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Dynamic Chiropractic – September 1, 2015, Vol. 33, Issue 17

Getting Down to Brass Tacks: The Neurophysiology of Spinal Manipulation

By Anthony Rosner, PhD, LLD [Hon.], LLC

Starting the new academic year off on a bold note, I would have to stick my neck out and say, in the language of racing aficionados who follow the horses (whose horses usually follow others, quoting Frank Sinatra from one of his movies), the fix is in.

Yes, folks, I would have to join a growing chorus to state that the effects of spinal manipulation are indeed systemic and bear further research on the varied responding organ systems extending well beyond the localized area of adjustment. Let's consider data from just two organ systems to lend some heft to this assertion.

The Nervous System

A wide variety of neurophysiologic studies are simply not possible to perform in humans; thus, animal models once again come to the forefront for providing the necessary evidence for chiropractic in the basic sciences. Table 1 provides some of the earlier outcome effects achieved in a variety of animals as the result of different types of interventions, all involving noxious stimuli.1-11 Quite distinct from pain are effects which extend far from the area of stimulation.

Table 1: Neural Responses To External Forces In Animal Models

Animal Intervention Effect Observed
Mouse2 Ligature implant around sciatic nerve Inflammation
Reduced nerve conduction velocity
Motor disturbances in gait
Rat3 External pressure on L6 Slower nerve conductivity
Rat1 Surgical clamp insertion with bending at T10-T11 Decreased blood pressure
Decreased renal nerve activity
Rat4 Ligature implant around sciatic nerve Changes in gait
Changes in nerve conduction velocity
Enzymatic changes in denervated muscles
Rabbit5 Manual manipulation Gastric smooth muscle inhibition
Dog6 Surgery plus glue injection into bilateral apophyseal joints in upper lateral spine Impairment of natural killer lymphocytes
Rabbit7 Miniature compression cuff around 1 sciatic nerve Decreased aldolase activity
Decreased lactic dehydrogenase activity
Cat8 Surgical preparations
Percutaneous bradykinin injections into motion segment
Slowly increasing excitatory discharges
Expansion of receptive fields
Hyper-responsiveness to subsequent stim
Rat9 Mustard oil injection into paraarticular space around C2-C3 joint Excitatory effects in muscles that were not local, including biphasic response
Cat10 T3 and T4 dorsal nerve stimulation Activated cardiac somatosympathetic reflexes
Rat11 Dorsal spinal afferent nerve stimulation Specific somatosympathetic reflex activity

With several of the investigations showing nerve conductivity is specifically affected,2-3,8 it is clear the nervous system provides an essential link between the experimentally produced aberrations and the physiological changes observed.

Additional investigations using rats have been able to elicit decreases in both mean arterial pressure and nerve blood flow following saline injections into the ipsilateral L4/L5 facet joint.12 Further experiments by the same investigator (Sato) demonstrated decreased gastric motility in response to a somatic stimulation (skin pinch).13

Thus, a wide range of stimuli are capable of producing physiological responses, providing a much broader canvas with which subluxations can be represented in experimental research – and again placing the nervous system at the center.

A complete description of the autonomic nervous system and its division into the sympathetic and parasympathetic branches is beyond the scope of this article, but is provided elsewhere.14 With regards to changes in neural function in response to either stress or manipulation, however, several observations can be brought to light:

  1. Insertion of a small pin into the IVF of the L4 and L5 vertebral joints of the experimental rat, mimicking a space-reducing lesion, produced thermal and mechanical hyperalgesia in the hind limb and increased the excitability of dorsal root ganglion cells.15-16 The same responses were observed with the injection of an inflammatory cocktail into the same region.17
  2. Reflex responses in paraspinal muscles were attenuated by activating Z-joint receptors in rats, regarding noxious stimulation of nerves in the intervertebral disc. Accordingly, there may be interaction between spinal joint receptors and the processing mechanisms for spinal reflexes.18
  3. Abnormal somatosensory evoked potentials from the paraspinal musculature were found to correlate with decreased pain responses after lumbar manipulation, possibly due to a central effect of sensory processing.19
  4. In a cohort of 12 subjects with a history of recurrent neck stiffness and/or neck pain, but no acute symptoms at the time of study, a single session of cervical spine manipulation revealed a significant decrease in the amplitude of two components of somatosensory evoked potentials, lasting 20 minutes following the intervention. The implication is cervical spine manipulation may alter cortical somatosensory processing and sensorimotor integration, shedding light upon the mechanisms for the relief of pain and restoration of functional ability, which are the most widely observed outcomes to treatment by spinal manipulation.20
  5. In subjects subjected to side-posture manipulation, both Hoffman reflex and M-wave responses displayed the greatest attenuation with actual manipulation, as opposed to a positioning maneuver.21
  6. Following SI joint manipulation, there was a decreased inhibitory effect of knee joint pathology on quadriceps muscle activity, suggesting an interaction between spinal manipulation and the inhibition of voluntary activities produced by pain.22
  7. Power spectrum analyses of patient electrocardiograms suggested alterations of sympathetic and parasympathetic activity produced by spinal manipulation.23-25
  8. In the experimental cat, muscle spindles and Golgi tendon organs in paraspinal muscles responded to vertebral loads with force-time profiles resembling those in spinal manipulation.26 The proprioreceptors displayed a unique response to the thrusting portion of the applied load, suggesting these receptors might contribute to the therapeutic effects of spinal manipulation.
  9. More recently, in 36 subjects with identifiable myofascial pain syndrome in the infraspinatus and gluteus medius muscles, spinal manipulation at the C5-C6 spinal segment significantly reduced the pressure-pain threshold in the infraspinatus muscle, but not the gluteus medius. There was no decrease in either muscle in the sham-treated group. The implication is that the primary physiological effect of spinal manipulation may be neurophysiological, rather than changed joint mechanics, in which spinal manipulation produces inhibitory mechanisms in the myofascial tissues.27

The Endocrine System

The evidence that chiropractic is effective in relieving pain is mentioned here in its possibly being mediated by two hormonal metabolites found to respond to spinal manipulation. Beta-endorphins (enkephalins) have been proposed to display a gating, palliative effect at the first synaptic relay in the spinal cord, limiting the transmission of pain information from the peripheral pain receptor to the brain.28 Investigations by Vernon29 revealed an approximately 8 percent increase in the level of plasma endorphins 5 minutes after a single rotary manipulation in asymptomatic men. This effect was not repeated in other studies;30-31 however, only Vernon's study employed measurements timed to more closely match the rapid postintervention physiologic events suggested by others32 and are more indicative of the short half-life of plasma beta-endorphin.33

Two specific forms of the prostaglandins, the hormones responsible for uterine contraction and suspected to be the cause of menstrual pain in dysmenorrhea, were found in a pilot study by Brennan to be suppressed together with menstrual pain after side-posture manipulation, as opposed to patients who received a low-force sham procedure.34 Inconclusive results were obtained in a follow-up full-scale randomized clinical trial;35 however, design flaws in that particular investigation may have substantially compromised its results.36

A 2014 trial addressing markers of pain perception adds further credibility to the significance of hormonal responses to spinal manipulation. Thirty asymptomatic subjects subjected to cervical, thoracic, or sham manipulation had blood samples taken before, immediately after and 2 hours after each intervention. Results were as follows:37

  • Neurotensin and oxytocin levels significantly increased after both cervical and thoracic manipulations.
  • Cortisol levels increased only in the cervical manipulation group.
  • No changes were found in orexin A [a neuropeptide] levels.
  • Two hours after the intervention, no differences were observed between the groups.

This is but a mere sampling of the fortunately deeper probes into what appear to be broader consequences of spinal manipulation than what may be regarded by the public. Chiropractors should be taking several bows with this emerging information, as well as making every effort to assure research along these lines can be adequately supported and disseminated.


