ARTICLE
Spinal manipulation has been an effective intervention for the management of various disorders. The mechanisms underlying the pain modulatory effects of spinal manipulation remain elusive. Although both biomechanical and neurophysiological phenomena have been thought to play a role in the observed clinical effects of spinal manipulation, a growing number of recent studies have indicated peripheral, spinal and supraspinal mechanisms of manipulation and suggested that the improved clinical outcomes are largely of neurophysiological origin. This article reviewed the relevance of various neurophysiological theories with respect to the findings of mechanistic studies that demonstrated neural responses following spinal manipulation. It also discussed whether these neural responses are associated with the possible neurophysiological mechanisms of spinal manipulation. The body of literature reviewed suggested some clear neurophysiological changes following spinal manipulation. Most of the early theories proposed to explain the analgesic and hypoalgesic effects of spinal manipulation were heavily focused on the biomechanical changes following the intervention. However, there has been a paradigm shift towards a neurophysiological mechanism of spinal manipulation, as an increasing number of recent studies have reported various neural effects of spinal manipulation. These studies have suggested a cascade of neurochemical responses in the central and peripheral nervous system following spinal manipulation. Hence, it has been hypothesised that the observed pain modulatory effects of spinal manipulation are largely due to neurophysiological mechanisms mediated by peripheral, spinal and supraspinal structures triggered by mechanical stimulus applied during the manipulative act. There has been a need for a comprehensive review that presents an updated framework based on the current knowledge and understanding of the neurophysiological effects of spinal manipulation. Therefore, the purpose of this article is to examine all the recent findings on the neurophysiological effects of spinal manipulation and review their relevance with respect to the improved clinical outcomes of spinal manipulation. Some excepts include the following: The evidence to date suggests that the effects of spinal manipulation are beyond biomechanical changes; in fact, a cascade of neurophysiological mechanisms may be initiated. There are four main theories of biomechanical changes elicited by spinal manipulation. These include (1) release of entrapped synovial folds or meniscoids; (2) restoration of buckled motion segments; (3) reduction of articular or periarticular adhesions; and (4) normalization of “hypertonic” muscle by reflexogenic effect. So far, only the muscular reflexogenic theory has some plausible evidence in support of its mechanical explanation. Biomechanical changes evoked as a result of spinal manipulation may induce neurophysiological responses by influencing the inflow of sensory input to the central nervous system (CNS). Moreover, the mechanical force applied during spinal manipulation could either stimulate or silence mechanosensitive and nociceptive afferent fibers in paraspinal tissues, including skin, muscles, disk, facet joints, tendons and ligaments. These inputs are thought to stimulate pain-processing mechanisms and other physiological systems connected to the nervous system. Neural responses arising from the nervous system due to mechanical stimuli might be due to alterations in peripheral sensory input from paraspinal tissues. Taken together, it can be said that changes in spinal biomechanics trigger the chain of neurophysiological responses that effects the therapeutic outcomes associated with spinal manipulation, and there is a potential for combined biomechanical and neurophysiological effects following spinal manipulation. Many authors have long postulated that spinal manipulation exerts its therapeutic effects through several neurophysiological mechanisms working on their own or in combination. These mechanisms involve complex interactions between the peripheral nervous system and the CNS and are thought to be activated when spinal manipulation stimulates paraspinal sensory afferents. These paraspinal sensory inputs are assumed to alter neural integration either by directly influencing reflex activity or by affecting central neural integration within motor, nociceptive and possibly autonomic neuronal pools. Implications for specific neural mechanisms of manipulation are suggested from associated neurophysiological responses, which have been observed in mechanistic studies. Over the past decades, a number of specific and nonspecific neural effects of spinal manipulation have been reported, including increased afferent discharge, central motor excitability, alterations in pain processing, reduction of temporal summation, stimulation of autonomic nervous system (ANS), lessening of pain perception and many more. These neural responses collectively implicate mechanisms mediated by the nervous system. The muscular reflexogenic response is an important theory that is frequently used to explain the mechanism of spinal manipulation. The muscles of the human body have some reflex responses, by means of their neural reflex arcs, to protect themselves from potentially harmful force. The reflexogenic effect is often explained using one of the prominent theories of pain, the pain-spasm-pain cycle, which suggests that pain causes muscular hyperactivity (spasm) and muscle spasm reflexively produces pain, establishing a self-perpetuating cycle. Spinal manipulation is thought to disrupt the pain-spasm-pain cycle by reducing muscle activity through reflex pathways. … It is evident that spinal manipulation results in neuromuscular responses, involves spinal reflex pathways and may reduce muscle hyperactivity. … It can be said that spinal manipulation may function via the pain model by attenuating stretch reflex hyperactivity and consequently reducing the hyperexcitability of γ motor neurons. The pain-adaptation model postulates that pain increases muscle activity when the muscle acts as antagonist but decreases it when acting as an agonist. The neural pathway proposed for this model involves feedback of nociceptive afferents projecting onto α motor neurons via both excitatory and inhibitory interneurons. The CNS is thought to control the function of these interneurons and provide motor command of whether to excite or inhibit the α-motoneuronal pool. In short, regardless of the exact neural pathways, it may be said that the α motor neuron excitability forms the basis in the mechanism of musculoskeletal pain, as the modulation of α motor neurons correlates with changes in muscle activation. …Spinal manipulation has been thought to relax or normalize hypertonic muscle through modulating α motor neuron activity. The Autonomic Nervous System (ANS) acts largely unconsciously and controls involuntary responses that maintain the body’s internal environment. It regulates several body processes (e.g., heart rate, respiratory rate, sweat and salivary secretion, blood pressure and pupillary response) and innervates various internal organs that have smooth muscle (e.g., heart, lungs, pupils, salivary, liver, kidneys, bladder and digestive glands). The system is regulated from the hypothalamus portion of the brain and is also in control of the underlying mechanisms during a fight-or flight response. The ANS also has potential interactions with the nociceptive (pain) system on multiple levels, which include the brain stem, fore brain, periphery and dorsal horn. Hence, any intervention that influences the functions of the ANS may have significant implications, as this may provide important mechanistic information and even shed some light on the possible neurophysiological mechanisms of that intervention. Autonomically mediated responses following spinal manipulation have been well established. It has been presumed that the effects of spinal manipulation on the ANS might lead to opioid independent analgesia, influencing the reflex neural outputs on the segmental and extra-segmental levels. Anatomically, the two complementary parts of the ANS include the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The interaction between these systems is known to influence the stress response of tissues. The SNS plays an active role in mediating the fight-or-flight response and serves as a medium for the efferent communication between the immune system and the CNS. It releases catecholamine as an end product, which modulates several immune parameters during acute and chronic inflammation. It has also been found that musculoskeletal abnormalities are associated with alterations in cutaneous patterns of sympathetic activity. This modulatory effect of the SNS on inflammation has been of special interest, as it may explain some of the neurophysiological effects observed after spinal manipulation. Some clear neurophysiological effects of spinal manipulation have been demonstrated. Source: https://chiro.org/research/ABSTRACTS/Spinal_Manipulation_Therapy.shtml