Classification of longissimus lumborum muscle spindle afferents in the anaesthetized cat

The Neurophysiological Impact of Experimentally-Induced Pain on Direct Muscle Spindle Afferent Response: A Scoping Review

Background: Musculoskeletal pain disorders are among the leading causes of years lived with disability worldwide representing a significant burden to society. Studies investigating a “nociceptive-fusimotor” relationship using experimentally-induced pain/noxious stimuli and muscle spindle afferent (MSA) response have been published over several decades. The purpose of this scoping review was to systematically identify and summarize research findings related to the impact of experimentally-induced pain or noxious stimulation on direct MSA discharge/response.

Methods: PubMed, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Cochrane and Embase were searched from database inception to August 2020. Eligible studies were: (a) published in English; (b) clinical or pre-clinical studies; (c) original data studies; (d) included the investigation of MSA response to experimentally-induced pain or noxious stimulation; (e) included quantification of at least one direct physiological measure associated with MSA activity/response. Two-phase screening procedures were conducted by a pair of independent reviewers and data extracted from eligible studies.

Results: The literature search resulted in 195 articles of which 23 met inclusion criteria. Six studies (26%) were classified as clinical and 17 (74%) as pre-clinical. Two clinical studies investigated the effects of sacral dermatome pin-pricking on MSA response, while the remaining 4 studies investigated the effects of tonic muscle and/or skin pain induced by injection/infusion of hypertonic saline into the tibialis anterior muscle or subdermal tissues. In pre-clinical studies, muscle pain was induced by injection of noxious substances or the surgical removal of the meniscus at the knee joint.

Conclusion: Clinical studies in awake humans reported that experimentally-induced pain did not affect, or else slightly decreased MSA spontaneous discharge and/or response during weak dorsiflexor muscle contraction, thus failing to support an excitatory nociceptive-fusimotor relationship. However, a majority of pre-clinical studies indicated that ipsilateral and contralateral muscle injection of noxious substances altered MSA resting discharge and/or response to stretch predominately through static fusimotor reflex mechanisms. Methodological differences (use of anesthesia, stretch methodology, etc.) may ultimately be responsible for the discrepancies between clinical and pre-clinical findings. Additional investigative efforts are needed to reconcile these discrepancies and to clearly establish or refute the existence of nociceptive-fusimotor relationship in muscular pain.

Stretch Receptor and Somatic Dysfunction: A Narrative Review

From its founding by Andrew Taylor Still, MD, DO, through the work of many contributors, one of the cornerstones of osteopathic medicine has been its ability to aid health by promoting neuromuscular homeostasis. As part of the understanding of osteopathic medicine since the time of Still, the proper functioning of stretch receptor organs (SROs) of skeletal muscle have been recognized as having a central role in this homeostasis. In doing so, the complexities of these numerous and vital sensors are described, including recent findings regarding their structure, function, and the nature of their neural connections. In their homeostatic role, SROs conduct information centrally for integration in proprioceptive and autonomic reflexes. By virtue of their integral role in muscle reflexes, they are putatively involved in somatic dysfunction and segmental facilitation. In reviewing some well-established knowledge regarding the SRO and introducing more recent scientific findings, an attempt is made to offer insights on how this knowledge may be applied to better understand somatic dysfunction.

Effect of changing lumbar stiffness by single facet joint dysfunction on the responsiveness of lumbar muscle spindles to vertebral movement

OBJECTIVE

Individuals experiencing low back pain often present clinically with intervertebral joint dysfunction. The purpose of this study was to determine whether relative changes in stiffness at a single spinal joint alters neural responsiveness of lumbar muscle spindles to either vertebral movement or position.

METHODS

Muscle spindle discharge was recorded in response to 1mm L6 ramp and hold movements (0.5mm/s) in the same animal for lumbar laminectomy-only (n=23), laminectomy & L5/6 facet screw (n=19), laminectomy & L5/6 facetectomy (n=5) conditions. Mean instantaneous frequency (MIF) was calculated for the ramp-up, hold, ramp-down and post-ramp phases during each joint condition.

