Enhancement of the conditioning lesion effect in rat sciatic motor axons after superimposition of conditioning and test lesions

In previous studies on sensory axons we reported that the effect of a conditioning lesion on increasing regeneration rate was enhanced if the two lesions were superimposed, rather than made at separate sites on the nerve, and proposed that this was due to the growth of axons through nerve predegenerated by the conditioning lesion. We now find that the regeneration of motor axons, determined by labeling with fast axonally transported protein, is also enhanced by superimposed conditioning and test lesions, to a greater extent than by separated lesions. However, the regeneration rate of the conditioned motor axons (5.40 +/- 0.44 mm/day) was less than that of conditioned sensory axons in the same nerves (6.65 +/- 0.56 mm/day). Recovery of motor function after the test lesion was assessed by computing a "sciatic functional index" from measurements of hind footprints made by the rats while walking. Recovery began earlier in the conditioned animals, with the time to half-maximum recovery being 13 days, compared with 18 days in animals that had received a test lesion only. In both groups of animals recovery was complete. Although these results are consistent with the proposal that regenerating motor axons elongate more rapidly through nerve predegenerated following the conditioning lesion, we cannot eliminate the possibility that the enhanced regeneration rate in motoneurons was a result of a more vigorous metabolic response to the conditioning lesion when placed more proximally on their axons.     

Changes in sensory inattention, directional motor neglect and "release" of the fixation reflex following a unilateral frontal lesion: a case report

We recorded eye movements to and away from visual stimuli from a patient with left-sided neglect following a right frontal infarct in order to determine (a) whether and to what extent his neglect was due to sensory inattention and directional motor neglect and (b) whether he had difficulty suppressing inappropriate eye movements to visual stimuli ("release" of visual grasp) as his sensory inattention declined. In the first testing session, conducted 5 days following his stroke, he often failed to move his eyes when a stimulus on the left required a rightward eye movement, but he consistently moved his eyes to a stimulus on the right. Thus, he showed contralateral but not ipsilateral sensory inattention. Initially, he also was impaired in making leftward eye movements when right stimuli were presented. Thus, he also showed a directional motor neglect. In subsequent tests, his left-sided sensory inattention as defined above decreased, and was no longer present three weeks following his stroke, nor in a follow-up test conducted almost 6 months following this stroke. In contrast, his directional motor neglect, as defined above, was still present in the follow-up test. As his left-sided sensory inattention declined, his tendency to move his eyes incorrectly to stimuli on the left side (the side contralateral to his lesion) when these stimuli required eye movements to the right became stronger ("release" of visual grasp); he continued to show this strong tendency in the test conducted almost 6 months following his stroke.     

Effects of trains of high-frequency stimulation of the premotor/supplementary motor area on conditioned corticomotor responses in hemicerebellectomized rats

We studied the effects of low- and high-frequency premotor electrical stimulations on conditioned corticomotor responses, intra-cortical facilitation (ICF) and spinal excitability in hemicerebellectomized rats (left side). Trains of stimulation were applied in prefrontal region rFr2 (the equivalent of the premotor/supplementary motor area in primates) at a rate of 1 Hz (low-frequency stimulation LFS) or 20 Hz (high-frequency stimulation HFS). Test stimuli on the motor cortex were preceded by a conditioning stimulus in contralateral sciatic nerve (two inter-stimulus intervals ISIs were studied: 5 ms or 45 ms). (A) At ISI-5, conditioning increased amplitudes of MEPs (motor evoked potentials) in the left motor cortex. This afferent facilitation was enhanced if preceded by trains of stimuli administered over the ipsilateral rFr2 area, and HFS had higher effects than LFS. The facilitation was lower for the right motor cortex, for both LFS and HFS. (B) At ISI-45, conditioned motor evoked responses were depressed as compared to unconditioned responses in the left motor cortex (afferent inhibition). Following LFS, the degree of inhibition was unchanged while it increased with HFS. At baseline, inhibition was enhanced in the right motor cortex. Interestingly, the afferent inhibition decreased significantly following HFS. (C) ICF was depressed in the right motor cortex, but increased similarly on both sides following LFS/HFS. These results (1) confirm the increased inhibition in the motor cortex contralaterally to the hemicerebellar ablation, (2) demonstrate for the first time that the cerebellum is necessary for tuning amplitudes of corticomotor responses following a peripheral nerve stimulation, (3) show that the application of LFS or HFS does not cancel the defect of excitability in the motor cortex for short ISIs, and (4) suggest that for longer ISIs, HFS could have interesting properties for the modulation of afferent inhibition in case of extensive cerebellar lesion. Our study underlines that cerebellar ablation impacts on the efficacy of combined peripheral-motor cortex stimulation in an ISI-dependent manner.     

