Decreased spontaneous activity and altered evoked nociceptive response of rat thalamic submedius neurons to lumbar vertebra thrust

Abstract

Objectives The thalamus is a central structure important to modulating and processing all mechanoreceptor input destined for the cortex. A large number of diverse mechanoreceptor endings are stimulated when a high velocity low amplitude thrust is delivered to the lumbar spine during spinal manipulation. The objective of this study was to determine if a lumbar thrust alters spontaneous and/or evoked nociceptive activity in medial thalamic submedius (Sm) neurons. Methods Extracellular recordings were obtained from 94 thalamic Sm neurons in 54 urethane-anesthetized adult Wistar rats. Spontaneous activity was recorded 5min before and after an L5 control (no thrust) and thrust (85% rat body weight; 100ms) procedure. In a subset of responsive nociceptive specific neurons, mean changes in noxious evoked response (10s pinch with clip; 795g) at 3 sites (tail, contra- and ipsilateral hindpaw) were determined following an L5 thrust. Mean changes in Sm spontaneous activity (60s bins) and evoked noxious response were compared using a mixed model repeated measures ANOVA with Bonferonni post hoc t-tests and paired t-tests, respectively. Results Compared to control, spontaneous Sm activity decreased 180–240s following the lumbar thrust (p<0.005). Inhibitory evoked responses were attenuated in the contralateral hindpaw following an L5 thrust compared to control (p<0.05). No other changes in spontaneous or noxious evoked Sm activity were found. Conclusions A delayed, but prolonged suppression of spontaneous Sm activity along with changes in noxious evoked inhibitory responses in the contralateral hindpaw following lumbar vertebra thrust suggest that thalamic submedius neurons may play a role in central pain modulation related to manual therapy intervention.

The contemporary model of vertebral column joint dysfunction and impact of high-velocity, low-amplitude controlled vertebral thrusts on neuromuscular function

“…HVLA thrusts delivered to segments with a CSMC problem, can improve vertebral column motor control (grey box of Fig. 2 ) by bombarding the CNS with mechanoreceptive input from the segment with a CSMC problem (Pickar and Wheeler 2001 ; Sung et al 2005 ; Pickar and Kang 2006 ; Pickar et al 2007 ; Cao et al 2013 ; Reed et al 2013a , b , 2014a , b , 2017a , b ; Reed and Pickar 2015 ), yet also impact whole body functions as well (orange box in Fig. 2 ) (Haavik and Murphy 2011 ; Holt 2014 ).…”

“…Multiple studies have explored the neurophysiological impacts of an HVLA thrust applied to the vertebral column (see Table 2 ) (Gyer et al 2019 ; Pickar 2002 ; Taylor et al 2010 ; Wirth et al 2019 ; Cao and Pickar 2014 ; Reed et al 2013a , b , 2017a , b ; Clark et al 2011 ). Animal studies that have explored the effects of spinal adjustments and spinal manipulation have provided both direct and indirect measures of altered proprioception (Cao et al 2013 ; Pickar and Kang 2006 ; Pickar et al 2007 ; Pickar and Wheeler 2001 ; Sung et al 2005 ; Reed et al 2013a , b , 2014a , b , 2017a , b ; Reed and Pickar 2015 ; Colloca et al 2006 , 2008 , 2012 ; Song et al 2006 , 2016 ; Duarte et al 2019 ). When an HVLA thrust is applied to the vertebral column, this will stretch the deep paraspinal muscles.…”

“…When an HVLA thrust is applied to the vertebral column, this will stretch the deep paraspinal muscles. Studies have shown that muscle spindles and Golgi tendon organs with receptive endings in the paraspinal muscles in anaesthetised animals respond to vertebral loads whose force–time profiles are similar to that of a load delivered during spinal adjustments and manipulation (see Table 2 ) (Pickar and Bolton 2012 ; Pickar and Wheeler 2001 ; Cao and Pickar 2014 ; Cao et al 2013 ; Pickar and Kang 2006 ; Pickar et al 2007 ; Sung et al 2005 ; Reed et al 2013a , b , 2014a , b ; 2017a , b ). Studies using animal models of spinal dysfunction, including creating hypermobile and hypomobile segments, have also clearly shown that HVLA spinal adjustments of these CSMC problem segments results in changes in paraspinal muscle spindle afferents (Reed et al 2013a , b ; Reed and Pickar 2015 ).…”

“…Spinal adjustments have recently also been shown to alter intersegmental range of motion (Anderst et al 2018 ), which in turn will also alter the ongoing stretch of these small intervertebral muscles, thus also induce changes in the sensory signalling from this part of the vertebral column even after the adjustment ends. Knowing this, and considering that both the velocity and relative position of the vertebral displacement appears to be encoded by afferent nerve activity from intervertebral muscles and that afferent nerves innervating the zygapophyseal joints do not contribute significantly to the signalling of vertebral displacement (Bolton and Holland 1996 , 1998 ), all this supports the contemporary view that CSMC problems lead to ongoing altered input from mechanoreceptors, such as muscle spindles and Golgi tendon organs, from the deep intervertebral muscle afferents to the CNS and that the main impact of spinal adjustments and manipulation is also due to altered afferent input from muscle spindles and Golgi tendon organs, from the deep intervertebral muscle both during and after the controlled HVLA vertebral thrusts (Alcantara et al 2013 ; Haavik and Murphy 2012 ; Henderson 2012 ; Kent 1996 ; Taylor et al 2010 ; Reed et al 2017a , b ; Cao and Pickar 2014 ). Further evidence for this model comes from studies that have shown proprioceptive improvements occur following spinal adjustments (Haavik and Murphy 2011 ; Holt et al 2016a , b ; Palmgren et al 2006 , 2009 ).…”

“…All of the above animal studies suggest that rapid stretch of the deep, small paraspinal muscles, that occurs during an HVLA adjustment, play a major role in the mechanisms of spinal adjustments, by bombarding the CNS with mechanoreceptive and proprioceptive input (Boal and Gillette 2004 ; Evans 2002 ; Haavik and Murphy 2012 ; Haavik 2014 ; Henderson 2012 ; Pickar 2002 ; Pickar and Bolton 2012 ; Potter et al 2013 ; Cao et al 2013 ; Pickar and Wheeler 2001 ; Pickar and Kang 2006 ; Pickar et al 2007 ; Sung et al 2005 ; Reed et al 2013a , b , 2014a , b , 2017a , b ; Reed and Pickar 2015 ). This explains why spinal adjustments improve vertebral column proprioception (Palmgren et al 2006 , 2009 ; Rogers 1997 ; Gong 2013 ; García-Pérez-Juana et al 2018 ).…”