J Manipulative Physiol Ther. 2010 (May); 33 (4): 261–272 ~ FULL TEXT
Heidi Haavik Taylor, PhD, BSc, Bernadette Murphy, PhD, DC
New Zealand College of Chiropractic,
Auckland, New Zealand.
OBJECTIVE: This study sought to investigate the influence of spinal dysfunction and spinal manipulation on the response of the central nervous system to a motor training task.
METHODS: The dual peripheral nerve stimulation somatosensory evoked potential (SEP) ratio technique was used in 11 subjects before and after a 20–minute typing task and again when the typing task was preceded with cervical spine manipulation. Somatosensory evoked potentials were recorded after median and ulnar nerve stimulation at the wrist (1 millisecond square wave pulse, 2.47 Hz, 1x motor threshold). The SEP ratios were calculated for the N9, N11, N13, P14–18, N20–P25, and P22–N30 peak complexes from SEP amplitudes obtained from simultaneous median and ulnar (MU) stimulation divided by the arithmetic sum of SEPs obtained from individual stimulation of the median (M) and ulnar (U) nerves.
RESULTS: There was a significant increase in the MU/M+U ratio for both cortical (ie, N20–P25 and P22–N30) SEP components after the 20–minute repetitive contraction task. This did not occur when the motor training task was preceded with spinal manipulation. Instead, there was a significant decrease in the MU/M+U ratio for the cortical P22–N30 SEP component. The ratio changes appear to be due to changes in the ability to suppress the dual input as concurrent changes in the MU amplitudes were observed.
DISCUSSION: This study suggests that cervical spine manipulation not only alters cortical integration of dual somatosensory input but also alters the way the central nervous system responds to subsequent motor training tasks.
CONCLUSION: These findings may help to clarify the mechanisms responsible for the effective relief of pain and restoration of functional ability documented after spinal manipulation and the mechanism involved in the initiation of overuse injuries.
Key Indexing Terms: Somatosensory Evoked Potentials, Neuronal Plasticity, Spinal Manipulation, Sensory Filtering, Sensorimotor Integration, Chiropractic
From the FULL TEXT Article:
A growing body of evidence suggests that the presence of spinal dysfunction of various kinds has an effect on central neural processing, and it has been suggested that spinal dysfunction could lead to maladaptive central plastic changes. [1, 2] Such plastic changes could result in abnormal responses to any subsequent input to the central nervous system (CNS). The reversal of such changes may be a mechanism by which spinal manipulation improves functional ability. Cervical manipulation to dysfunctional segments in a group of patients with recurrent neck pain and/or stiffness has been shown to alter cortical processing and sensorimotor integration, measured via somatosensory evoked potential (SEP) amplitude changes, for at least 20–30 minutes. [2, 3]
A somatosensory evoked potential is an electrical potential elicited by either physiologic or electrical stimulation of somatosensory receptors or their axons. These electrical potentials can be recorded at various sites along the pathway of the relevant peripheral nerve, or at its central projections, recorded over the spinal cord or via scalp recording electrodes. The SEP peaks are named based on the approximate latency in milliseconds (ms) from the time of peripheral nerve stimulation, and the different early peaks reflect somatosensory processing at various cortical and subcortical areas. For example, the N20 SEP peak reflects processing of afferent information at the level of the primary somatosensory cortex. [6–8] The N30 SEP peak appears to represent processing and central sensorimotor integration in areas that include the primary sensory cortex, primary motor cortex, premotor cortex, and deeper brain structures such as the basal ganglia. [9–16] The validity of the assignment of the neural generators of most of the early SEP peaks (occurring within 100 ms of nerve stimulation) has been determined by intracortical direct recording during surgery  or using dipole source localization and magnetoencephalography.  Interpeak, peak, and interside differences (mean ± SD) have good overall reproducibility. 
Somatosensory evoked potentials have been used to show altered cortical processing in clinical conditions. Tinazzi et al  recorded ulnar nerve SEPs in patients who had had carpal tunnel syndrome compressing the median nerve at baseline and after 4 weeks. They found significant increases in both cortical and brainstem SEP amplitudes after stimulation of the unaffected ulnar nerve, indicative of adaptive plasticity in the noncompressed ulnar nerve. This work indicates that conditions that change afferent input from one part of the body can affect processing in another. Changes in SEP peaks may reflect the perceptual changes seen in an early study where Murphy and Dawson  have demonstrated that patients with trigger points in their forearm muscles had an impaired ability to discriminate nonpainful intramuscular sensation delivered to those same muscles and that this could be improved by treating the trigger points.
