Neuroanatomical pathway of nociception originating in a low back muscle (multifidus) in the rat

Inhibiting the JNK Signaling Pathway Attenuates Hypersensitivity and Anxiety-Like Behavior in a Rat Model of Non-specific Chronic Low Back Pain

Low back pain (LBP) has become a leading cause of disability worldwide. Astrocyte activation in the spinal cord plays an important role in the maintenance of latent sensitization of dorsal horn neurons in LBP. However, the role of spinal c-Jun N-terminal kinase (JNK) in astrocytes in modulating pain behavior of LBP model rats and its neurobiological mechanism have not been elucidated. Here, we investigate the role of the JNK signaling pathway on hypersensitivity and anxiety-like behavior caused by repetitive nerve growth factor (NGF) injections in male non-specific LBP model rats. LBP was produced by two injections (day 0, day 5) of NGF into multifidus muscle of the low backs of rats. We observed prolonged mechanical and thermal hypersensitivity in the low backs or hindpaws. Persistent anxiety-like behavior was observed, together with astrocyte, p-JNK, and neuronal activation and upregulated expression of monocyte chemoattractant protein-1 (MCP-1), and chemokine (C-X-C motif) ligand 1 (CXCL1) proteins in the spinal L2 segment. Second, the JNK inhibitor SP600125 was intrathecally administrated in rats from day 10 to day 12. It attenuated mechanical and thermal hypersensitivity of the low back or hindpaws and anxiety-like behavior. Meanwhile, SP600125 decreased astrocyte and neuronal activation and the expression of MCP-1 and CXCL1 proteins. These results showed that hypersensitivity and anxiety-like behavior induced by NGF in LBP rats could be attenuated by the JNK inhibitor, together with downregulation of spinal astrocyte activation, neuron activation, and inflammatory cytokines. Our results indicate that intervening with the spinal JNK signaling pathway presents an effective therapeutic approach to alleviating LBP.

Persistent muscle hyperalgesia after adolescent stress is exacerbated by a mild-nociceptive input in adulthood and is associated with microglia activation

Non-specific low back pain (LBP) is a major global disease burden and childhood adversity predisposes to its development. The mechanisms are largely unknown. Here, we investigated if adversity in young rats augments mechanical hyperalgesia and how spinal cord microglia contribute to this. Adolescent rats underwent restraint stress, control animals were handled. In adulthood, all rats received two intramuscular injections of NGF/saline or both into the lumbar multifidus muscle. Stress induced in rats at adolescence lowered low back pressure pain threshold (PPT; p = 0.0001) and paw withdrawal threshold (PWT; p = 0.0007). The lowered muscle PPT persisted throughout adulthood (p = 0.012). A subsequent NGF in adulthood lowered only PPT (d = 0.87). Immunohistochemistry revealed changes in microglia morphology: stress followed by NGF induced a significant increase in ameboid state (p < 0.05). Repeated NGF injections without stress showed significantly increased cell size in surveilling and bushy states (p < 0.05). Thus, stress in adolescence induced persistent muscle hyperalgesia that can be enhanced by a mild-nociceptive input. The accompanying morphological changes in microglia differ between priming by adolescent stress and by nociceptive inputs. This novel rodent model shows that adolescent stress is a risk factor for the development of LBP in adulthood and that morphological changes in microglia are signs of spinal mechanisms involved.

Sex-Related Pain Behavioral Differences following Unilateral NGF Injections in a Rat Model of Low Back Pain

Low back pain (LBP) is a globally prevalent and costly societal problem with multifactorial etiologies and incompletely understood pathophysiological mechanisms. To address such shortcomings regarding the role of neurotrophins in the underlying mechanisms of pain, an LBP model was developed in rats involving two unilateral intramuscular injections of nerve growth factor (NGF) into deep trunk muscles. To date, behavioral investigations of this NGF-LBP model have been limited, especially as it pertains to female pain behaviors. This study compared mechanical sensitivity to noxious (hyperalgesia) and non-noxious (hypersensitivity) stimuli in control and NGF-injected male and female rats through pain resolution. Although the baseline testing revealed no differences between males and females, NGF-injected females demonstrated prolonged ipsilateral deep trunk mechanical hyperalgesia that resolved seven days later than males. Moreover, females showed bilateral trunk mechanical sensitivity to noxious and non-noxious stimuli compared to only ipsilateral behaviors in males. Sex differences were also observed in the severity of behavioral responses, with females displaying greater mean differences from baseline at several timepoints. Overall, these NGF-LBP behavioral findings mirror some of the sex differences reported in the clinical presentation of LBP and accentuate the translatability of this NGF-LBP model. Future studies using this LBP-NGF model could help to elucidate the neurobiological mechanisms responsible for the development, severity, and/or resolution of muscular LBP as well as to provide insights into the processes governing the transition from acute to chronic LBP.

