Brain region changes following a spinal cord injury

Brain-related complications are common in clinical practice after spinal cord injury (SCI); however, the molecular mechanisms of these complications are still unclear. Here, we reviewed the changes in the brain regions caused by SCI from three perspectives: imaging, molecular analysis, and electrophysiology. Imaging studies revealed abnormal functional connectivity, gray matter volume atrophy, and metabolic abnormalities in brain regions after SCI, leading to changes in the structure and function of brain regions. At the molecular level, chemokines, inflammatory factors, and damage-associated molecular patterns produced in the injured area were retrogradely transmitted through the corticospinal tract, cerebrospinal fluid, or blood circulation to the specific brain area to cause pathologic changes. Electrophysiologic recordings also suggested abnormal changes in brain electrical activity after SCI. Transcranial magnetic stimulation, transcranial direct current stimulation, and deep brain stimulation alleviated pain and improved motor function in patients with SCI; therefore, transcranial therapy may be a new strategy for the treatment of patients with SCI.

Recent advances in studying brain-behavior interactions using functional imaging: The primary startle response pathway and its affective modulation in humans

The startle response is a cross-species defensive reflex that is considered a key tool for cross-species translational emotion research. While the neural pathway mediating (affective) startle modulation has been extensively studied in rodents, human work on brain-behavior interactions has lagged in the past due to technical challenges, which have only recently been overcome through non-invasive simultaneous EMG-fMRI assessments. We illustrate key paradigms and methodological tools for startle response assessment in rodents and humans and review evidence for primary and modulatory neural circuits underlying startle responses and their affective modulation in humans. Based on this, we suggest a refined and integrative model for primary and modulatory startle response pathways in humans concluding that there is strong evidence from human work on the neurobiological pathway underlying the primary startle response while evidence for the modulatory pathway is still sparse. In addition, we provide methodological considerations to guide future work and provide an outlook on new and exciting perspectives enabled through technical and theoretical advances outlined in this work.

On the recovery of disorders of consciousness under intrathecal baclofen administration for severe spasticity-An observational study

BACKGROUND

Occasionally, patients show dramatic recovery from disorders of consciousness (DOC) under intrathecal baclofen (ITB), an established treatment option for severe supraspinal spasticity. Anecdotal explanations for ITB-related recovery of cognition include modulation of afferent impulses at the spinal level, thereby reducing spasticity-related proprioceptive information overload within cortico-thalamo-cortical connections.

OBJECTIVE

In this retrospective patient chart analysis, we assessed whether a reduction in spasticity would be associated with an increase in Coma Recovery Scale revised (CRS-R) scores in a larger sample of patients than previously published.

METHODS

From a hospital-based ITB treatment register, we extracted data from 26 patients with DOC and severe supraspinal spasticity who improved by >2 points on the Coma Recovery Scale revised (CRS-R) within 6 months after ITB treatment initiation. We assessed Modified Ashworth scale (MAS) scores and CRS-R scores on admission (PRE) and 3 and 6 months after initiation of ITB treatment (3M, 6M). We performed correlation analysis of the scores and their respective changes (PRE to 3M, 3M to 6M). We also correlated the time from acute event until ITB initiation to CRS-R scores at 3M and 6M.

RESULTS

ITB led to significant improvement in spasticity based on MAS scores, which did not correlate to the improvements seen in CRS-R total and subscale scores. Daily ITB dose did neither correlate to MAS scores nor to CRS-total scores in the whole patient group, but after 3 months, ITB dose correlated to some CRS-R subscale scores in some patient subgroups. Time until ITB treatment did not correlate to CRS-R scores later on.

CONCLUSIONS

Our data confirm that ITB may exert beneficial effects in selected DOC patients with respect to improved cognitive functions, which, however, do not correlate to its antispastic effect. The lack of correlation between time to ITB and CRS-R outcome, but significant CRS-R improvements following pump implantation, renders spontaneous remissions unlikely and leaves room for alternative pharmacological mechanisms.

