Parallel nociceptive reflex pathways with negative and positive feedback functions to foot extensors in the cat

Unsupervised Machine Learning on Motion Capture Data Uncovers Movement Strategies in Low Back Pain

Chronic low back pain (LBP) is a leading cause of disability and opioid prescriptions worldwide, representing a significant medical and socioeconomic problem. Clinical heterogeneity of LBP limits accurate diagnosis and precise treatment planning, culminating in poor patient outcomes. A current priority of LBP research is the development of objective, multidimensional assessment tools that subgroup LBP patients based on neurobiological pain mechanisms, to facilitate matching patients with the optimal therapies. Using unsupervised machine learning on full body biomechanics, including kinematics, dynamics, and muscle forces, captured with a marker-less depth camera, this study identified a forward-leaning sit-to-stand strategy (STS) as a discriminating movement biomarker for LBP subjects. A forward-leaning STS strategy, as opposed to a vertical rise strategy seen in the control participants, is less efficient and results in increased spinal loads. Inefficient STS with the subsequent higher spinal loading may be a biomarker of poor motor control in LBP patients as well as a potential source of the ongoing symptomology.

The nociceptive withdrawal response of the foot in the spinalized rat exhibits limited dependence on stimulus location

The nociceptive withdrawal response (NWR) of the limb is a protective, multi-joint movement in response to noxious stimulation of the homonymous limb. Previous studies in animal models differed as to the dependence of the response direction and magnitude on stimulus location. The specific aim of our research was to use three-dimensional high-speed video to determine whether movement of the foot in response to heat stimuli delivered to the foot and lower leg depended on the location of the stimulus. In particular, we sought to determine whether the movement strategy was categorical or continuous. In spinalized rats, localized, presumably nociceptive heat stimuli were delivered along three dimensions-circumferentially around the lower leg, circumferentially around the foot and along the plantar surface of the foot. Our results demonstrate that in spite of a wide range of stimulus locations over the hind foot and leg, response directions were restricted to two-rostral/medial/dorsal and caudal/medial/dorsal-directions, consistent with a categorical strategy. Further, the preference for these two directions was also reflected in the distance of the movement, which was greatest for stimuli directly opposite the preferred response directions. However, significant but weak dependencies of response direction and distance on stimulus location were found for all three dimensions of stimulus application, supporting a continuous strategy. Together, our results demonstrate, based on movement analysis, that the NWR employs a hybrid categorical-continuous strategy that may minimize the harmful consequences of noxious stimuli.

Identification of multisegmental nociceptive afferents that modulate locomotor circuits in the neonatal mouse spinal cord

Compared to proprioceptive afferent collateral projections, less is known about the anatomical, neurochemical, and functional basis of nociceptive collateral projections modulating lumbar central pattern generators (CPG). Quick response times are critical to ensure rapid escape from aversive stimuli. Furthermore, sensitization of nociceptive afferent pathways can contribute to a pathological activation of motor circuits. We investigated the extent and role of collaterals of capsaicin-sensitive nociceptive sacrocaudal afferent (nSCA) nerves that directly ascend several spinal segments in Lissauer's tract and the dorsal column and regulate motor activity. Anterograde tracing demonstrated direct multisegmental projections of the sacral dorsal root 4 (S4) afferent collaterals in Lissauer's tract and in the dorsal column. Subsets of the traced S4 afferent collaterals expressed transient receptor potential vanilloid 1 (TRPV1), which transduces a nociceptive response to capsaicin. Electrophysiological data revealed that S4 dorsal root stimulation could evoke regular rhythmic bursting activity, and our data suggested that capsaicin-sensitive collaterals contribute to CPG activation across multiple segments. Capsaicin's effect on S4-evoked locomotor activity was potent until the lumbar 5 (L5) segments, and diminished in rostral segments. Using calcium imaging we found elevated calcium transients within Lissauer's tract and dorsal column at L5 segments when compared to the calcium transients only within the dorsal column at the lumbar 2 (L2) segments, which were desensitized by capsaicin. We conclude that lumbar locomotor networks in the neonatal mouse spinal cord are targets for modulation by direct multisegmental nSCA, subsets of which express TRPV1 in Lissauer's tract and the dorsal column. J. Comp. Neurol. 521:2870-2887, 2013. © 2013 Wiley Periodicals, Inc.

