Neck Muscle Responses Research Articles

Vestibular Contributions to Primate Neck Postural Muscle Activity during Natural Motion.

To maintain stable posture of the head and body during our everyday activities, the brain integrates information across multiple sensory systems. Here, we examined how the primate vestibular system, independently and in combination with visual sensory input, contributes to the sensorimotor control of head posture across the range of dynamic motion experienced during daily life. We recorded activity of single motor units in the splenius capitis and sternocleidomastoid muscles in rhesus monkeys during yaw rotations spanning the physiological range of self-motion (up to 20 Hz) in darkness. Splenius capitis motor unit responses continued to increase with frequency up to 16 Hz in normal animals, and were strikingly absent following bilateral peripheral vestibular loss. To determine whether visual information modulated these vestibular-driven neck muscle responses, we experimentally controlled the correspondence between visual and vestibular cues of self-motion. Surprisingly, visual information did not influence motor unit responses in normal animals, nor did it substitute for absent vestibular feedback following bilateral peripheral vestibular loss. A comparison of muscle activity evoked by broadband versus sinusoidal head motion further revealed that low-frequency responses were attenuated when low- and high-frequency self-motion were experienced concurrently. Finally, we found that vestibular-evoked responses were enhanced by increased autonomic arousal, quantified via pupil size. Together, our findings directly establish the vestibular system's contribution to the sensorimotor control of head posture across the dynamic motion range experienced during everyday activities, as well as how vestibular, visual, and autonomic inputs are integrated for postural control.SIGNIFICANCE STATEMENT Our sensory systems enable us to maintain control of our posture and balance as we move through the world. Notably, the vestibular system senses motion of the head and sends motor commands, via vestibulospinal pathways, to axial and limb muscles to stabilize posture. By recording the activity of single motor units, here we show, for the first time, that the vestibular system contributes to the sensorimotor control of head posture across the dynamic motion range experienced during everyday activities. Our results further establish how vestibular, autonomic, and visual inputs are integrated for postural control. This information is essential for understanding both the mechanisms underlying the control of posture and balance, and the impact of the loss of sensory function.

First Human In Vivo Neuroelectrophysiology Recordings of Uncrossed Dentatothalamocortical White-Matter Connections: On the Fast Tract.

We aim to demonstrate intraoperative recording of cerebellar to cortical pathways that have not been previously recorded in humans, though imaged. We report 2 cases with intraoperative neurophysiologic mapping of cerebellocortical tracts. Direct electrical stimulation of subcortical cerebellum along with recordings of cortical evoked potential and motor muscle recordings was performed during surgery. MR tractography data from healthy participants were used to further illustrate the pathways. Neurophysiologic recordings showed large waveforms of evoked potentials in bilateral electrodes over premotor/motor cortices on stimulation of the dentate nucleus. EMG recordings showed responses in face and neck muscles on stimulation of the dentate nucleus at the motor threshold. We thus demonstrated first-in-human in vivo neurophysiologic evidence of cerebellum to cortex responses through an uncrossed dentatothalamocortical tract to the motor/premotor cortices. This technique provides a methodology for the direct mapping of the cerebellum and cerebello-cerebral connections. We hypothesize a direct structural connection from the dentate nucleus to the premotor and motor cortices, as well as to ipsilateral hemibody muscles, acting as a fast route of cerebellar output and back up for immediate motor responses. This will further help explain the modulatory effects of the cerebellum on motor, language, and cognitive functions.

Electromyographic Responses of Neck, Back, and Limb Outlet Muscles Associated With High-Velocity, Low-Amplitude Manual Cervical and Upper Thoracic Spinal Manipulation of Individuals With Mild Neck Disability: A Descriptive Observational Investigation