GABAB Receptor Agonist R-Baclofen Reverses Altered Auditory Reactivity and Filtering in the Cntnap2 Knock-Out Rat

Altered sensory information processing, and auditory processing, in particular, is a common impairment in individuals with autism spectrum disorder (ASD). One prominent hypothesis for the etiology of ASD is an imbalance between neuronal excitation and inhibition. The selective GABA_B receptor agonist R-Baclofen has been shown previously to improve social deficits and repetitive behaviors in several mouse models for neurodevelopmental disorders including ASD, and its formulation Arbaclofen has been shown to ameliorate social avoidance symptoms in some individuals with ASD. The present study investigated whether R-Baclofen can remediate ASD-related altered sensory processing reliant on excitation/inhibition imbalance in the auditory brainstem. To assess a possible excitation/inhibition imbalance in the startle-mediating brainstem underlying ASD-like auditory-evoked behaviors, we detected and quantified brain amino acid levels in the nucleus reticularis pontis caudalis (PnC) of rats with a homozygous loss-of-function mutation in the ASD-linked gene Contactin-associated protein-like 2 (Cntnap2) and their wildtype (WT) littermates using Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS). Abnormal behavioral read-outs of brainstem auditory signaling in Cntnap2 KO rats were accompanied by increased levels of GABA, glutamate, and glutamine in the PnC. We then compared the effect of R-Baclofen on behavioral read-outs of brainstem auditory signaling in Cntnap2 KO and WT rats. Auditory reactivity, sensory filtering, and sensorimotor gating were tested in form of acoustic startle response input-output functions, short-term habituation, and prepulse inhibition before and after acute administration of R-Baclofen (0.75, 1.5, and 3 mg/kg). Systemic R-Baclofen treatment improved disruptions in sensory filtering in Cntnap2 KO rats and suppressed exaggerated auditory startle responses, in particular to moderately loud sounds. Lower ASR thresholds in Cntnap2 KO rats were increased in a dose-dependent fashion, with the two higher doses bringing thresholds close to controls, whereas shorter ASR peak latencies at the threshold were further exacerbated. Impaired prepulse inhibition increased across various acoustic prepulse conditions after administration of R-Baclofen in Cntnap2 KO rats, whereas R-Baclofen did not affect prepulse inhibition in WT rats. Our findings suggest that GABA_B receptor agonists may be useful for pharmacologically targeting multiple aspects of sensory processing disruptions involving neuronal excitation/inhibition imbalances in ASD.

Effects of intrathecal baclofen therapy in subjects with disorders of consciousness: a reappraisal

Baclofen is a structural analogue of gamma-amino-butyric acid (GABA), which reduces spastic hypertonia of striated muscle due to a mechanism of GABA_B-ergic inhibition of mono- and polysynaptic reflexes at the spinal level. There are reports of patients with severe disorders of consciousness that presented a substantial improvement following intrathecal baclofen (ITB) administration for severe spasticity. The neural mechanisms underlying the clinical recovery after ITB have not yet been clarified. Baclofen could modulate sleep-wake cycles that may be dysregulated and thus interfere with alertness and awareness. The diminished proprioceptive and nociceptive sensory inputs may relieve thalamo-cortical neural networks involved in maintaining the consciousness of the self and the world. ITB treatment might also promote the recovery of an impaired GABAergic cortical tone, restoring the balance between excitatory and inhibitory cortical activity. Furthermore, glutamatergic synapses are directly or indirectly modulated by GABA_B-ergic receptors. Neurophysiological techniques (such as transcranial magnetic stimulation, electroencephalography, or the combination of both) can be helpful to explore the effects of intrathecal or oral baclofen on the modulation of neural cortical circuits in humans with disorders of consciousness.

