The effect of neck manipulation on excitability of the motor cortex
Neck manipulation is commonly used in the management of some musculoskeletal disorders to reduce pain and improve movement. There is, however, little understanding about the underlying mechanism. Recent research has alluded to a neurophysiological mechanism mediated through supraspinal pathways in the central nervous system, that may alter motor activity. The purpose of this study was to determine the effect of neck manipulation on the excitability of cortical motoneurons by means of activating corticospinal pathways to the flexor carpi radialis (FCR) muscle in an active motor system using the transcranial magnetic stimulation (TMS) technique.Motor evoked potentials (MEPs) were elicited by TMS and recorded in 20 normal subjects using established procedures. The peak-to-peak amplitude of MEPs were measured both before and after C6/7 manipulation and before and after neck positioning. Both interventions were applied to the normal subjects in random order on two different days. MEPs were recorded immediately after the intervention, then seven minutes and 14 minutes later to assess the time course of the effect. Five neck pain subjects participated in the manipulation experiments. The effects of manipulation, resulting in joint cavitation, were also explored. Two trials were undertaken before the intervention and these served as control measures. MEP data was represented in two ways. Firstly, 40 MEPs were averaged over 120 seconds; secondly, 15 MEPs were average over 60 seconds. A percentage change calculation was used to express the data relative to the baseline. Alterations in cortical excitability before and after manipulation were analysed by repeated measure analysis of variance (ANOVA) on the MEP data, and percentage change scores.Cortical neurons projecting to FCR were significantly facilitated up to 60 seconds after the manipulation of the non-painful segment, relative to baseline values and the positioning control. A small but significant latent increase in excitability was also observed 15 minutes after manipulation. The response to manipulation of the painful cervical segment was significantly different from that of the non-painful segment. When manipulation of the painful segment did not result in joint cavitation, an inhibitory effect was observed. In contrast, however, cortical motoneurons were facilitated when joint cavitation was associated with manipulation.In conclusion, motor activity is enhanced with neck manipulation when cortical motoneuron excitability is measured by TMS in human subjects performing a voluntary contraction. This may explain the clinically observed improvement in spinal motion and motor control with spinal manipulation. Further, joint cavitation may be important in signifying the success of the joint manipulation. The excitatory effect on cortical motoneurons is probably mediated through a transcortical pathway by means of the activation of muscle afferents with the manipulative thrust.These findings assist in understanding the neurophysiological mechanism underlying the effect of spinal manipulation.