Transient improvement in motor function and hemineglect by vestibular stimulation in a patient with right middle cerebral artery embolism]

A 62-year-old Japanese man presented left hemiparesis and left visuospatial hemineglect following a right hemispheric stroke. His CTs and MRIs of the brain revealed a large embolic infarction of the middle cerebral artery territory. A month after the cerebrovascular event, his weakness of the left lower limb almost recovered fully. However, his upper limb motor function was still disabled; in particular, his ability of finger flexion in the left hand was almost lost. Then, vestibular stimulation using either a cold caloric stimulation to the left ear or a warm caloric stimulation to the right ear was performed, and the effect on the hemineglect symptoms were assessed by a line bisection task. After vestibular stimulation, not only his hemineglect symptoms but also his motor functions of left upper limb transiently improved; he became able to make a fist. The improvement of his hemineglect symptoms was obtained by vestibular stimulation using either a cold or a warm caloric stimulation. However, the effect on the motor function was obtained only by the cold caloric stimulation applied to the left ear. Based on the effect of the vestibular stimulation, we postulates that the impairment of the motor function in the present patient is not only a paresis caused by the pyramidal tract lesion but also symptoms related to the hemineglect syndrome.     

Bilateral contributions to motor recovery in the monkey following lesions of the deep cerebellar nuclei

A unilateral lesion of the deep cerebellar nuclei in monkeys produced a transient inability to perform a reaching task with the limb ipsilateral to the lesion. The deficit recovered within 2 weeks following a time course having a initial rapid and a subsequent slower phase. After a second lesion of the cerebellar nuclei on the opposite side, the animals developed a bilateral deficit. Recovery occurred bilaterally after this second stage but following the slower rate observed after the first lesion. From these experiments we conclude that the initial, more rapid phase of the recovery after a unilateral cerebellar lesion depends upon intact contralateral cerebellar circuitry and that the slower rate of recovery was mediated by other parts of the motor system.     

Delayed induction of the cell body response and enhancement of regeneration following a condition/test lesion of frog peripheral nerve at 15 °C

Axons subjected to a prior injury exhibit an enhanced regenerative capability following a subsequent lesion. This phenomenon, known as the ‘conditioning effect’ has been demonstrated in both central and peripheral axons from a vareity of species, but the locus of enhancement has remained in doubt. Several reports have tentatively concluded that the effect involves processes located in the cell body of the injured axon, while other investigators have implicated local processes at the site of the injury. The present experiments made use of a previous observation, the apparent suppression of the cell body response in injured frog neurons when the animals are housed at 15 °C, to investigate the location of the conditioning effect. The effect occurred in frog neurons from ahe animals at 25 °C in the presence of a cell body response. The prior (conditioning) lesion produced a significant increase in regeneration rate (12–17%) and a significant decrease in the latency to the onset of regenerative outgrowth (12%). In contrast, regeneration was initially unaffected by the earlier conditioning lesion when the frogs were kept at 15 °C. After a 26–30 day interval, however, the rate of regeneration increased by 27%, coincident with the appearance of chromatolysis in the injured cell bodies. The ‘conditioned’ axons also continued to elongate, in contrast to singly-lesioned axons at 15 °C, and eventually reinnervated their target muscles. The cell body appeared to be the primary locus of the ‘conditioning effect’ in these experiments. This conclusion was further examined by measuring regenerative outgrowth in 15°C neurons in which a cell body response had been induced prior to the test lesion. This was accomplished by maintaining frogs at 25 °C for 7 days after the conditioning lesion. The animals were then returned to the 15 °C environment for an additional 7 days before producing the test lesion. With this paradigm, regenerative outgrowth was enhanced without the 26–30 day delay interval. These results also suggest that the conditioning effect stimulates processes in the cell body on the injured axon, but the experiments do not indicate which processes were affected.     