Jendrassik maneuver effect on spinal and brainstem reflexes

The effect of Jendrassik Maneuver (JM) has been extensively studied on monosynaptic reflexes in numerous muscles below the level at which the maneuver was performed. Here we hypothesize that the effect of JM could be observed also on other reflexes, indicating a widespread influence of performing a motor act such as the JM. We examined polysynaptic reflexes caudal (i.e., the withdrawal reflex of the lower extremities) and rostral (i.e., the blink reflex to supraorbital nerve stimulation) to the level of JM contraction. We have assessed soleus tendon (T) reflex; withdrawal reflex in tibialis anterior and soleus muscle; blink reflex (BR), blink reflex excitability recovery curve (BR-ER) and prepulse inhibition of the blink reflex. Our results showed that (1) T-reflex amplitude increased during JM and decreased just after and 15 min after JM; (2) no change in the withdrawal reflex; (3) R2 area of BR reduced significantly just after or 15 min after JM; (4) Prepulse inhibition in BR reduced significantly during JM; (5) no change in BR-ER. Our results indicate that JM leads to generalized effects on neural excitability at both caudal and rostral levels. Furthermore, JM has a selective effect on excitability of reflex circuitries.

Imbalanced Corticospinal and Reticulospinal Contributions to Spasticity in Humans with Spinal Cord Injury

Damage to the corticospinal and reticulospinal tract has been associated with spasticity in humans with upper motor neuron lesions. We hypothesized that these descending motor pathways distinctly contribute to the control of a spastic muscle in humans with incomplete spinal cord injury (SCI). To test this hypothesis, we examined motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the leg representation of the primary motor cortex, maximal voluntary contractions (MVCs), and the StartReact response (shortening in reaction time evoked by a startling stimulus) in the quadriceps femoris muscle in male and females with and without incomplete SCI. A total of 66.7% of the SCI participants showed symptoms of spasticity, whereas the other 33.3% showed no or low levels of spasticity. We found that participants with spasticity had smaller MEPs and MVCs and larger StartReact compared with participants with no or low spasticity and control subjects. These results were consistently present in spastic subjects but not in the other populations. Clinical scores of spasticity were negatively correlated with MEP-max and MVC values and positively correlated with shortening in reaction time. These findings provide evidence for lesser corticospinal and larger reticulospinal influences to spastic muscles in humans with SCI and suggest that these imbalanced contributions are important for motor recovery.SIGNIFICANCE STATEMENT Although spasticity is one of the most common symptoms manifested in humans with spinal cord injury (SCI) to date, its mechanisms of action remain poorly understood. We provide evidence, for the first time, of imbalanced contributions of the corticospinal and reticulospinal tract to control a spastic muscle in humans with chronic incomplete SCI. We found that participants with SCI with spasticity showed small corticospinal responses and maximal voluntary contractions and larger reticulospinal gain compared with participants with no or low spasticity and control subjects. These results were consistently present in spastic subjects but not in the other populations. We showed that imbalanced corticospinal and reticulospinal tract contributions are more pronounced in participants with chronic incomplete SCI with lesser recovery.

Brainstem reflex excitability after high-frequency repetitive transcranial magnetic stimulation in healthy and spinal cord injury subjects

BACKGROUND

The excitability of brainstem interneuronal circuits is partly under control from descending inputs. Since high frequency repetitive transcranial magnetic stimulation (rTMS) modulates cortical output, we hypothesized that it will also modulate brainstem functions. Such modulation may be different in healthy subjects than in subjects with spinal cord injury (SCI), submitted to an altered integration of body afferent inputs.

METHODS

In this randomized, double-blind, sham-controlled trial, we recruited 22 subjects with SCI assigned to either real (n = 11) or sham (n = 11) rTMS and nine healthy subjects, who served as control group, receiving both real (at 20 Hz, with double cone coil over vertex) and sham rTMS separated by at least one week. We recorded the blink reflex (BR) to supraorbital nerve (SON) electrical stimulation and its modification by another conditioning SON, to study the BR excitability recovery (BRER), or a prepulse electrical stimulus to the right index finger, to study the BR inhibition by prepulse (BRIP). Subjects were examined immediately before and after either sham or real rTMS.

RESULTS

Real but not sham rTMS significantly reduced the area of the BR R2 response in both SCI and healthy subjects. There were no changes in BRER and BRIP.