H-reflex disinhibition by lumbar muscle inflammation in a mouse model of spinal cord injury

Inflammation is a common comorbidity in patients with traumatic spinal cord injury (SCI). Recent reports indicate that inflammation hinders functional recovery in animal models of SCI. However, the spinal mechanisms underlying this alteration are currently unknown. Considering that spinal plasticity is a therapeutic target in patients and animal models of SCI, these mechanisms remain to be clarified. Using injections of complete Freund's adjuvant (CFA) in lumbar muscles as a model of persistent inflammation, the objective of this study was to assess the impact of inflammation on spinal reflex excitability after a complete midthoracic spinal transection in mice. To this end, the excitability of spinal reflexes was examined by measuring H-reflex frequency-dependent depression (FDD) on days 7, 14 and 28 following a complete spinal transection. H-reflex parameters were compared between spinal mice with CFA and control spinal mice. On day 7, lumbar muscle inflammation disinhibited the H-reflex, reflected by an attenuation of H-reflex FDD (p < 0.01), although this effect did not persist later on, either on day 14 or day 28. These results indicate that lumbar muscle inflammation alters spinal reflex excitability transiently in spinal mice. Considering that changes in spinal reflex excitability are associated with poor functional recovery after SCI, this implies that inflammation should be treated effectively to promote optimal recovery following SCI.

From "mechanical" to "neuropathic" back pain concept in FBSS patients. A systematic review based on factors leading to the chronification of pain (part C)

INTRODUCTION

Beyond initial lesions, any form of spinal (re)operation can cause direct potential aggression to the nervous system by contact with neural tissue or by imprinting a morphological change on the neural tissue. The potential consequences of nerve root injury affect both peripheral and axial dermatomal distribution. The hypothesis of a possible neuropathic aspect associated with the back pain component of failed back surgery syndrome (FBSS) therefore appears to be reasonable. Its pathophysiology remains unclear due to the permanent interplay between nociceptive and neuropathic pain components, resulting in the coexistence of physiological and pathological pain at the same anatomical site. This paper is designed to extensively review the fundamental mechanisms leading to chronification of pain and to suggest considering the emerging concept of "neuropathic back pain".

METHODS

Literature searches included an exhaustive review of 643 references and 74 book chapters updated by searching the major electronic databases from 1930 to August 2013.

RESULTS

Inflammatory and neuropathic back pain could be distinguished from pure nociceptive pain as a result of an increased activity and responsiveness of sensitized receptors at the peripheral nervous system and also as a consequence of increased afferent inflow to the central nervous system, moving to a new, more excitable "wind-up" state. This can be clinically translated to an amplified response to a moderate/intense stimulus (primary hyperalgesia) or an aversive sensation provoked by the activation of low-threshold mechanoreceptors through non-noxious stimuli, which defines allodynia. Activated non-neuronal cells including microglia have been found to be cellular intermediaries in mechanical allodynia. Major changes in the spinal cord are the loss of inhibitory mechanisms, resulting in an increased activity of interneurons or projection neurons and a structural reorganization of the central projection pattern. This abnormal excitability of sensory neurons is coupled to changes in the neurotransmitter phenotype, which could induce a resistance to conventional analgesic treatments.

CONCLUSION

A clear understanding of the factors leading to the chronification of back pain should help us to move to the choice of mechanism related pain treatments to improve outcomes in FBSS chronic condition.

Sensory and autonomic innervation of the cervical intervertebral disc in rats: the pathomechanics of chronic discogenic neck pain

STUDY DESIGN

An immunohistological analysis of the cervical intervertebral disc (IVD).

OBJECTIVE

To investigate sensory and autonomic innervation of the rat cervical IVD.

SUMMARY OF BACKGROUND DATA

Many clinicians are challenged with treating wide-ranging chronic neck pain. Several authors have reported that sympathetic nerves participate in chronic pain, and various sympathectomy procedures can effectively treat chronic pain.