One mechanism that would explain changes in processing from nearby body parts is changes in surround inhibition. Surround inhibition is a mechanism by which the CNS increases excitability to one area by increasing inhibition to adjacent areas to enhance neural processing. Changes in surround inhibition have been shown to be responsible for therapy-induced cortical reorganization in chronic stroke patients.  Abnormally low somatosensory surround inhibition has been demonstrated in subjects with dystonia, a movement disorder that causes twisting and abnormal postures during repetitive movements.  Decreasing surround inhibition in the short-term may be one mechanism that leads to motor learning but in certain individuals may also be the trigger for maladaptive plasticity. The dual SEP technique, which measures the ability of the CNS to appropriately suppress the response to simultaneous input from 2 peripheral nerves, as compared to the arithmetic sum of either nerve individually, has demonstrated that the CNS is less able to suppress dual somatosensory input after as little as 20 minutes of repetitive motor training.  However, it is unknown whether this reflects a natural process during skill acquisition or a maladaptive process that could over time lead to a degradation of the highly segregated sensorimotor cortical maps, such as has been demonstrated in primates after many weeks of highly articulated hand squeezing movements.  The presence of spinal dysfunction may represent one factor that could induce maladaptive central plastic changes as opposed to the adaptive plasticity of skill acquisition. It has been demonstrated that spinal manipulation of dysfunctional cervical joints can improve somatosensory “surround-inhibition-like” filtering of somatosensory information at the cortical level. 
This study used the dual SEP techniques to investigate the effect of manipulation, delivered to dysfunctional regions of the spine in a group of participants with recurrent neck pain and/or stiffness, on the CNS response to motor training. It was hypothesized that manipulation would change the way in which the CNS responded to the typing task.
The major finding in this study was that when a 20–minute session of thumb abductions is preceded by spinal manipulation of dysfunctional cervical joints, this altered the way in which the CNS responded to the motor training task. Motor training performed by subjects with reoccurring neck pain, but no acute symptoms at the time of the study, resulted in reduced suppression of the dual input at the cortical level for at least 20 minutes after the cessation of the typing task. This is in accordance with previous research  and may reflect the process responsible for use-dependent plastic changes in the sensorimotor cortex. However, when cervical spine manipulation of dysfunctional segments was performed before the same motor training task, there was a significantly increased suppression of the dual input at the cortical level (as reflected by a reduced frontal P22–N30 MU/M+U SEP peak ratio). This finding is similar to what has been shown after spinal manipulation alone  and suggests that spinal manipulation leads to an improved ability to integrate dual input from the upper limb. That this was also observed after the combined manipulation and motor training task in the current study suggests that the spinal manipulation not only results in altered sensorimotor integration but also alters the way the CNS responds to a functional task, such as a 20–minute repetitive thumb abduction task.
Implications of the Changes in Different SEP Peaks and Comparison to Other Work
Frontal P22–N30 SEP Changes
The changes observed in the current study after the combined intervention mainly occurred for the frontal N30 component of the SEP peaks. Most evidence suggests that this peak is related to a complex cortical and subcortical loop linking the basal ganglia, thalamus, premotor areas, and primary motor cortex. [12–16] The frontal N30 peak is therefore thought to reflect sensorimotor integration.  The decreased frontal N30 SEP peak ratio observed after the combined manipulation and motor training intervention in the current study therefore suggests that there may be an increase in “surround-like inhibition” or filtering of afferent information from the upper limb occurring somewhere in these cortical and subcortical loops linking the basal ganglia, thalamus, premotor areas, and primary motor cortex after spinal manipulation, as previously demonstrated,  and that this ability is maintained even after a 20–minute motor training task. Impaired surround-like inhibition before spinal manipulation may account for this finding. The SEP ratio changes appear to be due to an increased inhibition of the dual peripheral input, as the MU data significantly decreased for this SEP component after the manipulation + motor training intervention, and no significant changes in the M+U data was observed.
Parietal N20 SEP Changes
No significant changes were observed for the parietal N20–P25 SEP peak component after the motor task when it was preceded with spinal manipulation in the current study. The motor training intervention, before the manipulation, led to a significant increase in both the frontal P22–N30 and the central N20–P25 SEP peak MU/M+U ratios. The central N20 SEP component is generated in the primary somatosensory cortex (S1). [6–8] This suggests that S1 filtering of the dual input is reduced after motor training in subjects with chronic neck dysfunction and reoccurring pain. This finding may therefore reflect the initiation of a process that over time may result in the desegregation of the normally sharply defined somatosensory maps in S1, as observed by Byl et al  in the primate cortex after 20 weeks of repetitive contraction training. No significant changes occurred for the central N20–P25 SEP peak MU/M+U ratio after the motor training task when it was repeated on a different day after manipulation of the subjects' cervical spines. This further suggests that spinal dysfunction may represent one factor that promotes maladaptive plastic changes.