ObjectivesThe purpose of this study was to investigate the extent of electromyographic responses associated with manual high-velocity, low-amplitude (HVLA) spinal manipulation systematically applied to the upper and lower cervical and upper thoracic spines in a cohort with mild neck disability. MethodsThe study was a descriptive observational investigation, with all participants receiving the same interventions. Nineteen participants with mild neck disability received 6 manual HVLA manipulations to the cervical and upper thoracic spine. Bipolar surface electromyography electrode pairs were used to measure responses of 16 neck, back, and limb outlet muscles bilaterally. The number of electromyographic responses was then calculated. ResultsElectromyographic responses associated with cervical and thoracic manipulation occurred in a median of 4 of the 16 (range: 1-14) recorded muscles. Cervical spinal manipulation was associated with the highest rates of electromyographic responses in neck muscles, whereas responses in back muscles were highest after upper thoracic manipulation. ConclusionCervical spinal manipulation was associated with the highest rate of electromyographic responses in muscles of the cervical spine (sternocleidomastoid and splenius cervicis), whereas responses in back muscles (upper and middle trapezius, latissimus dorsi, and longissimus thoracis) were highest after upper thoracic manipulations. This result suggests that electromyographic muscular responses associated with spinal manipulation primarily occur locally (close to the target segment) rather than distally.

Neck Muscle and Head/Neck Kinematic Responses While Bracing Against the Steering Wheel During Front and Rear Impacts.

Drivers often react to an impending collision by bracing against the steering wheel. The goal of the present study was to quantify the effect of bracing on neck muscle activity and head/torso kinematics during low-speed front and rear impacts. Eleven seated subjects (3F, 8M) experienced multiple sled impacts (Δv = 0.77m/s; apeak = 19.9m/s2, Δt = 65.5ms) with their hands on the steering wheel in two conditions: relaxed and braced against the steering wheel. Electromyographic activity in eight neck muscles (sternohyoid, sternocleidomastoid, splenius capitis, semispinalis capitis, semispinalis cervicis, multifidus, levator scapulae, and trapezius) was recorded unilaterally with indwelling electrodes and normalized by maximum voluntary contraction (MVC) levels. Head and torso kinematics (linear acceleration, angular velocity, angular rotation, and retraction) were measured with sensors and motion tracking. Muscle and kinematic variables were compared between the relaxed and braced conditions using linear mixed models. We found that pre-impact bracing generated only small increases in the pre-impact muscle activity (< 5% MVC) when compared to the relaxed condition. Pre-impact bracing did not increase peak neck muscle responses during the impacts; instead it reduced peak trapezius and multifidus muscle activity by about half during front impacts. Bracing led to widespread changes in the peak amplitude and timing of the torso and head kinematics that were not consistent with a simple stiffening of the head/neck/torso system. Instead pre-impact bracing served to couple the torso more rigidly to the seat while not necessarily coupling the head more rigidly to the torso.

SPARC: Neural elements mediating side effects during cervical vagus nerve stimulation in the pig

Vagus nerve stimulation (VNS) is FDA‐approved and CE‐marked for a diverse set of conditions including epilepsy, depression, heart failure, and others. Despite widespread adoption and perceived efficacy, a post‐hoc analysis of patients treated for heart failure using VNS revealed that only 12% of patients showed stimulation evoked heart rate changes one‐year post‐implant. This finding is not unexpected given that A‐type nerve fibers have the lowest activation thresholds, and they innervate neck muscles, causing contractions that patients will not tolerate. Therefore, muscle responses at lower amplitudes may have prevented application of higher amplitudes required to achieve cardiac effects in clinical trials of VNS for heart failure. Understanding therapy‐limiting side effects to VNS and finding ways to mitigate them are critical for optimizing VNS for clinical therapies.We developed a swine model of VNS which closely approximates the size and fascicular organization of the cervical vagus nerve observed in humans. The clinical LivaNova stimulation electrode was used as advised by a practicing neurosurgeon. We hypothesized that neck muscle responses are mediated by somatic branches of the vagus nerve as opposed to direct electrical activation of the muscle fibers. Further, we hypothesized that stimulation evoked responses depend on the location of the stimulation electrode on the vagus nerve. We measured compound nerve action potentials using intrafascicular electrodes placed within the vagus to identify activation of different nerve fiber types by conduction velocity. The downstream effects of nerve fiber activation, i.e. changes in heart rate and neck muscle responses (EMG), were measured using an arterial catheter and intramuscular electrodes, respectively. We then used a neuromuscular junction blocking agent, as well as systematic transection of somatic vagus nerve branches, to specifically identify the pathways causing neck muscle contractions.Two pathways contributed to VNS‐evoked neck muscle responses: activation of nerve fibers in the recurrent laryngeal branch of the cervical vagus nerve at low amplitudes and in the superior laryngeal branch at higher amplitudes. Lesser activation of the superior laryngeal branch could be achieved by moving the stimulation electrode caudally down the length of the vagus. This work represents a step towards mitigating intolerable side effects of VNS, thereby allowing maximization of intended target engagement and therapeutic benefit.Support or Funding InformationThe Defense Advanced Research Projects Agency (DARPA) Biological Technologies Office (BTO) Targeted Neuroplasticity Training Program under the auspices of Doug Weber and Tristan McClure‐Begley through the Space and Naval Warfare Systems Command (SPAWAR) Systems Center with (SSC) Pacific grants no. N66001‐17‐2‐4010, and the NIH SPARC Program Award OT2 OD025340.