METHODS

The neuro-tracer Fluoro-gold (FG) was applied to the anterior surfaces of C5-C6 IVDs from 10 Sprague-Dawley rats to label the neurons of the innervating dorsal root ganglion (DRG), stellate ganglion (SG; sympathetic ganglion), and nodose ganglion (NG; parasympathetic ganglion). Seven days postsurgery, DRGs from level C1-C8, SG, and NG neurons were harvested, sectioned, and immunostained for calcitonin gene-related peptide (CGRP; a marker for peptide-containing neurons) and isolectin B4 (IB4; a marker for nonpeptide-containing neurons). The proportion of FG-labeled DRG neurons that were CGRP-immunoreactive (CGRP-IR), IB4-binding, and non-CGRP-IR and IB4-binding, and the proportion of FG-labeled SG neurons and NG neurons were calculated.

RESULTS

FG-labeled neurons innervating the C5-C6 IVD were distributed throughout the C2-C8 DRGs. The proportions of FG-labeled DRG neurons that were CGRP-IR, IB4-binding, non-CGRP-IR and IB4-binding, as well as SG neurons, and NG neurons were 20.6%, 3.3%, 55.7%, 8.9%, and 11.5%, respectively. The proportion of CGRP-IR FG-labeled DRG neurons was significantly higher than the proportion of IB4-binding FG-labeled DRG neurons at each level (P < 0.05).

CONCLUSION

The C5-C6 IVD was innervated multisegmentally from neurons of the C2-C8 DRG, SG, and NG. Overall, 79.6% of the nerve fibers innervating the IVD were sensory nerves and 20.4% were autonomic nerves. Furthermore, 23.9% of the nerve fibers innervating the IVD were afferent sensory pain-related nerves, 8.9% were efferent sympathetic nerves, and 11.5% were efferent parasympathetic nerves. These findings may explain the wide-ranging and chronic discogenic pain that occurs via the somatosensory and autonomic nervous system.

What is different about spinal pain?

BACKGROUND

The mechanisms subserving deep spinal pain have not been studied as well as those related to the skin and to deep pain in peripheral limb structures. The clinical phenomenology of deep spinal pain presents unique features which call for investigations which can explain these at a mechanistic level.

METHODS

Targeted searches of the literature were conducted and the relevant materials reviewed for applicability to the thesis that deep spinal pain is distinctive from deep pain in the peripheral limb structures. Topics related to the neuroanatomy and neurophysiology of deep spinal pain were organized in a hierarchical format for content review.

RESULTS

Since the 1980's the innervation characteristics of the spinal joints and deep muscles have been elucidated. Afferent connections subserving pain have been identified in a distinctive somatotopic organization within the spinal cord whereby afferents from deep spinal tissues terminate primarily in the lateral dorsal horn while those from deep peripheral tissues terminate primarily in the medial dorsal horn. Mechanisms underlying the clinical phenomena of referred pain from the spine, poor localization of spinal pain and chronicity of spine pain have emerged from the literature and are reviewed here, especially emphasizing the somatotopic organization and hyperconvergence of dorsal horn "low back (spinal) neurons". Taken together, these findings provide preliminary support for the hypothesis that deep spine pain is different from deep pain arising from peripheral limb structures.

CONCLUSIONS

This thesis addressed the question "what is different about spine pain?" Neuroanatomic and neurophysiologic findings from studies in the last twenty years provide preliminary support for the thesis that deep spine pain is different from deep pain arising from peripheral limb structures.

Lumbar facet joint compressive injury induces lasting changes in local structure, nociceptive scores, and inflammatory mediators in a novel rat model

Objective. To develop a novel animal model of persisting lumbar facet joint pain. Methods. Sprague Dawley rats were anaesthetized and the right lumbar (L5/L6) facet joint was exposed and compressed to ~1 mm with modified clamps applied for three minutes; sham-operated and naïve animals were used as control groups. After five days, animals were tested for hind-paw sensitivity using von Frey filaments and axial deep tissue sensitivity by algometer on assigned days up to 28 days. Animals were sacrificed at selected times for histological and biochemical analysis. Results. Histological sections revealed site-specific loss of cartilage in model animals only. Tactile hypersensitivity was observed for the ipsi- and contralateral paws lasting 28 days. The threshold at which deep tissue pressure just elicited vocalization was obtained at three lumbar levels; sensitivity at L1 > L3/4 > L6. Biochemical analyses revealed increases in proinflammatory cytokines, especially TNF-α, IL-1α, and IL-1β. Conclusions. These data suggest that compression of a facet joint induces a novel model of local cartilage loss accompanied by increased sensitivity to mechanical stimuli and by increases in inflammatory mediators. This new model may be useful for studies on mechanisms and treatment of lumbar facet joint pain and osteoarthritis.