Reflex transmission to lumbar α-motoneurones in the mouse similar and different to those in the cat

Investigation and interpretation of defective motor circuitries in transgenic mice required further basic results from wild-type mice. Therefore, we investigated the lumbar motor reflex pattern in anaesthetised mice using intracellular motoneuronal recording and monosynaptic reflex testing. Thresholds and latencies in mice were similar to those in cats: thresholds for monosynaptic (group I) EPSPs were slightly above 1T (T=threshold for the lowest threshold fibres), around 1.5T for group II EPSPs and above 10T for group III EPSPs; group I EPSPs were maximal with a stimulus strength around 2T, group II EPSPs were maximal with 5-8T; latencies to the group I incoming volley were below 1ms for monosynaptic group I EPSPs, around 3ms for polysynaptic group II EPSPs and above 4ms for polysynaptic group III EPSPs. In contrast to reflex actions in the cat, monosynaptic gastrocnemius-soleus reflexes were facilitated by conditioning stimulation of the peroneal, sural and tibial nerves, i.e. by a variety of different, probably flexor reflex afferents. This facilitation persisted after high lumbar spinalisation indicating an independency to supraspinal influences. Nociceptive muscle afferents facilitated the peroneal monosynaptic reflex while nociceptive cutaneous afferents from the foot sole inhibited the ipsilateral but facilitated the contralateral peroneal reflex.

Nociceptive transmission to rat primary somatosensory cortex--comparison of sedative and analgesic effects

CO(2)-laser C-fibre evoked cortical potentials (LCEPs) is a potentially useful animal model for studies of pain mechanisms. A potential confounding factor when assessing analgesic effects of systemically administered drugs using LCEP is sedation. This study aims to clarify: 1) the relation between level of anaesthesia and magnitude of LCEP, 2) the effects of a sedative and an analgesic on LCEP and dominant EEG frequency 3) the effects of a sedative and analgesic on LCEP when dominant EEG frequency is kept stable. LCEP and EEG were recorded in isoflurane/nitrous-oxide anaesthetized rats. Increasing isoflurane level gradually reduced LCEPs and lowered dominant EEG frequencies. Systemic midazolam (10 μmol/kg) profoundly reduced LCEP (19% of control) and lowered dominant EEG frequency. Similarly, morphine 1 and 3 mg/kg reduced LCEP (39%, 12% of control, respectively) and decreased EEG frequency. When keeping the dominant EEG frequency stable, midazolam caused no significant change of LCEP. Under these premises, morphine at 3 mg/kg, but not 1 mg/kg, caused a significant LCEP reduction (26% of control). In conclusion, the present data indicate that the sedative effects should be accounted for when assessing the analgesic effects of drug. Furthermore, it is suggested that LCEP, given that changes in EEG induced by sedation are compensated for, can provide information about the analgesic properties of systemically administrated drugs.

Sensory-induced activation of pattern generators in the absence of supraspinal control

Sacrocaudal afferent (SCA) stimulation is used in this work to study neural pathways involved in sensory-activation of central pattern generators (CPGs) in the isolated spinal cord of the neonatal rat. Surgical manipulations of the white matter funiculi and confocal imaging of back-labeled funicular pathways suggest that the CPGs are activated during SCA stimulation by crossed and uncrossed multifunicular projections of sacral neurons and that activation of short projecting proprioneurons is sufficient for the generation of the rhythm by SCA stimulation. The versatile organization of the pathways involved in the SCA-induced rhythm makes it a potent and durable activator of the CPGs in the absence of descending control from the brain. The significance of our findings and their potential clinical use are discussed.

Locomotor networks are targets of modulation by sensory transient receptor potential vanilloid 1 and transient receptor potential melastatin 8 channels