RESULTS

Mean MIFs were not significantly different between the laminectomy-only and the other two types of joint dysfunction for the ramp-up, hold, ramp-down, or post-ramp phases.

CONCLUSION

Stiffness changes caused by single facet joint dysfunction failed to alter spindle responses during slow 1mm ramp and hold movements of the L6 vertebra.

Using vertebral movement and intact paraspinal muscles to determine the distribution of intrafusal fiber innervation of muscle spindle afferents in the anesthetized cat

Increasing our knowledge regarding intrafusal fiber distribution and physiology of paraspinal proprioceptors may provide key insights regarding proprioceptive deficits in trunk control associated with low back pain and lead to more effective clinical intervention. The use of vertebral movement as a means to reliably stretch paraspinal muscles would greatly facilitate physiological study of paraspinal muscle proprioceptors where muscle tendon isolation is either very difficult or impossible. The effects of succinylcholine (SCh) on 194 muscle spindle afferents from lumbar longissimus or multifidus muscles in response to computer-controlled, ramp-and-hold movements of the L(6) vertebra were investigated in anesthetized cats. Paraspinal muscles were stretched by moving the L(6) vertebra 1.5-1.7 mm in the dorsal-ventral direction. Initial frequency (IF), dynamic difference (DD), their changes (∆) following SCh injection (100-400 μg kg(-1)), and post-SCh dynamic difference (SChDD) were measured. Muscle spindle intrafusal fiber terminations were classified as primary or secondary fibers as well as bag(1) (b(1)c), bag(2) (b(2)c), b(1)b(2)c, or chain (c) fibers. Intrafusal fiber subpopulations were distinguished using logarithmic transformation of SChDD and ∆IF distributions as established by previous investigators. Increases in DD indicate strength of b(1)c influence while increases in IF indicate strength of b(2)c influence. Out of 194 afferents, 46.9 % of afferents terminated on b(2)c fibers, 46.4 % on b(1)b(2)c fibers, 1 % on b(1)c fibers, and 5.7 % terminated on c fibers. Based on these intrafusal fiber subpopulation distributions, controlled vertebral movement can effectively substitute for direct tendon stretch and allow further investigation of paraspinal proprioceptors in this anatomically complex body region.

Plane of vertebral movement eliciting muscle lengthening history in the low back influences the decrease in muscle spindle responsiveness of the cat

Proprioceptive feedback is thought to play a significant role in controlling both lumbopelvic and intervertebral orientations. In the lumbar spine, a vertebra's positional history along the dorsal-ventral axis has been shown to alter the position, movement, and velocity sensitivity of muscle spindles in the multifidus and longissimus muscles. These effects appear due to muscle history. Because spinal motion segments have up to 6 degrees of freedom for movement, we were interested in whether the axis along which the history is applied differentially affects paraspinal muscle spindles. We tested the null hypothesis that the loading axis, which creates a vertebra's positional history, has no effect on a lumbar muscle spindle's subsequent response to vertebral position or movement. Identical displacements were applied along three orthogonal axes directly at the L(6) spinous process using a feedback motor system under displacement control. Single-unit nerve activity was recorded from 60 muscle spindle afferents in teased filaments from L(6) dorsal rootlets innervating intact longissimus or multifidus muscles of deeply anesthetized cats. Muscle lengthening histories along the caudal-cranial and dorsal-ventral axis, compared with the left-right axis, produced significantly greater reductions in spindle responses to vertebral position and movement. The spinal anatomy suggested that the effect of a lengthening history is greatest when that history had occurred along an axis lying within the anatomical plane of the facet joint. Speculation is made that the interaction between normal spinal mechanics and the inherent thixotropic property of muscle spindles poses a challenge for feedback and feedforward motor control of the lumbar spine.