Spike train characteristics of single motor units in the human masseter muscle

Single motor unit (SMU) spike trains were recorded from the masseter muscles of human subjects while they isometrically produced interocclusal force. The majority of SMUs increased their firing rate as interocclusal force was increased, but the rate changes declined in magnitude at each successive force level and eventually reached constant values. This indicated a progressive decline in the ability of motoneurons to increase firing rate in response to increases in excitatory drive, and the evidence suggests that this is due to temporal summation of afterhyperpolarization conductances. There was a strong tendency for SMUs with low recruitment thresholds to show the largest rate changes per kilogram change in force, which may be a consequence of the decreasing input resistances associated with increases in cell size. There were also monotonic decreases in standard deviations and skewness of the interspike interval (ISI) distributions as firing rate increased, which indicates that synaptic drive to the motoneurons was increasing monotonically. The graph of standard deviation mean ISI revealed less tendency for masseter motoneurons to maintain minimum discharge variability as compared to spinal motoneurons; the lack of recurrent inhibition to masseter motoneurons may be responsible.     

Identification of neurons producing long-term potentiation in the cat motor cortex: Intracellular recordings and labeling

Intracellular, in vivo recordings were used to identify and subsequently to label neurons in the cat motor cortex in which long-term potentiation (LTP) was induced. Thirty-nine motor cortical neurons that produced excitatory postsynaptic potentials (EPSPs) in response to microstimulation in areas 1--2 (SI) or in area 5a (SIII) were studied. Amplitudes of EPSPs produced in response to test stimulation (1 Hz) were recorded before and after tetanic stimulation (200 Hz, 20 seconds). In 25/39 cells (64%), EPSP amplitudes were significantly increased following the tetanic stimulation (65 ± 51% average increase), and remained at the potentiated level as long as stable recordings could be maintained (20 ± 18 minutes, maximum = 90 minutes). LTP was induced exclusively in cells that produced monosynpatic EPSPs in response to area 1--2 or area 5a stimulation. Of the 39 analyzed cells, 13 were labeled by intracellular injections of 5% biocytin. Neurons in which LTP was induced included both pyramidal and nonpyramidal cells and were located exclusively in layers II or III of the motor cortex; cells in deeper cortical layers were not potentiated. These findings indicate that various corticocortical inputs can increase the efficacy of synaptic transmission in a subset of motor cortical neurons. We propose that this plasticity in synaptic transmission constitutes one of the bases of motor learning and memory.     

Effects of cyclosporin-A on immune response,tissue protection and motor function of rats subjected to spinal cord injury

The aim of this work was to test the effect of cyclosporin-A (CsA) on some immunological, morphological and functional aspects developed after spinal cord injury. The specific cellular immune response against spinal cord constituents, the amount of spared tissue and myelination at the site of injury, and the motor function outcome were assessed in a first series of experiments. Rats were subjected to spinal cord compression and treated with cyclosporin-A before lesion and during the entire study. A specific lymphocyte response against spinal cord antigens was found in untreated spinal cord injured rats but not in cyclosporine-A treated injured rats. A significantly better myelination index was also found in injured cyclosporin-A-treated rats, as compared to untreated animals. The amount of spared spinal cord tissue at the epicenter was not significantly different comparing CsA-treated with vehicle-treated rats. Looking for a potential therapeutic use of CsA, in a second series of experiments, rats were subjected to spinal cord contusion and treated with cyclosporin-A from 1 to 72 h after lesion. Motor recovery and red nuclei neurons survival, were evaluated, and found to be significantly better in spinal cord injured rats treated with cyclosporin-A than in injured-untreated rats. This work confirms the existence of an autoimmune cellular reaction after injury that can be inhibited by cyclosporin-A treatment. Furthermore, cyclosporin-A promotes neuroprotection by diminishing both demyelination and neuronal cell death, resulting in a better motor outcome after spinal cord injury.     