It is well recognized that proprioceptive afferent inputs can control the timing and pattern of locomotion. C and Adelta afferents can also affect locomotion but an unresolved issue is the identity of the subsets of these afferents that encode defined modalities. Over the last decade, the transient receptor potential (TRP) ion channels have emerged as a family of non-selective cation conductances that can label specific subsets of afferents. We focus on a class of TRPs known as ThermoTRPs which are well known to be sensor receptors that transduce changes in heat and cold. ThermoTRPs are known to help encode somatosensation and painful stimuli, and receptors have been found on C and Adelta afferents with central projections onto dorsal horn laminae. Here we show, using in vitro neonatal mouse spinal cord preparations, that activation of both spinal and peripheral transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential melastatin 8 (TRPM8) afferent terminals modulates central pattern generators (CPGs). Capsaicin or menthol and cooling modulated both sacrocaudal afferent (SCA) evoked and monoaminergic drug-induced rhythmic locomotor-like activity in spinal cords from wild type but not TRPV1-null (trpv1(-/-)) or TRPM8-null (trpm8(-/-)) mice, respectively. Capsaicin induced an initial increase in excitability of the lumbar motor networks, while menthol or cooling caused a decrease in excitability. Capsaicin and menthol actions on CPGs involved excitatory and inhibitory glutamatergic mechanisms, respectively. These results for the first time show that dedicated pathways of somatosensation and pain identified by TRPV1 or TRPM8 can target spinal locomotor CPGs.

Group III and IV muscle afferents differentially affect the motor cortex and motoneurones in humans

The influence of group III and IV muscle afferents on human motor pathways is poorly understood. We used experimental muscle pain to investigate their effects at cortical and spinal levels. In two studies, electromyographic (EMG) responses in elbow flexors and extensors to stimulation of the motor cortex (MEPs) and corticospinal tract (CMEPs) were evoked before, during, and after infusion of hypertonic saline into biceps brachii to evoke deep pain. In study 1, MEPs and CMEPs were evoked in relaxed muscles and during contractions to a constant elbow flexion force. In study 2, responses were evoked during elbow flexion and extension to a constant level of biceps or triceps brachii EMG, respectively. During pain, the size of CMEPs in relaxed biceps and triceps increased (by approximately 47% and approximately 56%, respectively; P < 0.05). MEPs did not change with pain, but relative to CMEPs, they decreased in biceps (by approximately 34%) and triceps (by approximately 43%; P < 0.05). During flexion with constant force, ongoing background EMG and MEPs decreased for biceps during pain (by approximately 14% and 15%; P < 0.05). During flexion with a constant EMG level, CMEPs in biceps and triceps increased during pain (by approximately 30% and approximately 26%, respectively; P < 0.05) and relative to CMEPs, MEPs decreased for both muscles (by approximately 20% and approximately 17%; P < 0.05). For extension, CMEPs in triceps increased during pain (by approximately 22%) whereas MEPs decreased (by approximately 15%; P < 0.05). Activity in group III and IV muscle afferents produced by hypertonic saline facilitates motoneurones innervating elbow flexor and extensor muscles but depresses motor cortical cells projecting to these muscles.

Differential Effects of Opioids on Sacrocaudal Afferent Pathways and Central Pattern Generators in the Neonatal Rat Spinal Cord

The effects of opioids on sacrocaudal afferent (SCA) pathways and the pattern-generating circuitry of the thoracolumbar and sacrocaudal segments of the spinal cord were studied in isolated spinal cord and brain stem-spinal cord preparations of the neonatal rat. The locomotor and tail moving rhythm produced by activation of nociceptive and nonnociceptive sacrocaudal afferents was completely blocked by specific application of the μ-opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate salt (DAMGO) to the sacrocaudal but not the thoracolumbar segments of the spinal cord. The rhythmic activity could be restored after addition of the opioid receptor antagonist naloxone to the experimental chamber. The opioid block of the SCA-induced rhythm is not due to impaired rhythmogenic capacity of the spinal cord because a robust rhythmic activity could be initiated in the thoracolumbar and sacrocaudal segments in the presence of DAMGO, either by stimulation of the ventromedial medulla or by bath application of N-methyl-d-aspartate/serotonin. We suggest that the opioid block of the SCA-induced rhythm involves suppression of synaptic transmission through sacrocaudal interneurons interposed between SCA and the pattern-generating circuitry. The expression of μ opioid receptors in several groups of dorsal, intermediate and ventral horn interneurons in the sacrocaudal segments of the cord, documented in this study, provides an anatomical basis for this suggestion.

Effects of electrically induced muscle contraction on flexion reflex in human spinal cord injury

STUDY DESIGN

Flexion reflex study in motor complete human spinal cord injury (SCI).

OBJECTIVES

To examine changes in the magnitude of the flexion reflex following functional electrical stimulation (FES) of the rectus femoris (RF) muscle.