Processing afferent proprioceptive information at the main cuneate nucleus of anesthetized cats

Medial lemniscal activity decreases before and during movement, suggesting prethalamic modulation, but the underlying mechanisms are largely unknown. Here we studied the mechanisms underlying proprioceptive transmission at the midventral cuneate nucleus (mvCN) of anesthetized cats using standard extracellular recordings combined with electrical stimulation and microiontophoresis. Dual simultaneous recordings from mvCN and rostroventral cuneate (rvCN) proprioceptive neurons demonstrated that microstimulation through the rvCN recording electrode induced dual effects on mvCN projection cells: potentiation when both neurons had excitatory receptive fields in muscles acting at the same joint, and inhibition when rvCN and mvCN cells had receptive fields located in different joints. GABA and/or glycine consistently abolished mvCN spontaneous and sensory-evoked activity, an effect reversed by bicuculline and strychnine, respectively; and immunohistochemistry data revealed that cells possessing strychnine-sensitive glycine receptors were uniformly distributed throughout the cuneate nucleus. It was also found that proprioceptive mvCN projection cells sent ipsilateral collaterals to the nucleus reticularis gigantocellularis and the mesencephalic locomotor region, and had slower antidromic conduction speeds than cutaneous fibers from the more dorsally located cluster region. The data suggest that (1) the rvCN-mvCM network is functionally related to joints rather than to single muscles producing an overall potentiation of proprioceptive feedback from a moving forelimb joint while inhibiting, through GABAergic and glycinergic interneurons, deep muscular feedback from other forelimb joints; and (2) mvCN projection cells collateralizing to or through the ipsilateral reticular formation allow for bilateral spreading of ascending proprioceptive feedback information.

Lengthening but not shortening history of paraspinal muscle spindles in the low back alters their dynamic sensitivity

Proprioception is considered important for maintaining spinal stability and for controlling posture and movement in the low back. Previous studies demonstrate the presence of thixotropic properties in lumbar muscle spindles, wherein a vertebra's positional history alters spindle responsiveness to position and movement. This study investigated whether a vertebra's movement history affects the velocity sensitivity of paraspinal muscle spindles in the low back. Afferent activity from multifidus and longissimus muscle spindles was recorded in the L(6) dorsal root in 30 anesthetized cats. To alter movement history, a feedback-controlled motor attached to the L(6) spinous process held (conditioned for 4 s) the L(6) vertebra at an intermediate position or at positions that either lengthened or shortened the muscles. With the vertebra returned to the intermediate position, resting spindle discharge was measured over the next 0.5 s (static test) and then during a dynamic test consisting of ramp vertebral movement at four velocities (0.2, 0.5, 1.0, 2.0 mm/s). Spindle activity during the tests was measured relative to hold-intermediate. For both tests, hold-long decreased and hold-short increased muscle spindle responsiveness. For the static test position responsiveness was not different among the velocity protocols for either hold-long or hold-short (P = 0.42 and 0.24, respectively). During the dynamic test, hold-long conditioning significantly decreased [F((3,119)) = 7.99, P < 0.001] spindle responsiveness to increasing velocity. Mean velocity sensitivity was 4.44, 3.39, and 1.41 (impulses/s)/(mm/s) for the hold-short, hold-intermediate, and hold-long protocols, respectively. The nearly 2.5-fold decrease in velocity sensitivity following hold-long was significantly less than that for either hold-intermediate (P = 0.005) or hold-short conditioning (P < 0.001). Hold-short conditioning had little effect on velocity responses during the dynamic test [F((3,119)) = 0.23, P = 0.87]. In conclusion, only movement histories that stretch but not shorten muscle spindles alter their velocity sensitivity. In the low back, forward flexion and lateral bending postures would likely be the most provocative.

Distribution pattern of muscle fiber types in the perivertebral musculature of two different sized species of mice

Many physiological parameters scale with body size. Regarding limb muscles, it has been shown that the demands for relatively faster muscles, less postural work, and greater heat production in small mammals are met by lower proportions of Type I and conversely higher proportions of Type II fibers. To investigate possible adaptations of the perivertebral musculature, we investigated the proportion, spatial distribution, and cross-sectional area (csa) of the different muscle fiber types in the laboratory and harvest mouse. Serial cross sections from the posterior thoracic to the lumbo-sacral region were prepared and Type I, IIA, and IIB fibers identified using enzymehistochemistry. The general distribution of Type I and IIB fibers, as well as the more or less equal distribution of IIA fibers, resembles the pattern found in other mammals. However, the overall proportion of Type I fibers was very low in the laboratory mouse and particularly low in the harvest mouse. Muscular adaptations to a small body size were met primarily by increased Type IIA fiber proportions. Thereby, not all muscles or muscle regions similarly reflected the expected scaling effects. However, our results clearly show that body size is a critical factor when fiber-type proportions are compared among different sized mammals.