  1. Sato A, Swenson RS. Sympathetic nervous system response to mechanical stress of the spinal column in rats. JMPT, 1984;7(3):141-147.
  2. Triano J, Luttges M. Subtle intermittent mechanical irritation of the sciatic nerve of mice. JMPT, 1980;3(2):75-80.
  3. Israel V. Changes in nerve physiology in the rat after induced subluxation. M. Sc. Thesis, summarized in Articulations, Aug 1983:9-10.
  4. Christiansen J, Meyer J. Altered metabolic enzyme activities in fast and slow twitch muscles due to induced sciatic neuropathy in the rat. JMPT, 1987;10(5):227-231.
  5. DeBoer KF, Schutz M, McKnight ME. Acute effects of spinal manipulation on gastrointestinal myoelectric activity in conscious rabbits. Manual Med, 1988;3:85-94.
  6. Brennan PC, Kokjohn K, Triano JJ, Fritz TE, Wardrip CL, Hondras MA. Immunologic correlates of reduced spinal mobility; preliminary observations of a dog model. Proceed Int Conf Spinal Manip, 1991:118-121.
  7. Christiansen JA, Beals S, Burnham G, Magnani M, Urbanek S. Enzyme changes in rabbit muscles due to chronic compressive nerve irritation. Proceed World Fed Chiro Congress, 1991.
  8. Gillette RG, Kramis RC, Roberts WJ. Characterization of spinal somatosensory neurons having receptive fields on lumbar tissues of cats. Pain, 1993;54(1):85-98.
  9. Hu JW, Yu X-M, Vernon H, Sessle BJ. Excitatory effects on neck and jaw muscle activity of inflammatory irritant injections into cervical paraspinal tissues. Pain, 1993;55:243-250.
  10. Sato A, Sato Y, Swenson RS. Effects of morphine on somatocardiac sympathetic reflexes in spinalized cats. J Autonomic Nerv Sys, 1985;12:175-184.
  11. Araki T, Ito K, Kurosawa M, Sato A. The somato-adrenal medullary reflexes in rats. J Autonomic Nerv Sys, 1981;3:161-170.
  12. Budgell B, Holtz H, Sato A. Spinovisceral reflexes evoked by noxious and innocuous stimulation of the lumbar spine. J Neuromusculoskel Sys, 1995;3(3):122-131.
  13. Sato A, Sato Y, Shimado F, Torigata Y. Change in gastric motility produced by nociceptive stimulation of the skin in rats. Brain Res, 1975;87:151-159.
  14. Cauwenbergs P. Vertebral Subluxation and the Anatomic Relationship of the Autonomic Nervous System. In: Gatterman MI [Ed]. Foundations of Chiropractic Subluxation. St. Louis, MO: Mosby-Year Book, Inc., 1995, pp. 234-266.
  15. Song XJ, Xu DS, Vizcarrra C, Rupert RL. Onset and recovery of hyperalgesia and hyperexcitability of sensory neurons following intervertebral foramen volume reduction and restoration. JMPT, 2003;26(7):426-436.
  16. Song XJ, Vizcarra C, Xu D-S, Rupert RL, Wong Z-N. Hyperalgesia and neural excitability following injuries to central and peripheral branches of axons and somata of dorsal root ganglion neurons. J Neurophys, 2003;89:2185-2193.
  17. Song XJ, Gan Q, Cao Jun-Li, Wang Z-B, Rupert RL. Spinal manipulation reduces pain and hyperalgesia after lumbar intervertebral foramen inflammation in the rat. JMPT, 2006;29(1):5-13.
  18. Indahl A, Kaigle AM, Reikeras O, Holm SH. Interaction between the porcine lumbar intervertebral disc, zygapophyseal joints, and paraspinal muscles. Spine, 1997;22:834-2840.
  19. Zhu Y, Haldeman S, Starr A, Seffinger MA, Su SH. Paraspinal evoked cerebral potentials in patients with unilateral low back pain. Spine, 1993;18:1096-1102.