CONCLUSION

rTMS over the vertex modulates brainstem reflexes with no significant differences between SCI and healthy subjects.

Pattern of startle reflex to somatosensory stimuli changes after spinal cord injury

INTRODUCTION

Spinal cord injury (SCI) may cause functional changes at various levels in central and peripheral nervous systems. One of these changes is increased excitability above the lesion such as enhanced auditory startle responses (ASR). Startle response may also be obtained after somatosensory stimulus (startle reflex to somatosensory stimuli, SSS). In this study, we investigated changes of both ASR and SSS in SCI.

METHOD

We examined ASR and SSS in 14 patients with SCI and 18 age-matched healthy volunteers. SSS responses were recorded from orbicularis oculi (O.oc), sternocleidomastoid (SCM) and biceps brachii (BB) muscles by electrical stimulation of median nerve at the wrist. ASR was evoked by binaural auditory stimuli and recorded from O.oc, masseter, SCM and BB muscles. Probability, latency, amplitude and duration of responses were compared between two groups for each muscle.

RESULTS

Presence of response over O.oc after somatosensory stimuli was decreased in patients compared to controls (P = 0.004). There were no differences in SSS responses of other muscles. ASR latency was shorter in masseter, SCM and BB in patients with SCI, but only BB had significantly reduced latency (P = 0.033). The duration of O.oc response was longer and the amplitude of SCM was larger in patients with SCI (P = 0.037 and P = 0.015, respectively).

CONCLUSION

ASR is enhanced after SCI whereas SSS of eye muscles is hypoactive and pattern of SSS after median stimulation changes in SCI.

Supraspinal respiratory plasticity following acute cervical spinal cord injury

Impaired breathing is a devastating result of high cervical spinal cord injuries (SCI) due to partial or full denervation of phrenic motoneurons, which innervate the diaphragm - a primary muscle of respiration. Consequently, people with cervical level injuries often become dependent on assisted ventilation and are susceptible to secondary complications. However, there is mounting evidence for limited spontaneous recovery of respiratory function following injury, demonstrating the neuroplastic potential of respiratory networks. Although many studies have shown such plasticity at the level of the spinal cord, much less is known about the changes occurring at supraspinal levels post-SCI. The goal of this study was to determine functional reorganization of respiratory neurons in the medulla acutely (>4h) following high cervical SCI. Experiments were conducted in decerebrate, unanesthetized, vagus intact and artificially ventilated rats. In this preparation, spontaneous recovery of ipsilateral phrenic nerve activity was observed within 4 to 6h following an incomplete, C2 hemisection (C2Hx). Electrophysiological mapping of the ventrolateral medulla showed a reorganization of inspiratory and expiratory sites ipsilateral to injury. These changes included i) decreased respiratory activity within the caudal ventral respiratory group (cVRG; location of bulbospinal expiratory neurons); ii) increased proportion of expiratory phase activity within the rostral ventral respiratory group (rVRG; location of inspiratory bulbo-spinal neurons); iii) increased respiratory activity within ventral reticular nuclei, including lateral reticular (LRN) and paragigantocellular (LPGi) nuclei. We conclude that disruption of descending and ascending connections between the medulla and spinal cord leads to immediate functional reorganization within the supraspinal respiratory network, including neurons within the ventral respiratory column and adjacent reticular nuclei.