Dynamic responsiveness of lumbar paraspinal muscle spindles during vertebral movement in the cat

Muscle spindles provide essential information for appropriate motor control. In appendicular muscles, much is known about their position and movement sensitivities, but little is known about the axial muscles of the low back. We investigated the dynamic responsiveness of lumbar paraspinal muscle spindle afferents from L(6) dorsal root filaments during constant velocity movement of the L(6) vertebra (the feline has seven lumbar vertebrae) in Nembutal-anesthetized cats. Actuations of 1 mm applied at the L(6) spinous process were delivered at 0.5, 1.0 and 2.0 mm/s. The slow velocity component was measured as the slope of the relationship between displacement during the constant velocity ramp and instantaneous discharge frequency. The quick velocity component was the slope's intercept at zero displacement. The peak component was determined as the highest discharge rates occurring near the end of the ramp compared with control. The slow velocity component over the three increasing velocities was 23.9 (9.9), 21.6 (9.6) and 20.5 (9.5) imp/(s mm) [mean (SD)], respectively. The quick velocity component was 28.4 (8.6), 31.4 (9.8) and 35.8 (10.6) imp/s, respectively. These measures of dynamic responsiveness were at least 5-10 times higher compared with values reported for appendicular muscle spindles. The peak component's velocity sensitivity was 2.9 (imp/s)/(mm/s) [0.2, 5.5, lower, upper 95% confidence interval] similar to that for cervical paraspinal muscles as well as appendicular muscles. Increased dynamic responsiveness of lumbar paraspinal muscle spindles may insure central driving to insure control of intervertebral motion during changes in spinal orientation. It may also contribute to large, rapid and potentially injurious increases in paraspinal muscle activity during sudden and unexpected muscle stretch.

Metabolic profile of the perivertebral muscles in small therian mammals: implications for the evolution of the mammalian trunk musculature

In order to gain a better understanding of the ancestral properties of the perivertebral muscles of mammals, this study investigated the fiber type composition of these muscles in six small, extant therians (two metatherians and four eutherians) similar in body shape to early mammals. Despite a few species-specific differences, the investigated species were very similar in their overall distribution of fiber types indicating similar functional demands on the back muscles in mammals of this body size and shape. Deep and short, mono- or multisegmental muscles (i.e., mm. interspinales, intermammillares, rotatores et intertransversarii) consistently showed the highest percentage of slow, oxidative fibers implying a function as local stabilizers of the vertebral column. Superficial and large, polysegmental muscles (i.e., mm. multifidus, sacrospinalis, iliopsoas et psoas minor) were predominantly composed of fast, glycolytic fibers suggesting they function to both globally stabilize and mobilize the spine during rapid non-locomotor and locomotor activities. Some muscles contained striking accumulations of oxidative fibers in specific regions (mm. longissimus et quadratus lumborum). These regions are hypothesized to function independently from the rest of the muscle belly and may be comparable in their functionality to regionalized limb muscles. The deep, central oxidative region in the m. longissimus lumborum appears to be a general feature of mammals and likely serves a proprioceptive function to control the postural equilibrium of the pelvic girdle and lumbar spine. The potential functions of the m. quadratus lumborum during ventilation and ventral stabilization of the vertebral column are discussed. Because representatives of the stem lineage of mammals were comparable in their body proportions and probably also locomotor parameters to the species investigated here, I suggest that the described fiber type distribution is representative of the ancestral condition in mammals. The origin of mammals was associated with a substantial enlargement of the epaxial muscles and the addition of subvertebral muscle mass. Because this novel muscle mass is mainly composed of fast, glycolytic fibers in extant species, it is plausible that these changes were associated with the evolution of increased sagittal mobility in the posterior trunk region in the therapsid ancestors of mammals. The caudally increasing role of sagittal bending in body propulsion is consistent with the overall increase in the percentage of glycolytic fibers in the cranio-caudal direction. The evolution of mammals was also associated with a loss of ribs in the posterior region of the trunk. This loss of ribs is thought to have decreased the stability of the posterior trunk, which may explain the observed greater oxidative capacity of the caudal local stabilizers. The increased need for postural feedback in the more mobile lumbar region may also explain the evolution of the proprioceptive system in the m. longissimus lumborum. Furthermore, the anatomical subdivision of the transversospinal muscle into several smaller muscle entities is suggested to facilitate their functional specialization.