Neural Mechanisms Underlying High-Frequency Vestibulocollic Reflexes In Humans And Monkeys.

The vestibulocollic reflex is a compensatory response that stabilizes the head in space. During everyday activities, this stabilizing response is evoked by head movements that typically span frequencies from 0 to 30 Hz. Transient head impacts, however, can elicit head movements with frequency content up to 300-400 Hz, raising the question whether vestibular pathways contribute to head stabilization at such high frequencies. Here, we first established that electrical vestibular stimulation modulates human neck motor unit (MU) activity at sinusoidal frequencies up to 300 Hz, but that sensitivity increases with frequency up to a low-pass cutoff of ∼70-80 Hz. To examine the neural substrates underlying the low-pass dynamics of vestibulocollic reflexes, we then recorded vestibular afferent responses to the same electrical stimuli in monkeys. Vestibular afferents also responded to electrical stimuli up to 300 Hz, but in contrast to MUs their sensitivity increased with frequency up to the afferent resting firing rate (∼100-150 Hz) and at higher frequencies afferents tended to phase-lock to the vestibular stimulus. This latter nonlinearity, however, was not transmitted to neck motoneurons, which instead showed minimal phase-locking that decreased at frequencies >75 Hz. Similar to human data, we validated that monkey muscle activity also exhibited low-pass filtered vestibulocollic reflex dynamics. Together, our results show that neck MUs are activated by high-frequency signals encoded by primary vestibular afferents, but undergo low-pass filtering at intermediate stages in the vestibulocollic reflex. These high-frequency contributions to vestibular-evoked neck muscle responses could stabilize the head during unexpected head transients.SIGNIFICANCE STATEMENT Vestibular-evoked neck muscle responses rely on accurate encoding and transmission of head movement information to stabilize the head in space. Unexpected transient events, such as head impacts, are likely to push the limits of these neural pathways since their high-frequency features (0-300 Hz) extend beyond the frequency bandwidth of head movements experienced during everyday activities (0-30 Hz). Here, we demonstrate that vestibular primary afferents encode high-frequency stimuli through frequency-dependent increases in sensitivity and phase-locking. When transmitted to neck motoneurons, these signals undergo low-pass filtering that limits neck motoneuron phase-locking in response to stimuli >75 Hz. This study provides insight into the neural dynamics producing vestibulocollic reflexes, which may respond to high-frequency transient events to stabilize the head.

Reflex Responses of Neck, Back, and Limb Muscles to High-Velocity, Low-Amplitude Manual Cervical and Upper Thoracic Spinal Manipulation of Asymptomatic Individuals—A Descriptive Study

ObjectiveThe purpose of this research was to determine the extent of reflex responses after spinal manipulative therapy (SMT) of the cervical and upper thoracic spine. MethodsEleven asymptomatic participants received 6 commonly used SMTs to the cervical and upper thoracic spine. Bipolar surface electromyography electrodes were used to measure reflex responses of 16 neck, back, and proximal limb muscles bilaterally. The percentage of occurrence and the extent of reflex responses of these muscles were determined. ResultsReflex responses after cervical SMT were typically present in all neck and most back muscles, whereas responses in the outlets to the arm and leg were less frequent. This trend was similar, although decreased in magnitude, after thoracic SMT. ConclusionReflex responses were greatest after upper cervical SMT and lowest with thoracic SMT.

Neck muscle responses of driver and front seat passenger during frontal-oblique collisions.