Clinical and Research Implications
The Effects of Spinal Manipulation on Sensorimotor Integration and Neural Plasticity
Somatosensory information is very important for motor control. It can be integrated at multiple levels of the CNS, from simple spinal or cortical reflex loops to highly complex networks involving cortical and subcortical circuits. [32–38] These various sensorimotor processing circuits that make up the sensorimotor integration system continuously monitor and respond to all peripheral input by appropriately altering connectivity and the strength of synaptic connections. The retention of such alterations is thought to underpin sensorimotor skill acquisition. However, these alterations may also in some circumstances, such as after an injury or the sustained performance of repetitive muscular activity, become maladaptive plastic changes that are thought to be responsible for initiating and perpetuating certain movement disorders and chronic pain syndromes. [24,39–51] Furthermore, joint dysfunction originating from an injury may be a cause of ongoing pain and loss of function due to maladaptive sensorimotor integration from a hyperafferentiation of the CNS from the dysfunctional joints and associated structures. [52–58]
Does Repetitive Muscular Activity and Joint Dysfunction Lead to Maladaptive Plasticity?
A previous study has shown that a 20–minute repetitive thumb abduction task leads to altered central integration of dual input, as measured by changes in cortical SEP ratios.  Reduced cortical somatosensory filtering was demonstrated for both primary somatosensory cortex (N20) and areas linked to sensorimotor integration (N30).  It is possible that the N20 (S1) changes reflect the mechanism responsible for altering the boundaries of cortical sensory maps, thus, altering the way the CNS perceives and processes information from adjacent body parts. It may also reflect the initiation of a process that over time leads to a degradation of the fine somatosensory maps in S1, which has been observed in animals for 20 weeks of behavioral training.  Furthermore, it is possible that the N30 changes observed in this study are related to the intracortical inhibitory changes shown previously with both single and paired pulse TMS. [59, 60] This could be reflecting a natural part of motor skill acquisition. However, if these changes are not gained appropriately, they could also reflect the mechanisms by which maladaptive plastic changes develop, resulting in inappropriate motor control similar to that seen in movement disorders such as dystonia.
The key question, therefore, is what circumstances or preexisting patient characteristics need to be present for the process of natural skill acquisition to go wrong and lead to maladaptive plastic changes instead. One possible characteristic is the presence of abnormally functioning spinal segments. As joint dysfunction can lead to altered afferent input to the CNS, [52–58] the presence of joint dysfunction would therefore alter the background milieu into which subsequent sensory input is received and processed. The current study indirectly investigated whether sensory filtering of information is adversely affected when joint dysfunction is present, by determining if manipulation of dysfunctional cervical joints would improve sensory filtering in the CNS. If the reduced sensory filtering after motor training represents a process that could lead to maladaptive plastic changes in the CNS and spinal dysfunction is a component that enables this dysfunction to develop, then this process may be avoided when joint dysfunction is addressed by appropriate manual treatment. The results of this study suggests that this is possible, as an improved ability to filter somatosensory information in sensorimotor integration circuits was observed after the same 20–minute motor training task, when this was preceded with spinal manipulation of the subjects' dysfunctional cervical joints. This finding was similar to what has been previously observed after spinal manipulation alone  and indicates that spinal manipulation improves gating or filtering of sensory information, an ability the CNS retains even after the motor training intervention.
It is, however, important that further research is carried out to ensure that manipulation does not simply interfere with the process of skill acquisition. This is unlikely, as no such reports exist in the literature. On the contrary, some evidence suggests that upper cervical manipulation leads to improvement in a complex reaction-time task,  suggesting improved cognitive function and skill acquisition after manipulation. Overall, this research has significant implications for understanding the role of altered afferent input from joints and muscles on the ability to appropriately integrate somatosensory input.
Although the sample size was small (11 subjects), this is common in experimental human neurophysiologic studies. As such strongly significant changes were seen in certain SEP peaks with such a small sample size this suggests that these effects are very robust, as it is nearly impossible to make a type I error with a small sample size because the variance is usually very high.
These observations suggest that spinal manipulation of dysfunctional cervical joints may improve suppression of SEPs evoked by dual upper limb nerve stimulation at the level of the motor cortex, premotor areas, and/or subcortical areas such as basal ganglia and/or thalamus, as previously shown,3 and that this improved somatosensory filtering is maintained after a 20–minute repetitive motor training task. Further studies are needed to elucidate the role and mechanisms of these cortical changes, and their relationship to a patient's clinical presentation and their ability to perform daily tasks.
Funding Sources and Potential Conflicts of Interest
The head author was supported with a Bright Futures Top Achievers Doctoral Scholarship awarded by the New Zealand Government. No conflicts of interest were reported for this study.
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