Figure 1.  Chris Colloca, D.C. delivers a chiropractic adjustment as Robert Gunzburg, M.D., Ph.D. assists with the research protocol in Antwerp, Belgium.  Compound action potentials obtained from the spinal nerve roots, neuromuscular responses (needle EMG), and bone movement were quantified simultaneously during the research.

Neurophysiological Research

The last two decades have brought about exciting advances in neurophysiology as it relates to the spine, disease, and it’s treatment.  Summarizing the work in this field helps to put the relevance to chiropractic into perspective while understanding what is yet to be done.  Noteworthy are the contributions of doctors of chiropractic in advancing the field of neurophysiology from the research that is being conducted at chiropractic institutions and through grants from Federal agencies and generous non-profit chiropractic foundations.

Characterizing Afferents

            In the 1960s, Wykes’s classic work identified afferents in articular tissues in animal preparations (Wyke, 1967; Wyke, 1979).   In the decades to follow, advances in immunohistochemistry led to the visualization of afferent fibers and receptive endings in most tissues associated with the spine (Cavanaugh et al., 1989; Cavanaugh et al., 1996). As time went on this line of investigation was continued in humans which identified the presence of mechanoreceptive and nociceptive nerves in the spinal joints (McLain, 1994; McLain and Pickar, 1998; Jiang et al., 1995; Grob et al., 1995).

Emphasizing the importance of this line of study, McLain stated, “The presence of mechanoreceptive and nociceptive nerve endings in cervical facet capsules proves that these tissues are monitored by the CNS and implies that neural input from the facets is important to proprioception and pain sensation in the spine.”  McLain’s statements reinforce the concept that the very presence of neural structures in spinal tissues provides the evidence of the integration between the spinal joints and the nervous system. 

Research of this kind has helped to reposition chiropractic theories away from the thought process that subluxation created nerve interference by spinal nerve root compression, and toward theories that were more consistent with the scientific knowledge and clinical presentation of patients.  Notably, it’s rare that patients with referred pain symptoms suffer nerve root compression.  Rather, research has found that such patients commonly suffer spinal pain who’s origin lies in nociceptive stimulation of the nerves located in the spinal ligaments, facet joints, or intervertebral discs (Aprill and Bogduk, 1992; Moneta et al., 1994; Schwarzer et al., 1995).  Just how abnormal static (loads on spinal joints) and dynamic (motions) biomechanics [vertebral subluxation] serve to influence the nervous system is of great interest in chiropractic research.

From a chiropractic standpoint, the vertebral subluxation is the mainstay of chiropractic care, and the establishment of scientific evidence that quantifies the relationship between the spine, it’s alignment, and the nervous system is of fundamental importance.  Yet, until recently, little was known about how mechanical stimulation caused discharge of afferent fibers.  It’s been known that the mechanoreceptive afferents in spinal joints and tissues contribute to both the proprioception of the individual, and similarly, the presence of nociceptive afferents provides the individual with a mechanism to inform the nervous system of potentially harmful stimuli (pain).  Yet, little is known about just how mechanical stimulation of spinal joints as performed in chiropractic and other treatments results in therapeutic benefit.

            Study at the Palmer Center for Chiropractic Research by Joel Pickar, D.C., Ph.D. has contributed much needed research to the field of neurophysiology through the use of an animal model.  In this manner, mechanosensitive afferent nerve fibers have been identified in the spinal joints and adjacent tissues (Pickar and McLain, 1995; Pickar and Wheeler, 2001).  Further, the reaction of these afferents to mechanical stimulation (simulating spinal manipulation) has demonstrated the role of these nerves in providing efferent input to the adjacent musculature (Kang et al., 2002).  This research helps us to understand how the spinal joints assist in regulating spinal function through coordination with the paraspinal musculature via reflexes, and further demonstrates how such dysfunction may contribute to the biomechanical changes associated with spinal disorders and back pain.