Startle Response in Progressive Myoclonic Epilepsy

Cortical reflex myoclonus is a typical feature of progressive myoclonic epilepsy (PME) in which it is accompanied by other types of mostly drug-resistant seizures and progressive neurological signs. Although PME is characterized by cortical hyperexcitability, studies have demonstrated atrophy and degenerative changes in the brainstem in various types of PME. Thus, we have questioned whether any stimuli may trigger a hyperactive response of brainstem reticular formation in PME and investigated the startle reflex in individuals with PME. We recorded the auditory startle response (ASR) and the startle response to somatosensory inputs (SSS) in patients with PME, and compared the results with healthy volunteers and patients with other types of drug-resistant epilepsy. All patients were using antiepileptic drugs (AEDs), 12 were on multiple AEDs. The probability of ASR was significantly lower and mean onset latency was longer in patients with PME compared with other groups. SSS responses over all muscles were low in both the PME and drug-resistant epilepsy groups; however, the differences were not statistically significant. The presence of a response over the biceps brachii muscle was zero in the PME group and showed a borderline difference compared with the other groups. Decreased probability and prolonged latencies of ASR in PME indicate inhibition of reflex circuit. A trend for decreased responses of SSS suggests hypoactive SSS in both PME and other epilepsy groups. Hypoactive ASR in PME and hypoactive SSS in both PME and other epilepsies may be attributed to the degeneration of pontine reticular nuclei in PME and functional inhibition by AEDs in both disorders.

Acoustic startle reflex in patients with chronic stroke at different stages of motor recovery: a pilot study

BACKGROUND

Acoustic startle reflex (ASR) can be used as a tool to examine reticulospinal excitability. The potential role of reticulospinal mechanisms in the development of spasticity has been suggested but not tested.

OBJECTIVE

To examine reticulospinal excitability at different stages of motor recovery in patients with chronic stroke using the ASR.

METHODS

Sixteen subjects with hemiplegic stroke participated in the study. We examined ASR responses at rest and contralateral motor overflow during voluntary elbow flexion.

RESULTS

ASR responses in impaired biceps muscles showed different patterns at different stages. In subjects without spasticity, ASR responses were less frequent (10% on impaired side) and had normal duration (<200 ms). In subjects with spasticity, the responses were more frequent (58.3% on impaired side) and longer lasting (up to 1 minute). However, no correlation between exaggerated reflex responses and Modified Ashworth Scale (MAS) scores was observed. During voluntary elbow flexion on the impaired side, similar positive linear force-electromyogram (EMG) relationships were found in subjects with and without spasticity. Electromyographic activity of the resting nonimpaired limb increased proportionally in subjects with spasticity (r = 0.6313, P = .0004), but no such correlation was found in subjects without spasticity (r = 0.0191, P = .9612).

CONCLUSIONS

Preliminary findings of exaggerated ASR responses and associated contralateral overflow only in spastic biceps muscles in patients with chronic stroke suggest the important role of reticulospinal mechanisms in the development of spasticity.

Trigemino-cervical reflex in spinal cord injury

Abnormal enhancement of polysynaptic brainstem reflexes has been previously reported in patients with spinal cord injury (SCI). We aimed to investigate trigemino-cervical reflex (TCR) in SCI since it may reflect alterations in the connections of trigeminal proprioceptive system and cervical motoneurons. Consecutive 14 patients with SCI and 16 healthy subjects were included in this study. All patients were in the chronic phase. TCR was recorded over sternocleidomastoid (SCM) and splenius capitis (SC) muscles by stimulation of infraorbital nerve. We measured onset latency, amplitudes and durations of responses and compared between groups. We obtained stable responses over both muscles after one sided stimulation in healthy volunteers whereas probability of TCR was decreased in patients over both SCM (78.6% vs. 100%, p=0.050) and SC (71.4% vs. 100%, p=0.022). The absence of TCR was related to use of oral baclofen (≥50mg/day). However, when present, responses of SCI group had higher amplitudes and were more persistent. We demonstrated that TCR probability was similar to healthy subjects in SCI patients who used no or low dose oral baclofen. But it had higher amplitudes and longer durations. It was not obtained in only two patients who used oral baclofen more than 50mg/day.

Differential effect of baclofen on cortical and spinal inhibitory circuits

OBJECTIVE

The cutaneous silent period (SP) is a spinal inhibitory reflex, which suppresses activity in spinal motor nuclei. Transcranial magnetic stimulation (TMS) elicits a cortical SP, which represents GABA(B) receptor-mediated inhibition of cortical excitability. Baclofen as a strong GABA(B) agonist effectively reduces muscle hypertonia, however, it is not known whether intrathecal baclofen (ITB) may modulate spinal inhibitory circuits.