Position sensitivity of feline paraspinal muscle spindles to vertebral movement in the lumbar spine

Muscle spindles contribute to sensorimotor control by supplying feedback regarding muscle length and consequently information about joint position. While substantial study has been devoted to determining the position sensitivity of spindles in limb muscles, there appears to be no data on their sensitivity in the low back. We determined the relationship between lumbar paraspinal muscle spindle discharge and paraspinal muscle lengthening estimated from controlled cranialward movement of the L(6) vertebra in anesthetized cats. Ramp (0.4 mm/s) and hold displacements (0.2, 0.4, 0.6, 0.8, and 1.2 mm for 2.5 s) were applied at the L(6) spinous process. Position sensitivity was defined as the slope of the relationship between the estimated increase in muscle length and mean instantaneous frequency at each length. To enable comparisons with appendicular muscle spindles where joint angle was measured, we also calculated sensitivity in terms of the L(6) and L(7) intervertebral flexion angle (IVA). This angle was estimated from measurements of facet joint capsule strain (FJC) based on a previously established relationship between IVA and FJC strain in the cat lumbar vertebral column during lumbar flexion. Single-unit recordings were obtained from 12 muscle spindle afferents. Longissimus and multifidus muscles contained the receptive field of 10 and 2 afferents, respectively. Mean position sensitivity was 16.3 imp.s(-1).mm(-1) [10.6-22.1, 95% confidence interval (CI), P < 0.001]. Mean angular sensitivity was 5.2 imp.s(-1). degrees (-1) (2.6-8.0, P < 0.003). These slope estimates were more than 3.5 times greater compared with appendicular muscle spindles, and their CIs did not contain previous slope estimates for the sensitivity of appendicular spindles from the literature. Potential reasons for and the significance of the apparently high position sensitivity in the lumbar spine are discussed.

Spinal projection of spindle afferents of the longissimus lumborum muscles of the cat

The connections and monosynaptic projections of muscle spindle afferents of individual heads of the longissimus lumborum have been studied in cats by natural stimulation, by electrical stimulation and by spike-triggered averaging from single identified afferents. The spindle afferents were classified by sensitivity to vibration and by the effect of succinylcholine on their response to ramp-and-hold muscle stretches. Axonal conduction and synaptic effects were recorded as field potentials and focal synaptic potentials during systematic exploration of the spinal cord in segments L1 to L4 with extracellular metal microelectrodes, singly and in linear arrays. Ascending branches of afferent axons within the cord had a significantly higher mean conduction velocity (CV: 56.5 m s(-1)) than descending branches (40.8 m s(-1)). The CV of ascending branches was significantly positively correlated with a measure of the strength of intrafusal bag(2) muscle fibre contacts, but not to a measure of bag(1) contacts. Two sites of monosynaptic excitatory projection in the cord were identified, namely to the intermediate region (laminae V, VI and VII) and to ventral horn region (laminae VIII and IX). In tests of 154 single afferents, signs of central projection were detected for 60, providing 122 regions of maximum negative focal synaptic potentials (FSPs) of mean amplitude 7.51 microV. Their longitudinal spacing indicated that axons gave off descending collaterals at intervals of 1.5-3.5 mm. Based on the amplitude of FSPs, the projection of secondary afferents is stronger than that of primaries in the intermediate region and possibly also in the ventral horn region. Evidence is also presented that spindle afferent input from different heads of the longissimus converges into any given spinal segment and that input in one spinal root projects to adjacent segments. It is concluded that the organization of the longissimus monosynaptic spindle input favours relatively tonic and diffuse stretch reflexes.