Exercise-induced pain requires NMDA receptor activation in the medullary raphe nuclei

PURPOSE

Pain in response to physical activity is common in people with chronic musculoskeletal pain and is likely a barrier to regular exercise, which would lead to a sedentary lifestyle. We recently developed a model of exercise-induced pain that is associated with increased activation of neurons in the medullary raphe nuclei, i.e., the nucleus raphe obscurus (NRO) and nucleus raphe pallidus (NRP). Because the NRO and NRP not only modulate motor output but also respond to noxious stimuli, we hypothesized that the NRO and NRP were key nuclei in the interaction between pain and exercise. We tested whether exercise enhances hyperalgesia through activation of N-methyl D-aspartate (NMDA) receptors in the NRO/NRP.

METHODS

Muscle insult was induced by two injections of pH 5.0 saline 5 d apart into one gastrocnemius muscle. We initially tested whether hyperalgesia developed in mice injected with acidic saline (pH 5.0) into the gastrocnemius muscle immediately after a 30-min or 2-h exercise task or 2 h after a 2-h exercise task. Next, we tested whether blockade of NMDA receptors in the NRO/NRP during the exercise task prevented the development of exercise-induced hyperalgesia. Finally, we evaluated changes in phosphorylation of the NR1 subunit of the NMDA receptor (pNR1) after the exercise task at times in which muscle insult was given in behavioral experiments, i.e., immediately after a 30-min or 2-h exercise task or 2 h after the 2-h exercise task.

RESULTS

All exercise conditions enhanced nociception (hyperalgesia) after combining with two injections of pH 5.0 saline. Microinjection of AP5 (1.0-0.1 nmol; 2-amino-5-phophonopenanoate) dose-dependently prevented the development of exercise-induced hyperalgesia. All exercise conditions increased pNR1 in the NRO and NRP.

CONCLUSIONS

Thus, exercise-induced pain in sedentary mice is associated with increased phosphorylation and activation of NMDA receptors in the NRO/NRP, suggesting that changes in central excitability mediate an interaction between unaccustomed exercise and pain.

Investigation of dichotomizing sensory nerve fibers projecting to the lumbar multifidus muscles and intervertebral disc or facet joint or sacroiliac joint in rats

STUDY DESIGN

Immunohistological analysis of dichotomizing sensory nerve fibers projecting to the lumbar multifidus muscles and intervertebral disc (IVD), facet joint (FJ), or sacroiliac joint (SIJ) in rats.

OBJECTIVE

To elucidate dichotomizing sensory nerve fibers projecting to the lumbar multifidus muscles and to IVDs, FJs, or SIJs.

SUMMARY OF BACKGROUND DATA

Clinically, the origin of low back pain remains unknown. Multiple studies have identified lumbar muscles, IVDs, FJs, and SIJs as sources of low back pain. Pain may originate directly from lumbar muscles or be referred from the spine, or both. Dorsal root ganglion (DRG) neurons with dichotomizing axons have been reported in several species and are thought to be related to referred pain.

METHODS

We used 2 neurotracers, 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) and fluorogold (FG), in this double-labeling study involving 30 Sprague Dawley rats. DiI was applied to lumbar multifidus muscles in all rats. Simultaneously, FG was applied to the anterior left portion of L5-L6 IVDs in the IVD group (n = 10), to the left L5-L6 FJs in the FJ group (n = 10), and to the left SIJs in the SIJ group (n = 10). Fourteen days after surgery, left DRGs from L1 to L6 were harvested, sectioned, and observed under a fluorescence microscope.

RESULTS

We verified the existence of double-labeled DRG neurons (i.e., dichotomizing sensory nerve fibers) projecting to lumbar multifidus muscles and to IVDs, FJs, or SIJs, depending on the group. The proportion of double-labeled cells in all DiI-labeled DRG neurons was higher in the FJ group (6.8%) and SIJ group (7.1%) than in the IVD group (3.1%) (P < 0.05).