BackgroundLow-velocity motor vehicle crashes often lead to severe and chronic neck disorders also referred to as whiplash-associated disorders (WAD). The etiology of WAD is still not fully understood. Many studies using a real or simulated collision scenario have focused on rear-end collisions, whereas the kinematics and muscular responses during frontal-oblique collisions have hardly been investigated. In particular for rear-end collisions, drivers were shown to have a higher WAD risk than front seat passengers. Yet, independently from the impact direction, neither the muscular nor the kinematic responses of drivers and front seat passengers have been compared to date, although some findings indicate that the neck muscles have the potential to alter the head and neck kinematics, and that the level of neck muscle activity during impact may be relevant for the emergence of WAD.ObjectiveIn this study, we quantitatively examined the subjects’ neck muscle activity during low-velocity left-frontal-oblique impacts to gain further insights into the neuromuscular mechanism underlying whiplash-like perturbations that may lead to WAD.MethodsIn a within-subject study design, we varied several impact parameters to investigate their effect on neck muscle response amplitude and delay. Fifty-two subjects experienced at least ten collisions while controlling for the following parameters: change in velocity Δv (3 / 6 km/h), seating position (driver / front seat passenger), and deliberate pre-tension of the musculature (tense / relaxed) to account for a potential difference between an expected and an unexpected crash. Ten of the 52 subjects additionally ran the same experimental conditions as above, but without wearing a safety belt.FindingsThere were significant main effects of Δv and muscle pre-tension on the reflex amplitude but not of seating position. As for the reflex delay, there was a significant main effect of muscle pre-tension, but neither of Δv nor of seating position. Moreover, neither the safety belt nor its asymmetrical orientation had an influence on the reflexive responses of the occupants.ConclusionIn summary, we did not find any significant differences in the reflex amplitude and delay of the neck musculature between drivers and front seat passengers. We therefore concluded that an increased risk of the driver sustaining WAD in frontal-oblique collisions, if it exists, cannot be due to differences in the reflexive responses.

Hyperexcitability of brain stem pathways in cerebral palsy.

Individuals with cerebral palsy (CP) experience impairments in the control of head and neck movements, suggesting dysfunction in brain stem circuitry. To examine if brain stem circuitry is altered in CP, we compared reflexes evoked in the sternocleidomastoid (SCM) muscle by trigeminal nerve stimulation in adults with CP and in age/sex-matched controls. Increasing the intensity of trigeminal nerve stimulation produced progressive increases in the long-latency suppression of ongoing SCM electromyography in controls. In contrast, participants with CP showed progressively increased facilitation around the same reflex window, suggesting heightened excitability of brain stem pathways. We also examined if there was altered activation of cortico-brain stem pathways in response to prenatal injury of the brain. Motor-evoked potentials (MEPs) in the SCM that were conditioned by a prior trigeminal afferent stimulation were more facilitated in CP compared with controls, especially in ipsilateral MEPs that are likely mediated by corticoreticulospinal pathways. In some participants with CP, but not in controls, a combined trigeminal nerve and cortical stimulation near threshold intensities produced large, long-lasting responses in both the SCM and biceps brachii muscles. We propose that the enhanced excitatory responses evoked from trigeminal and cortical inputs in CP are produced by heightened excitability of brain stem circuits, resulting in the augmented activation of reticulospinal pathways. Enhanced activation of reticulospinal pathways in response to early injury of the corticospinal tract may provide a compensated activation of the spinal cord or, alternatively, contribute to impairments in the precise control of head and neck functions. NEW & NOTEWORTHY This is the first study to show that in adults with spastic cerebral palsy, activation of brain stem circuits by cortical and/or trigeminal afferents produces excitatory responses in anterior neck muscles compared with inhibitory responses in age/sex-matched controls. This may reflect a more excitable reticulospinal tract in response to early brain injury to provide a compensated activation of postural muscles. On the other hand, a hyperexcitable brain stem may contribute to impairments in the precise control of head and neck functions.

