Measureable changes in the neuro-endocrinal mechanism following spinal manipulation. 
Kovanur Sampath K, Mani R, Cotter JD, Tumilty S.

The autonomic nervous system and the hypothamlamic-pituitary-adrenal axis have been shown to be dysfunctional in a number of chronic pain disorders. Spinal manipulation is a therapeutic technique used by manual therapists, which may have widespread neuro-physiological effects. The autonomic nervous system has been implicated to modulate these effects.
(Med Hypothesis. 2015 Dec;85(6):819-24. doi:10.1016/j.mehy.2015.10.003 Epub2015 Oct 14.) 

The role of spinal manipulation in addressing disordered sensorimotor integration and altered motor control. 
Haavik H1, Murphy B.

Results show a significant change in the amptitude of parietal (sensory) and frontal (motor) SEP components following the single session of cervical spine manipulation. No SEP changes were observed in the passive head movement control condition. The author states, "This study suggests that cervical spine manipulation may alter cortical somatosensory processing and sensorimotor integration.
(J Electromyogr Kinesiol. 2012 Oct;22(5):768-76. doi: 10.1016/j.jelekin.2012.02.012.)

Changes in H-reflex and V-waves following spinal manipulation.
Niazi IK, Turker KS, Flavel S, Kinget M, Duehr J, Haavik H.

"What we can say based on this study is that we adjust subluxations, we improve strength, we prevent fatigue and we change the way the brain drives our muscles." "When we adjust dysfunctional segments or subluxations, we change the way the brain processes what's going on in our bodies. Hence we are able to better process all that sensory information from through-out the body, and control the body in a better way."
(Exp Brain Res. 2015 Apr,233(4):1165-73. doi: 10.1007/s00221-014-4193-5.)

Cerebral metabolic changes in men after chiropractic spinal manipulation for neck pain.
Ogura T1, Tashiro M, Masud M, Watanuki S, Shibuya K, Yamaguchi K, Itoh M, Fukuda H, Yanai K.

Increased glucose metabolism was observed in the inferior prefrontal cortex, anterior cingulated cortex, and middle temporal gyrus, and decreased glucose metabolism was found in the cerebellar vermis and visual association cortex, in the treatment condition (P<.001). Comparisons of questionnaires indicated a lower stress level and better quality of life in the treatment condition. A significantly lower VAS (pain scale) was noted after Chiropractic Spine Manipulation. Cervical muscle tone and salivary amylase (measure of sympathetic/stress nervous system) were decreased after Chiropractic Spinal Manipulation. 
(Altern Ther Health Med. 2011 Nov-Dec;17(6):12-7.)