  20. Haavik-Taylor H, Murphy B. Cervical spine manipulation alters sensorimotor integration: a somatosensory evoked potential study. Clin Neurophys, 2006.
  21. Dishman JD, Dougherty PE, Burke JR. Evaluation of the effect of postural perturbation on motoneural activity following various methods of lumbar spinal manipulation. Spine J, 2005;5:650-659.
  22. Suter E, McMorland G, Herzog W, Bray R. Conservative lower back treatment reduces inhibition in knee extensor muscles: a randomized controlled trial. JMPT, 2000;23:76-80.
  23. Budgell B, Hirano F. Innocuous mechanical stimulation of the neck and alterations in heart-rate variability in healthy young adults. Autonomic Neurosci: Basic & Clinical, 2001;91:96-99.
  24. Budgell B, Polus B. The effects of thoracic manipulation on heart rate variability: a controlled crossover trial. JMPT, 2006;29(8):603-610.
  25. Welch A, Boone R. Sympathetic and parasympathetic responses to specific diversified adjustments to chiropractic vertebral subluxations of the cervical and thoracic spine. J Chiro Med, 2008;7:86-93.
  26. Pickar JG, Wheeler JD. Response of muscle proprioceptors to spinal manipulative-like loads in the anesthetized cat. JMPT, 2001;24(1):2-11.
  27. Srberly J, Vernon H, Lee D, Polgar M. Immediate effects of spinal manipulative therapy on regional anti-nociceptive effects in myofascial tissues in healthy young adults. JMPT, 2013;36(6):333-341.
  28. Iversen LL. "The Chemistry of the Brain." Scientific American, 1979;241(3):134-149.
  29. Vernon HT, Dhami MSI, Howley TP, Annett R. Spinal manipulation and beta-endorphin: a controlled study of the effect of a spinal manipulation on plasma beta-endorphin levels in normal males. JMPT, 1986;9(2):115-123.
  30. Christian GF, Sissons D, How HY, Jamison J, Alder B, Fullerton M, Funder JW. Immunoreactive ACTH, beta-endorphin, and cortisol levels in plasma following spinal manipulative therapy. Spine, 1988;13(12):1411-1417.
  31. Luisetto G, Spano D, Steiner W, Tagliaro F, Darling P, Campacci R. Plasma Levels of Beta-Endorphin and Calcitonin Before and After Manipulative Treatment of Patients With Cervical Arthrosis and Barre's Syndrome. In: Mazarelli JP [Ed], Chiropractic Interprofessional Research. Torino, ITALY: Edizioni Minerva, 1982, pp. 47-52.
  32. Herzog W, Scheele D, Conway PJ. Electromyographic responses of back and limb muscles associated with spinal manipulative therapy. Spine, 1999;24(2):146-153.
  33. Strand F. Physiology: A Systems Approach. New York, NY: Collier McMillan, 1983.
  34. Kokjohn K, Schmid DM, Triano JJ, Brennan PC. The effect of spinal manipulation on pain and prostaglandin levels in women with primary dysmenorrhea. JMPT, 1992;15(5):279-285.
  35. Hondras M, Brennan PC. The effect of spinal manipulation on pain and prostaglandin levels in women with primary dysmenorrhea. Pain, 1999;81:105-114.
  36. Rosner A. Fables of foibles: inherent problems with RCTs. JMPT, 2003;26(7):460-467.
  37. Plaza-Manzano G, Molina-Ortegra F, Lomas-Vega R, MarAnez-Arnat A, Achalandabaso A, Hita-Contreras F. Changes in biochemical markers of pain perception and stress response after spinal manipulation. J Orthoped, Sports & Phys Ther, 2014;44(4):231-239.

Click here for previous articles by Anthony Rosner, PhD, LLD [Hon.], LLC.

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