Neck muscle vibration produces diverse responses in balance and gait speed between individuals with and without neck pain

BackgroundNeck muscle vibration can influence cervical proprioception and sensorimotor function. It is hypothesized to affect motor performance differently in persons with and without neck pain. Study designCross-sectional study. ObjectiveTo clarify the extent to which vibration-induced motor responses of neck muscles affect static standing balance and gait speed in persons with and without neck pain. MethodsThirty participants with chronic neck pain and 30 healthy controls were recruited. Balance and gait were measured before and after 30 s of suboccipital neck muscle vibration. Balance was measured in a confortable stance with eyes closed using a swaymeter and gait using the timed 10 m walk test. ResultsAt baseline, neck pain participants had greater postural sway, particularly in the anterior-posterior direction and slower gait speed than healthy controls (p < 0.001). Immediately after vibration, neck pain participants displayed decreased postural sway, and increased gait speed (p < 0.001). Healthy controls had increased postural sway and decreased gait speed (p < 0.001). ConclusionNeck muscle vibration improved standing balance and gait speed in participants with neck pain but reduced performance in healthy controls. The study supports the importance of cervical proprioceptive information in postural control. Use of vibration could be considered neck pain patients' rehabilitation.

Alterations in the Mechanical Response of Deep Dorsal Neck Muscles in Individuals Experiencing Whiplash-Associated Disorders Compared to Healthy Controls: An Ultrasound Study.

The aim of this study was to investigate and compare the mechanical responses of dorsal neck muscles in individuals with whiplash-associated disorders (WAD) versus healthy individuals. This study included 36 individuals with WAD (26 women and 10 men) and 36 healthy controls (26 women and 10 men). Ultrasound imaging with speckle tracking was used to measure deformation and deformation rate in five dorsal neck muscles during a neck extension task. Compared with controls, individuals with WAD showed higher deformations of the semispinalis cervicis (P = 0.02) and multifidus (P = 0.002) muscles and higher deformation rates (P = 0.03 and 0.0001, respectively). Among individuals with WAD, multifidus deformation and deformation rate were significantly associated with pain, disability, and fatigue (r = 0.31-0.46, P = 0.0001-0.01). These findings indicate that the mechanical responses of the deep dorsal neck muscles differ between individuals with WAD and healthy controls, possibly reflecting that these muscles use altered strategies while performing a neck extension task. This finding provides new insight into neck muscles pathology in patients with chronic WAD and may help improve rehabilitation programs. Complete the self-assessment activity and evaluation online at http://www.physiatry.org/JournalCME CME OBJECTIVES: Upon completion of this article, the reader should be able to: (1) Summarize the mechanical responses of dorsal neck muscles during loading of the neck muscles via an extension task in individuals with chronic whiplash associated disorders and healthy volunteers; (2) Differentiate mechanical responses between five dorsal neck muscles while loading the neck via an extension task; and (3) Describe the relationships between the mechanical responses of the dorsal neck muscles with the patients' perception of neck pain, disability, and fatigue. Advanced ACCREDITATION: The Association of Academic Physiatrists is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.The Association of Academic Physiatrists designates this Journal-based CME activity for a maximum of 0.5 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity.

The Role of Neck Muscle Activities on the Risk of Mild Traumatic Brain Injury in American Football.

Concussion, or mild traumatic brain injury (mTBI), is frequently associated with sports activities. It has generally been accepted that neck strengthening exercises are effective as a preventive strategy for reducing sports-related concussion risks. However, the interpretation of the link between neck strength and concussion risks remains unclear. In this study, a typical helmeted head-to-head impact in American football was simulated using the head and neck complex finite element (FE) model. The impact scenario selected was previously reported in lab-controlled incident reconstructions from high-speed video footages of the National Football League using two head-neck complexes taken from Hybrid III dummies. Four different muscle activation strategies were designed to represent no muscle response, a reactive muscle response, a pre-activation response, and response due to stronger muscle strength. Head kinematics and various head/brain injury risk predictors were selected as response variables to compare the effects of neck muscles on the risk of sustaining the concussion. Simulation results indicated that active responses of neck muscles could effectively reduce the risk of brain injury. Also, anticipatory muscle activation played a dominant role on impact outcomes. Increased neck strength can decrease the time to compress the neck and its effects on reducing brain injury risks need to be further studied.

Effects of Shoulder Taping on Discomfort and Electromyographic Responses of the Neck While Texting on a Touchscreen Smartphone.