CONCLUSION

Our results document the presence of dichotomizing sensory nerve fibers projecting to lumbar multifidus muscles and to IVDs, FJs, and SIJs. Referred low back muscle pain may reflect disorders of lumbar posterior structures, such as FJs and SIJs, rather than disorders of lumbar anterior structures, such as IVDs.

Differences in damage to CGRP immunoreactive sensory nerves after two lumbar surgical approaches: investigation using humans and rats

STUDY DESIGN

Immunohistochemical study.

OBJECTIVE

To evaluate invasive surgical approaches by analyzing the number of sensory nerve fibers at 2 back muscle sites in rats and humans and the number of injured nerve fibers innervating these 2 sites after muscle injury in rats. SUMMARY OF BACKGROUND DATA.: Several minimally invasive approaches have recently become popular in the treatment of lumbar spine disorders. Minimally invasive surgery (MIS) is not invasive to back muscle and is thought to reduce low back pain. Muscle damage has been generally evaluated by magnetic resonance imaging (MRI); however, damage to sensory nerve fibers in and around back muscle that is directly related to pain has apparently not been explored.

METHODS

Human muscle at L4-L5 was obtained from the paraspinous process (during a midline approach) and from paraspinal back muscle (during a Wiltse paraspinal approach) (n = 10 each). The muscle was sectioned and immunostained for calcitonin gene-related peptide (CGRP). To detect dorsal root ganglion (DRG) neurons innervating back muscle in rats, Fluoro-Gold (FG) was applied to the same 2 sites on the lower back muscle at L4-L5 (only application, n = 12; application of FG + muscle injury, n = 12). DRGs were harvested and immunostained for CGRP and activating transcription factor-3 (ATF-3: marker for nerve injury). The numbers of FG-labeled CGRP-immunoreactive or FG-labeled ATF-3-immunoreactive DRG neurons innervating the 2 sites were counted and compared.

RESULTS

CGRP-immunoreactive sensory nerve fibers were found at the 2 sites. The average number of CGRP-immunoreactive sensory nerve fibers in muscle obtained in a midline approach was significantly higher than that in muscle obtained in a Wiltse paraspinal approach (P < 0.01). The numbers of FG-labeled CGRP- and ATF-3-immunoreactive DRG neurons innervating paraspinous process muscle were significantly greater than those innervating paraspinal back muscle in rats (P < 0.01).

CONCLUSION

There are more CGRP-immunoreactive sensory nerve fibers and DRG neurons innervating muscle in the midline approach area than in the Wiltse paraspinal approach area in humans and rats. There are more ATF-3-immunoreactive DRG neurons innervating muscle in the midline approach area than in the Wiltse paraspinal approach area after muscle injury in rats. This result may show the differences in sensory nerve injury during the 2 surgical approaches.

Sensory innervation of the nonspecialized connective tissues in the low back of the rat

Chronic musculoskeletal pain, including low back pain, is a worldwide debilitating condition; however, the mechanisms that underlie its development remain poorly understood. Pathological neuroplastic changes in the sensory innervation of connective tissue may contribute to the development of nonspecific chronic low back pain. Progress in understanding such potentially important abnormalities is hampered by limited knowledge of connective tissue's normal sensory innervation. The goal of this study was to evaluate and quantify the sensory nerve fibers terminating within the nonspecialized connective tissues in the low back of the rat. With 3-dimensional reconstructions of thick (30-80 μm) tissue sections we have for the first time conclusively identified sensory nerve fiber terminations within the collagen matrix of connective tissue in the low back. Using dye labeling techniques with Fast Blue, presumptive dorsal root ganglia cells that innervate the low back were identified. Of the Fast Blue-labeled cells, 60-88% also expressed calcitonin gene-related peptide (CGRP) immunoreactivity. Based on the immunolabeling with CGRP and the approximate size of these nerve fibers (≤2 μm) we hypothesize that they are Aδ or C fibers and thus may play a role in the development of chronic pain.

Historical overview and update on subluxation theories()

OBJECTIVE

This article presents a personal view of the historical evolution of theories of subluxation in the chiropractic profession.

