Withdrawal reflexes in the upper limb adapt to arm posture and stimulus location

Exploring the effect of capsaicin-induced central sensitization on the upper limb nociceptive withdrawal reflex threshold

The nociceptive withdrawal reflex (NWR) threshold is commonly employed in the lower limb to assess clinical and experimentally induced pain. However, no studies to date have investigated changes in spinal nociception in the upper limb, via the NWR threshold, following experimentally induced central sensitization (CS). We tested the hypothesis that experimentally induced CS of the C5-C6 spinal segment significantly reduces NWR thresholds in muscles of the upper limb. Upper limb NWR thresholds from 20 young, healthy adults were assessed by applying noxious electrical stimuli to the right index finger and recording muscle activity from the biceps brachii (BI), triceps brachii (TRI), flexor carpi ulnaris (WF), and extensor carpi radialis longus (WE) muscles via surface electromyography. Topical cream (either 0.075% capsaicin, or control) was applied to the C5-C6 dermatome of the lateral forearm (50 cm²). NWR thresholds were compared at baseline, and four 10-min intervals after topical application. WF muscle NWR thresholds were significantly reduced in the capsaicin session compared to control, while TRI muscle NWR thresholds were significantly reduced 40 min after capsaicin application only (p < 0.05). There were no significant differences for BI or WE muscle NWR thresholds. We observed poor to moderate test-retest reliability for all upper limb NWR thresholds, a key contributor to the selective reduction in NWR thresholds among muscles. Accordingly, while our findings demonstrate some comparability to previously reported lower limb NWR studies, we concurrently report limitations of the upper limb NWR technique. Further exploration of optimal parameters for upper limb NWR acquisition is needed.

The nociceptive flexion reflex: a scoping review and proposed standardized methodology for acquisition in those affected by chronic pain

The nociceptive flexion reflex (NFR) is used in neurophysiological research as an objective measure of nociception. NFR thresholds are reduced in numerous chronic pain pathologies, which are indicative of common central hyperexcitability within conditions. However, variation exists in both the NFR assessment and determinants of NFR threshold among research groups. Our purpose was to provide a review of the recent literature to (a) confirm the NFR threshold's efficacy in identifying those with chronic pain compared to controls and (b) provide a narrative synthesis on the current methodology used to assess the NFR in clinical populations. We conducted a review of multiple databases (MEDLINE, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Google Scholar and Cochrane Library), including articles that reported controlled clinical studies of humans, in English, comparing NFR thresholds within chronic pain conditions to matched control subjects, published since the last NFR review in 2010. Our search resulted in nine studies included in our narrative synthesis and eight studies included in a meta-analysis. There was a significant pooled standardized mean difference in NFR threshold between chronic pain conditions and controls (-0.94, 95% confidence interval (CI) -1.33 to -0.55, p < 0.0001), with substantial heterogeneity of pooled estimates (I 2 = 87%, τ 2 = 0.41, Q = 76.13, the degrees of freedom (df) = 11, p < 0.0001). Significant variations in participant positioning, stimulation parameters and determinants of the NFR threshold were evident among included studies. We provided a narrative synthesis on the methodologies of included studies, as a recommendation for future studies in the assessment of the NFR in chronic pain.

The nociceptive withdrawal response of the tail in the spinalized rat employs a hybrid categorical-continuous spatial mapping strategy

Complexity in movement planning, arising from diverse temporal and spatial sources, places a computational burden on the central nervous system. However, the efficacy with which humans can perform natural, highly trained movements suggests that they have evolved effective behavioral strategies that simplify the computational burden. The specific aim of our research was to use three-dimensional high-speed video to determine whether the tail nociceptive withdrawal response (NWR) to noxious heat stimuli delivered at locations that varied both circumferentially and rostral-caudally on the tail depended on the location of the stimulus in spinalized rats. In particular, we sought to determine whether the movement strategy was categorical (limited number of directions) or continuous (any variation in stimulus location results in a variation in response direction). In spinalized rats, localized, noxious heat stimuli were delivered at eight locations circumferentially around the tail and at five rostral-caudal levels. Our results demonstrate that at all rostral-caudal levels, response movement direction was bimodal regardless of circumferential stimulus location-either ~ 64° left or right of ventral. However, in spite of tight clustering, movement direction varied significantly but weakly according to circumferential location, in that responses to stimuli were more lateral for lateral stimulus locations. In contrast, changes in stimulus level strongly affected movement direction, in that a localized bend response closely matched the level of the stimulus. Together, our results demonstrate, based on movement analysis in spinalized rats, that the NWR employs a hybrid categorical-continuous strategy that may minimize the harmful consequences of noxious stimuli.