BackgroundProlonged neck flexion during smartphone use is known as a factor of neck pain and alteration of neck muscle activity. Studies on the effects of shoulder taping on neck discomfort and neck muscle responses while texting on a smartphone are still lacking. The aim of this study was to examine the effects of shoulder taping on neck discomfort using a numerical rating scale, and neck muscle activity and fatigue using a surface electromyography during a texting task on a touchscreen smartphone.MethodsTwenty-five healthy adolescents used the dominant hand to perform a 30-minute texting task using a touchscreen smartphone at two separate times under one of the following two conditions: taping across the upper trapezius muscle and no taping. Neck discomfort, normalized root mean square, and normalized median frequency slopes for upper trapezius, cervical erector spinae, and sternocleidomastoid muscles were recorded.ResultsThe results revealed that shoulder taping provided significantly lower neck discomfort than no taping (p < 0.001). However, shoulder taping did not significantly alter normalized root mean square and normalized median frequency slope values of all muscles when compared with no taping controls.ConclusionShoulder taping reduces neck discomfort but does not affect neck muscle activity and fatigue while texting on a touchscreen smartphone.

Protein Kinase C Mediates the Corticosterone-induced Sensitization of Dorsal Root Ganglion Neurons Innervating the Rat Stomach.

Background/AimsGastric hypersensitivity contributes to abdominal pain in patients with functional dyspepsia. Recent studies showed that hormones induced by stress are correlated with visceral hypersensitivity. However, the precise mechanisms underlying gastric hypersensitivity remain largely unknown. The aim of the present study was designed to investigate the roles of corticosterone (CORT) on excitability of dorsal root ganglion (DRG) neurons innervating the stomach.MethodsDRG neurons innervating the stomach were labeled by DiI injection into the stomach wall. Patch clamp recordings were employed to examine neural excitability and voltage-gated sodium channel currents. Electromyograph technique was used to determine the responses of neck muscles to gastric distension.ResultsIncubation of acutely isolated DRG neurons with CORT significantly depolarized action potential threshold and enhanced the number of action potentials induced by current stimulation of the neuron. Under voltage-clamp mode, incubation of CORT enhanced voltage-gated sodium current density of the recorded neurons. Pre-incubation of GF109203X, an inhibitor of protein kinase C, blocked the CORT-induced hyperexcitability and potentiation of sodium currents. However, pre-incubation of H-89, an inhibitor of protein kinase A, did not alter the sodium current density. More importantly, intraperitoneal injection of CORT produced gastric hypersensitivity of healthy rats, which was blocked by pre-administration of GF109203X but not H-89.ConclusionsOur data strongly suggest that CORT rapidly enhanced neuronal excitability and sodium channel functions, which is most likely mediated by protein kinase C but not protein kinase A signaling pathway in DRG neurons innervating the stomach, thus underlying the gastric hypersensitivity induced by CORT injection.

Transcranial Magnetic Stimulation of the Prefrontal Cortex in Awake Nonhuman Primates Evokes a Polysynaptic Neck Muscle Response That Reflects Oculomotor Activity at the Time of Stimulation

Transcranial magnetic stimulation (TMS) has emerged as an important technique in cognitive neuroscience, permitting causal inferences about the contribution of a given brain area to behavior. Despite widespread use, exactly how TMS influences neural activity throughout an interconnected network, and how such influences ultimately change behavior, remain unclear. The oculomotor system of nonhuman primates (NHPs) offers a potential animal model to bridge this gap. Here, based on results suggesting that neck muscle activity provides a sensitive indicator of oculomotor activation, we show that single pulses of TMS over the frontal eye fields (FEFs) in awake NHPs evoked rapid (within ∼25 ms) and fairly consistent (∼50-75% of all trials) expression of a contralateral head-turning synergy. This neck muscle response resembled that evoked by subsaccadic electrical microstimulation of the FEF. Systematic variation in TMS location revealed that this response could also be evoked from the dorsolateral prefrontal cortex (dlPFC). Combining TMS with an oculomotor task revealed state dependency, with TMS evoking larger neck muscle responses when the stimulated area was actively engaged. Together, these results advance the suitability of the NHP oculomotor system as an animal model for TMS. The polysynaptic neck muscle response evoked by TMS of the prefrontal cortex is a quantifiable trial-by-trial reflection of oculomotor activation, comparable to the monosynaptic motor-evoked potential evoked by TMS of primary motor cortex. Our results also speak to a role for both the FEF and dlPFC in head orienting, presumably via subcortical connections with the superior colliculus.