DISCUSSION

TWO MAJOR THEMES EMERGE FROM THIS REVIEW: those related to the mechanical behavior of the spine and those related to the neurologic implications of these mechanical issues. Chiropractic subluxation theory is one of the few health-related theories whereby these mechanical and neurologic theories have been unified into a comprehensive theory of disorder of spinal function. For this disorder, doctors of chiropractic have used the term subluxation. These theories, and their unification in the "subluxation concept," have undergone evolution in the profession's history.

CONCLUSION

The "subluxation concept" currently faces challenges, which are briefly reviewed in this article. The only way forward is to strengthen our efforts to investigate the "subluxation concept" with high-quality scientific studies including animal models and human clinical studies.

Primary sensory neurons with dichotomizing axons projecting to the facet joint and the low back muscle in rats

BACKGROUND

Clinically, the origin of low back pain is unknown. The pain may originate from the lumbar muscles directly, or it may be referred pain from the spine. Dorsal root ganglion (DRG) neurons with dichotomizing axons have been reported in several species and are thought to be related to referred pain. However, these neurons, which have dichotomizing axons to the lumbar facet joints and to the lumbar muscle, have not been fully investigated.

METHODS

Two neurotracers - 1,1'-dioctadecyl-3,3,3',3'- tetramethyl-indocarbocyanine perchlorate (DiI) and fluorogold (FG) - were used in the present double-labeling study. DiI crystals were placed in the right L5/6 facet joint, and FG was applied to right multifidus muscles at the L5 level in 10 rats. Two weeks later, bilateral DRGs from L1 through L6 were harvested, sectioned, and observed under a fluorescence microscope.

RESULTS

DiI-labeled DRG neurons innervating the L5/6 facet joint (5.17% of the total DRG neurons) were distributed from L1 to L6. FG-labeled DRG neurons innervating the lower back muscle (15.9% of the total) were also distributed from L1 to L6. Double-labeled DRG neurons were found from L1 to L6. The ratio of total double-labeled/total DiI-labeled DRG neurons was 17% and that of total double-labeled/total FG-labeled DRG neurons was 7%. Approximately 17% of all DRG neurons innervating the facet joints had other axons that extended to the lower back muscle.

CONCLUSIONS

This finding provides a possible neuroanatomical explanation for referred low back muscle pain from the lower facet joints.

[Manual medicine]

The techniques of manual medicine provide the basis for the palpatory recognition of dysfunction, primarily in the musculoskeletal system. The essential criterion of manual medicine is inclusion of the segmental level of function. One criterion of evaluation is, most notably, segmental hypomobility. The purpose of manual diagnostics is to determine segmental blockage. Manual therapy techniques lead first to improved function of the segment and, consequently, to improved function of the whole organism. Manual medicine focuses on the (peri)articular and myofascial levels as well as on the nervous control level of the motion segment and may interact with therapeutic methods. For therapeutic purposes, the joint may be manipulated with impulse as well as without therapeutic impulse for soft techniques. Different muscle relaxation techniques are used on the myofascial system to achieve improved function of reflectory or structurally altered musculature and of the myofascial system.

Dorsal horn neurons having input from low back structures in rats

The mechanisms of nociception in the low back are poorly understood, partly because systematic recordings from dorsal horn neurons with input from the low back are largely missing. The purpose of this investigation was to (1) identify spinal segments and dorsal horn neurons receiving input from the low back, (2) test the effect of nerve growth factor (NGF) injected into the multifidus muscle (MF) on the neurons' responsiveness, and (3) study the influence of a chronic MF inflammation on the responses. In rats, microelectrode recordings were made in the segments L2, L3, and L5 to find dorsal horn neurons having input from the low back (LB neurons). In control animals, the proportion of LB neurons in L2 was larger than in L3 and L5. Most LB neurons had a convergent input from several tissues. Injections of NGF into MF increased the proportion of LB neurons significantly. A chronic MF inflammation likewise increased the proportion of LB neurons and the input convergence. The centers of the neurons' receptive fields (RFs) were consistently located 2-3 segments caudally relative to their recording site. The results show that (1) input convergence from various tissues is common for LB neurons, (2) the input from structures of the low back is processed 2-3 segments cranially relative to the vertebral level of the RFs, and (3) the responsiveness of LB neurons is increased during a pathologic alteration of the MF. The above findings may be relevant for some cases of chronic low back pain in patients.