Neuromuscular Reflexes

            While research in animal models provides a controlled environment to perform basic science neurophysiological experiments, the results from this line of research are not easily extrapolated to human patients.  Research into the neuromuscular responses of chiropractic adjustments was first conducted by Walter Herzog, Ph.D. and co-workers at the University of Calgary. Using asymptomatic subjects, Herzog et al. measured the neuromuscular responses of the adjacent musculature during high-velocity low-amplitude (HVLA) spinal manipulation (SM) and reported that the speed of the thrust had the more of an effect upon the elicitation of neuromuscular responses than joint cavitation (Suter et al., 1994; Herzog et al., 1995; Herzog, 1996a; Herzog, 1996b).  Therefore, it appears that the production of the reflex response depends directly on the rate of change in force and deformation during the treatment rather than on the force or stretch magnitude itself (Herzog et al., 1999).

            Building on the work of Herzog et al., we conducted experiments in my private practice on patients with low back pain beginning in 1998.  We hypothesized that there may be differences in the neuromuscular response of actual patients as opposed to the asymptomatic subjects that Dr. Herzog’s group had so often used.  We found that patients with more frequent back pain symptoms had larger amplitude neuromuscular responses during the adjustments that we performed using an Activator II Adjusting Instrument (AAI) (Colloca and Keller, 2001b; Colloca and Keller, 2001a).  In further work, we aim to better understand the association between neuromuscular responses, spinal biomechanics, and clinical outcome.  We have also undertaken research to investigate how adjustments may act to inhibit hyperactive muscles.  For this research, we will be collaborating with researchers at the Department of Orthopaedics at the University of Gothenburg in Sweden this summer.

Neurophysiological Study

            We have also had a very unique and exciting opportunity to collaborate with Robert Gunzburg, M.D., Ph.D., a renowned orthopedic surgeon, author and researcher from Belgium, to investigate the neurophysiological and biomechanical responses of chiropractic adjustments (Figure 1).  As the spine and nerve roots are exposed during decompressive surgery, we were able to use intraoperative monitoring to measure spinal nerve root responses during our research protocol (Colloca et al., 2000).  More recently, we expanded our research design to measure spinal nerve root responses, neuromuscular responses, and spinal motions simultaneously during the protocol (Figure 2) (Colloca et al., 2002; Keller et al., 2002).  We hope to be able to better understand how applied force and line of drive act to influence neurophysiological responses in patients with spinal disorders with this line of investigation.

Figure 2.  (Top to Bottom) Posteroanterior spinal motion (Accel-z; from accelerometer mounted to pin implanted into spinous process of L4), right (EMG 1 & 3) and left (EMG 2 & 4) neuromuscular responses (from needle EMG electrodes inserted into the medial multifidus muscles at L4), and right (Nerve1) and left (Nerve2) nerve root compound action potentials  (from bipolar platinum electrodes cradling the dorsal nerve roots at S1) are shown for a thrust delivered to the skin overlying the left transverse process of L5.  Left sided neuromuscular and neurophysiological responses are observed concomitant with the spinal motion that took place during the adjustment.

H-Reflex – Motoneuron Excitability

            Don Dishman, D.C., Msc, Associate Professor of Anatomy & Research at New York Chiropractic College and his team of researchers have conducted some exciting work to investigate the effects of SM on motoneuron excitability.  Using a novel research methodology, Dishman et al. administer transcranial magnetic stimulation (TMS) to the head and concomitantly measure compound muscle action potentials known as motor-evoked potentials (MEPs).  The amplitude of the MEP elicited in the target muscle by TMS will reflect changes in excitability of the alpha motoneuron pool and thus can be used to study pre-post SM changes in the central motor system.   With publications in Spine (Dishman and Bulbulian, 2000), Electromyographic Clinics of Neurophysiology (Dishman and Bulbulian, 2001), and JMPT (the Scott Haldeman award winning paper presented at the World Federation of Chiropractic 6^th Biennnial Congress) (Dishman et al., 2002) Dishman et al. have demonstrated a physiologic effect resulting from SM.  Dr. Dishman’s group is currently investigating the effects of different kinds of SM and patient populations to better understand the mechanisms of chiropractic adjustments.

            The past two decades has presented an exciting time for chiropractic research on several fronts.  With continued work from the many laboratories in this field, far reaching advances will continue to take place in the field of neurophysiological research relevant to chiropractic.  No longer can critics say that there is no evidence that an adjustment influences the nervous system.  Just how our chiropractic techniques work, however, leaves much work to be done.  In the next issue, I will discuss the relevant neurophysiological research in the field of somato-visceral reflexes.

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