Presynaptic actions of transcranial and local direct current stimulation in the red nucleus.

The main aim of the present study was to examine to what extent long-lasting subcortical actions of transcranial direct current stimulation (tDCS) may be related to its presynaptic actions. This was investigated in the red nucleus, where tDCS was recently demonstrated to facilitate transmission between interpositorubral and rubrospinal neurons. Changes in the excitability of preterminal axonal branches of interpositorubral neurons close to rubrospinal neurons were investigated during and after tDCS (0.2 mA) applied over the sensorimotor cortical area in deeply anaesthetized rats and cats. As a measure of the excitability, we used the probability of antidromic activation of individual interpositorubral neurons by electrical stimuli applied in the red nucleus. Our second aim was to compare effects of weak (≤1 μA) direct current applied within the red nucleus with effects of tDCS to allow the use of local depolarization in a further analysis of mechanisms of tDCS instead of widespread and more difficult to control depolarization evoked by distant electrodes. Local cathodal polarization was found to replicate all effects of cathodal tDCS hitherto demonstrated in the rat, including long-lasting facilitation of trans-synaptically evoked descending volleys and trisynaptically evoked EMG responses in neck muscles. It also replicated all effects of anodal tDCS in the cat. In both species, it increased the excitability of preterminal axonal branches of interpositorubral neurons up to 1 h post-tDCS. Local anodal polarization evoked opposite effects. We thus show that presynaptic actions of polarizing direct current may contribute to both immediate and prolonged effects of tDCS.

Loud preimpact tones reduce the cervical multifidus muscle response during rear-end collisions: a potential method for reducing whiplash injuries

Background contextNeck muscle responses after unexpected rear-end collisions consist of a stereotypical combination of postural and startle responses. Prior work using surface electromyography (EMG) has shown that the superficial neck muscle responses can be attenuated when a loud tone (105 dB) is presented 250 milliseconds before impact, but the accompanying response of the deeper multifidus muscles remains unknown. Quantifying this response in multifidus is important because this muscle attaches directly to the cervical facet capsule and can potentially increase the strain in the capsule during an impact and contribute to whiplash injury. PurposeTo investigate if a loud preimpact tone decreases the cervical multifidus muscle response during rear-end perturbations. Study designAfter approval by the University Clinical Ethics Review Board, human volunteers experienced a series of three whiplash-like perturbations. Patient sampleTwelve subjects with no history of neurologic disorders or whiplash injury were recruited to participate in this experiment. Outcome measuresBilateral indwelling EMG of multifidus at the C4 and C6 levels, surface EMG of sternocleidomastoid (SCM) and C4 paraspinals (PARAs), and kinematics of the head/neck were measured. MethodsSubjects experienced three whiplash-like perturbations (peak acceleration of 19.5 m/s2) preceded by either no tone or a loud tone (105 dB) presented 250 milliseconds before sled acceleration onset. ResultsThe loud tone decreased the muscle activity of C6 multifidus (42%) and C4 PARAs (30%), but did not affect the C4 multifidus or SCM activity. Peak head kinematic responses (extension angle: 6%, retraction: 9%, linear forward acceleration: 9%, and angular acceleration in extension: 13%) were also decreased by the loud preimpact tone. ConclusionsThe attenuation of peak C6 multifidus activity and head kinematic responses suggests that a loud preimpact tone may reduce the strain in the cervical facet capsule, which may reduce the risk of whiplash injury during rear-end collisions.

