, the impact of rTMS is upon a distributed network of structures connected to the primary site of application.
The application of mustard oil (MO), a small-fibre excitant and inflammatory irritant, into neck paraspinal muscles of the rat has been shown to produce a significant reflexive increase in electromyographic (EMG) activity in both neck and jaw muscles. It is possible that this nociceptive reflex activity is influenced by muscle length since recent evidence indicates that abnormal neck posture may be associated with cervical musculoskeletal disorders and pain. Therefore, the aim of this study was to test if muscle length modulates this nociceptive reflex response. Three different experimental procedures were employed in rats under halothane anesthesia: (1) MO injected into the left deep neck muscles with the rat placed in a straight body position (Straight group, n = 7); (2) MO injected into lengthened left deep neck muscles with the rat's neck rotated 45 degrees to the right with respect to the head (Stretched group, n = 11); and (3) MO injected into the right deep neck muscles with the rat's neck rotated 45 degrees to the right (Relaxed group, n = 9). The EMG activity of the deep neck, trapezius, and digastric muscles was bilaterally recorded, rectified and integrated into area under the curve (AUC). Control injections of the vehicle, mineral oil, did not evoke any muscle activity but MO evoked EMG activity in the ipsilateral deep neck and trapezius muscles of the Stretched group that was significantly greater than that evoked in the same muscles in the Straight and Relaxed groups. Also, the MO-evoked EMG activity in the contralateral deep neck muscles of the Stretched and Relaxed groups was greater than that of the corresponding muscles in the Straight group. The MO-evoked activity in the digastric, a jaw muscle whose length was not changed, did not show any significant difference between the three groups. These findings indicate that MO application to the rat deep neck muscles results in a larger nociceptive reflex in deep neck and trapezius muscles when they are stretched. This enhanced muscle activity could be associated with changes in the susceptibility of the neck muscles to pain or damage.
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.