METHODS

We evaluated clinical and neurophysiological effects of ITB in ten patients with severe spasticity due to spinal cord injury (n = 9) and chronic progressive multiple sclerosis (n = 1). Neurophysiological assessment included H reflex and cutaneous and cortical SPs, before and 15, 30, 60, 90, 120, and 180 min after ITB bolus administration.

RESULTS

ITB suppressed soleus H reflex as early as 15 min after lumbar bolus injection; MAS scores declined after 1 h. Cortical SP end latency and duration increased progressively with a significant maximum 3h following ITB bolus, whereas cutaneous SP latency and duration did not change significantly.

CONCLUSION

The present findings suggest that baclofen does not affect the cutaneous SP, but prolongs the cortical SP.

SIGNIFICANCE

The spinal inhibitory circuitry of the cutaneous SP is not modulated by GABA(B) receptor-mediated activity, in contrast to the cortical inhibitory circuitry of the cortical SP, which is subject to powerful GABA(B) control.

Narcolepsy-cataplexy: deficient prepulse inhibition of blink reflex suggests pedunculopontine involvement

Hypocretin (orexin) deficiency plays a major role in the pathophysiology of narcolepsy-cataplexy. In animal models, hypocretinergic projections to the pedunculopontine nucleus are directly involved in muscle tone regulation mediating muscle atonia - a hallmark of cataplexy. We hypothesized that pedunculopontine nucleus function, tested with prepulse inhibition of the blink reflex, is altered in human narcolepsy-cataplexy. Twenty patients with narcolepsy-cataplexy and 20 healthy controls underwent a neurophysiological study of pedunculopontine nucleus function. Blink reflex, prepulse inhibition of the blink reflex and blink reflex excitability recovery were measured. Blink reflex characteristics (R1 latency and amplitude, and R2 and R2c latency and area under the curve) did not differ between patients and controls (P > 0.05). Prepulse stimulation significantly increased R2 and R2c latencies and reduced R2 and R2c areas in patients and controls. However, the R2 and R2c area suppression was significantly less in patients than in controls (to 69.8 ± 14.4 and 74.9 ± 12.6%, respectively, versus 34.5 ± 28.6 and 43.3 ± 29.5%, respectively; each P < 0.001). Blink reflex excitability recovery, as measured by paired-pulse stimulation, which is not mediated via the pedunculopontine nucleus, did not differ between patients and controls (P > 0.05). Our data showed that prepulse inhibition is reduced in narcolepsy-cataplexy, whereas unconditioned blink reflex and its excitability recovery are normal. Because the pedunculopontine nucleus is important for prepulse inhibition, these results suggest its functional involvement in narcolepsy-cataplexy.

GABAergic neural activity involved in salicylate-induced auditory cortex gain enhancement

Although high doses of sodium salicylate impair cochlear function, it paradoxically enhances sound-evoked activity in the auditory cortex (AC) and augments acoustic startle reflex responses, neural and behavioral metrics associated with hyperexcitability and hyperacusis. To explore the neural mechanisms underlying salicylate (SS)-induced hyperexcitability and "increased central gain," we examined the effects of GABA receptor agonists and antagonists on SS-induced hyperexcitability in the AC and startle reflex responses. Consistent with our previous findings, local or systemic application of SS significantly increased the amplitude of sound-evoked AC neural activity, but generally reduced spontaneous activity in the AC. Systemic injection of SS also significantly increased the acoustic startle reflex. S-baclofen or R-baclofen, GABA-B agonists, which suppressed sound-evoked AC neural firing rate and local field potentials, also suppressed the SS-induced enhancement of the AC field potential and the acoustic startle reflex. Local application of vigabatrin, which enhances GABA concentration in the brain, suppressed the SS-induced enhancement of AC firing rate. Systemic injection of vigabatrin also reduced the SS-induced enhancement of acoustic startle reflex. Collectively, these results suggest that the sound-evoked behavioral and neural hyperactivity induced by SS may arise from a SS-induced suppression of GABAergic inhibition in the AC.