SETTING

Bioengineering Unit, University of Strathclyde, Glasgow, Scotland, UK.

METHODS

The flexion reflex was evoked by electrical stimulation of the sural nerve, and was recorded in the tibialis anterior (TA) muscle. RF muscle conditioning stimulation was performed at 0.7, 1, and 2 times motor threshold ( x MT) over a range of conditioning test intervals.

RESULTS

The incidence of the early component of the flexion reflex (<100 ms) was low, suggesting that this reflex component might be suppressed in SCI. The long latency flexion reflex component (>120 ms) was observed in all subjects during control conditions and following sensorimotor conditioning. FES applied to the RF muscle (above and below MT) in the main induced a significant early and long lasting depression of the long latency flexion reflex.

CONCLUSION

The depression of the flexion reflex was a result of multisensory actions on flexion reflex pathways resulting from the direct and indirect (mechanical) consequences of electrically induced muscle contraction on cutaneous and muscle afferents. Our findings emphasize the importance of sensory feedback mechanisms in modulating flexion reflex excitability, and highlight the need for rehabilitation professionals to consider the central actions of FES-induced afferent feedback when incorporating FES into a rehabilitation program.

SPONSORSHIP

State Scholarships Foundation (IKY) of Hellas.

Contribution of force feedback to ankle extensor activity in decerebrate walking cats

Previous investigations have demonstrated that feedback from ankle extensor group Ib afferents, arising from force-sensitive Golgi tendon organs, contributes to ankle extensor activity during the stance phase of walking in the cat. The objective of this investigation was to gain insight into the magnitude of this contribution by determining the loop gain of the positive force feedback pathway. Loop gain is the relative contribution of force feedback to total muscle activity and force. In decerebrate cats, the isolated medial gastrocnemius muscle (MG) was held at different lengths during sequences of rhythmic contractions associated with walking in the other three legs. We found that MG muscle activity and force increased at longer muscle lengths. A number of observations indicated that this length dependence was not due to feedback from muscle spindles. In particular, activity in group Ia afferents was insensitive to changes in muscle length during the MG bursts, and electrical stimulation of group II afferents had no influence on the magnitude of burst activity in other ankle extensors. We concluded that the homonymous positive force feedback pathway was isolated from other afferent pathways, allowing the use of a simple model of the neuromuscular system to estimate the pathway loop gain. This gain ranged from 0.2 at short muscle lengths to 0.5 at longer muscle lengths, demonstrating that force feedback was of modest importance at short muscle lengths, accounting for 20% of total activity and force, and of substantial importance at long muscle lengths, accounting for 50%. This length dependence was due to the intrinsic force-length property of muscle. The gain of the pathway that converts muscle force to motoneuron depolarization was independent of length. We discuss the relevance of this conclusion to the generation of ankle extensor activity in intact walking cats. These findings emphasize the general importance of feedback in generating ankle extensor activity during walking in the cat.

Properties of an Adult Spinal Sensorimotor Circuit Shaped Through Early Postnatal Experience

During development, information about the three-dimensional shape and mechanical properties of the body is laid down in the synaptic connectivity of sensorimotor systems through adaptive mechanisms. This functional adaptation occurs through alteration of connection properties. Here, we characterize the differences between strong and weak connections in the nociceptive withdrawal reflex in adult decerebrate spinal rats, representing the preserved end product of the developmental adaptation process. Stronger excitatory reflex connections from the skin onto a muscle had relatively higher gain in their input-output relations, shorter onset latencies (up to ∼150 ms) and lower trial-to-trial variability in relation to response amplitude (SD ∼ mean1/2) than weaker pathways. Although inhibitory and excitatory nociceptive receptive fields of a muscle overlap to some degree, the results indicate that the inhibitory input is not a major determinant of the gain distribution within the excitatory receptive field and vice versa. The N-methyl-d-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonovalerate (0.1–1 μg), applied topically on the spinal cord reduced the gain, whereas the response amplitude was mainly reduced by an absolute number by the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor antagonist, 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (1–10 μg). The results indicate that NMDA receptors have a critical role in gain regulation in the nociceptive withdrawal reflex system. It is suggested that after normal postnatal experience-dependent adaptation, the number of connections from a given skin site onto the reflex encoding interneurons is a major determinant of the difference in gain.