The nociceptive withdrawal reflex of the trunk is organized with unique muscle receptive fields and motor strategies

Noxious stimuli induce a nociceptive withdrawal reflex (NWR) to protect the tissue from injury. Although the NWR was once considered as a stereotyped response, previous studies report distinct responses depending on the stimulation site and context for limbs. We aimed to determine whether noxious stimuli over the trunk produced adaptable complex NWR. We hypothesized that organization of the NWR of the trunk muscle would vary with the site of noxious input and would differ between body and spine postures, which modify the potential for specific muscles to remove threat. Fourteen participants were tested in sitting and three lumbar spine postures in side lying (neutral, flexion and extension). Noxious electrical stimuli were applied over the sacrum, spinous process of L3 and T12, lateral side of the 8th rib and anterior midline. NWR latency and amplitude were recorded with surface electromyography (EMG) electrodes over different trunk muscles. Distinct patterns of muscle activation depended on the stimulation site and were consistent with motor strategies needed to withdraw from the noxious stimuli. The NWR pattern differed between body positions, with less modulation observed in sitting than side lying. Spine posture did not affect the NWR organisation. Our results suggest the circuits controlling trunk muscle NWR presents with adaptability as a function of stimulation site and body position by utilizing the great complexity of the trunk muscle system to produce an efficient protective response. This suggests that the central nervous system (CNS) uses multiple adaptable strategies that are unique depending on which context the noxious stimuli are applied.

Effect of movement-related pain on behaviour and corticospinal excitability changes associated with arm movement preparation

KEY POINTS

Experimental pain or its anticipation influence motor preparation processes as well as upcoming movement execution, but the underlying physiological mechanisms remain unknown. Our results showed that movement-related pain modulates corticospinal excitability during motor preparation. In accordance with the pain adaptation theory, corticospinal excitability was higher when the muscle has an antagonist (vs. an agonist) role for the upcoming movement associated with pain. Anticipation of movement-related pain also affects motor initiation and execution, with slower movement initiation (longer reaction times) and faster movement execution compared to movements that do not evoke pain. These results confirm the implementation of protective strategies during motor preparation known to be relevant for acute pain, but which may potentially have detrimental long-term consequences and lead to the development of chronic pain.

ABSTRACT

When a movement repeatedly generates pain, we anticipate movement-related pain and establish self-protective strategies during motor preparation, but the underlying mechanisms remains poorly understood. The current study investigated the effect of movement-related pain anticipation on the modulation of behaviour and corticospinal excitability during the preparation of arm movements. Participants completed an instructed-delay reaction-time (RT) task consisting of elbow flexions and extensions instructed by visual cues. Nociceptive laser stimulations (unconditioned stimuli) were applied to the lateral epicondyle during movement execution in a specific direction (CS+) but not in the other (CS-), depending on experimental group. During motor preparation, transcranial magnetic stimulation was used to measure corticospinal excitability in the biceps brachii (BB). RT and peak end-point velocity were also measured. Neurophysiological results revealed an opposite modulation of corticospinal excitability in BB depending on whether it plays an agonist (i.e. flexion) or antagonist (i.e. extension) role for the CS+ movements (P < 0.001). Moreover, behavioural results showed that for the CS+ movements RT did not change relative to baseline, whereas the CS- movements were initiated more quickly (P = 0.023) and the CS+ flexion movements were faster relative to the CS- flexion movements (P < 0.001). This is consistent with the pain adaptation theory which proposes that in order to protect the body from further pain, agonist muscle activity is reduced and antagonist muscle activity is increased. If these strategies are initially relevant and lead to short-term pain alleviation, they may potentially have detrimental long-term consequences and lead to the development of chronic pain.

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.