Modulation of neck muscle activity induced by intra-oral stimulation in humans

ObjectiveTo investigate the effect of painful electrical stimuli applied to intra-oral tissues around the teeth on the neck muscle activity in healthy humans. MethodsElectromyographic (EMG) responses of the dorsal neck muscles evoked by intra-oral electrical stimulation were recorded before and after local anesthesia to the stimulus site in 17 healthy volunteers. ResultsInhibition of dorsal neck muscle EMG activities on average 80% compared to baseline level was observed with a latency around 50ms after the electrical stimulation before anesthesia, and the EMG activity inhibition decreased after anesthesia of the intra-oral stimulus site. The perceived intensity of the electrical stimuli as scored on a visual analogue scale (VAS) was 6.1±0.4cm before anesthesia and 1.5±0.2cm after anesthesia. ConclusionIntra-oral stimulation can inhibit neck muscle activity. This modulation might be attributed mainly to nociceptive afferent nerves however, non-nociceptive fibers could also be responsible. SignificanceIntra-oral information including nociceptive activity can inhibit neck muscle activity. From a clinical viewpoint, the present findings demonstrate the neural connectivity between the trigeminal region and the cervical region raising the possibility that orofacial pain conditions could influence head, neck and shoulder activity.

Frequency response of vestibular reflexes in neck, back, and lower limb muscles

Vestibular pathways form short-latency disynaptic connections with neck motoneurons, whereas they form longer-latency disynaptic and polysynaptic connections with lower limb motoneurons. We quantified frequency responses of vestibular reflexes in neck, back, and lower limb muscles to explain between-muscle differences. Two hypotheses were evaluated: 1) that muscle-specific motor-unit properties influence the bandwidth of vestibular reflexes; and 2) that frequency responses of vestibular reflexes differ between neck, back, and lower limb muscles because of neural filtering. Subjects were exposed to electrical vestibular stimuli over bandwidths of 0-25 and 0-75 Hz while recording activity in sternocleidomastoid, splenius capitis, erector spinae, soleus, and medial gastrocnemius muscles. Coherence between stimulus and muscle activity revealed markedly larger vestibular reflex bandwidths in neck muscles (0-70 Hz) than back (0-15 Hz) or lower limb muscles (0-20 Hz). In addition, vestibular reflexes in back and lower limb muscles undergo low-pass filtering compared with neck-muscle responses, which span a broader dynamic range. These results suggest that the wider bandwidth of head-neck biomechanics requires a vestibular influence on neck-muscle activation across a larger dynamic range than lower limb muscles. A computational model of vestibular afferents and a motoneuron pool indicates that motor-unit properties are not primary contributors to the bandwidth filtering of vestibular reflexes in different muscles. Instead, our experimental findings suggest that pathway-dependent neural filtering, not captured in our model, contributes to these muscle-specific responses. Furthermore, gain-phase discontinuities in the neck-muscle vestibular reflexes provide evidence of destructive interaction between different reflex components, likely via indirect vestibular-motor pathways.

Distribution of concussion related symptoms after whiplash injury in risk strata

Abstract Background/aims The presence and severity of concussion related symptoms after acute whiplash injury are debatable. In this study we examine the distribution and development of the burden of concussion related symptoms in whiplash patients. Methods Consecutively 141 acute whiplash patients and 40 ankle injured controls were recruited from emergency units and were examined after 1 week, 1, 3, 6, 12 months obtaining neck/head VAS score, number-of-non-painful complaints, epidemiological, social, psychological data and neurological examination, active neck mobility, and furthermore muscle tenderness and pain response, strength and duration of neck muscles. Risk factors derived (reduced CROM, intense neckpain/headache, multiple non-pain complaints) were applied in a Risk Assessment Score and divided into 7 risk-strata (refer Kasch et al., Spine 2011). After 1 week and 6 months whiplash patients fulfilled the Rivermead Oxford Head Injury questionnaire (10 items, score from 0 = no change to 4 = very marked change, Danish Version, total score range from 0 to 40). Results 138 acute whiplash patients fulfilled the Rivermead Oxford Head Injury questionnaire after 1 week and 111 after 6 months. The distribution was markedly different in the risk strata, in stratum 7 the sum score was a factor 10 higher than stratum 1 after 1 week (Kruskal-Wallis, p &lt; 1.4 × 10–7) and remained high 6 months after injury (p &lt; 0.05). Conclusion Mild concussion symptoms do not necessarily reflect eventual concussion, but are found after injuries with no direct head trauma and amnesia e.g. whiplash injuries. The Risk Assessment Score fits nicely with the burden of concussion related symptoms after whiplash injury.