Decline in reactive fiber growth in the dentate gyrus of aged rats compared to young adult rats following entorhinal cortex removal

The reaction of septal and commissural-associational afferents in the dentate gyrus was examined at various times following a unilateral entorhinal lesion in 2- and 3-month-old, 12- to 18-month-old and 25–30-month-old rats. The response of septohippocampal fibers was examined histochemically by staining for acetylcholinesterase (AChE) activity; and that of commissural-associational fibers by the Holmes' fiber stain. In 2- and 3-month-old rats, AChE staining fibers, which project to the outer three-fourths of the molecular layer of the dentate gyrus, increased their staining intensity within 5–6 days following lesion of the entorhinal cortex. The rate of the response and the eventual magnitude declined progressively with the age of the subject. In 2- and 3-month-old rats, the commissural-associational fiber plexus appeared to expand partially into the entorhinal zone within 6 days following the lesion. This response also decreased progressively in rate and magnitude with age. Animals in the oldest age group showed at 12 days after the lesion a greater variability in the expansion of the commissural-associational fiber plexus than all younger groups.Astrocytes in the dentate molecular layer appeared to become more abundant and more hypertrophied in unoperated animals with age. The appearance of astrocytes in 25- to 30-month-old rats was similar to that seen in 2- and 3-month-old animals following an entorhinal lesion. An entorhinal lesion in the aged animals did not appear to cause a marked change in the appearance of astrocytes.     

Motor evoked potentials in Wilson's disease: early and late motor responses

Motor evoked potentials following magnetic stimulation of the motor cortex and the spinal roots were studied in 13 patients with Wilson's disease (WD). The fast-conducting motor pathway function was normal in 9 patients. The cortically-evoked motor responses were absent in patients with cerebral white matter lesion and akinetic rigid syndrome. In addition to the primary or early motor response, late motor response was common in WD and occurred with a latency of 70-90 msec for the upper limb and 90-110 msec for the lower limb. The cause of the late motor response is unknown, but may be an enhanced late response infrequently observed in normal subjects.     

Whisker motor cortex reorganization after superior colliculus output suppression in adult rats

The effect of unilateral superior colliculus (SC) output suppression on the ipsilateral whisker motor cortex (WMC) was studied at different time points after tetrodotoxin and quinolinic acid injections, in adult rats. The WMC output was assessed by mapping the movement evoked by intracortical microstimulation (ICMS) and by recording the ICMS‐evoked electromyographic (EMG) responses from contralateral whisker muscles. At 1 h after SC injections, the WMC showed: (i) a strong decrease in contralateral whisker sites, (ii) a strong increase in ipsilateral whisker sites and in ineffective sites, and (iii) a strong increase in threshold current values. At 6 h after injections, the WMC size had shrunk to 60% of the control value and forelimb representation had expanded into the lateral part of the normal WMC. Thereafter, the size of the WMC recovered, returning to nearly normal 12 h later (94% of control) and persisted unchanged over time (1–3 weeks). The ICMS‐evoked EMG response area decreased at 1 h after SC lesion and had recovered its baseline value 12 h later. Conversely, the latency of ICMS‐evoked EMG responses had increased by 1 h and continued to increase for as long as 3 weeks following the lesion. These findings provide physiological evidence that SC output suppression persistently withdrew the direct excitatory drive from whisker motoneurons and induced changes in the WMC. We suggest that the changes in the WMC are a form of reversible short‐term reorganization that is induced by SC lesion. The persistent latency increase in the ICMS‐evoked EMG response suggested that the recovery of basic WMC excitability did not take place with the recovery of normal explorative behaviour.     

A conditioning lesion enhances sympathetic neurite outgrowth

Axonal regeneration can be influenced by a conditioning lesion (an axonal injury made prior to a second test lesion). Previously, sympathetic neurons in vivo were shown to respond to a conditioning lesion with decreased neurite outgrowth, in contrast to the enhanced outgrowth observed in all other peripheral neurons examined. The present experiments tested the effects of a conditioning lesion on neurite outgrowth in vitro from the superior cervical ganglion (SCG) and the impact of several factors on that response. Ganglia axotomized 1 week earlier and then explanted in Matrigel or collagen gel responded with a significant increase in neurite extension compared to sham-operated ganglia. A distal axotomy produced by unilateral removal of the salivary glands (sialectomy) caused an increase in neurite outgrowth similar to that of a proximal axotomy. These conditioning lesions induced both an increase in the rate of elongation, and, in the case of the proximally axotomized SCG, a shorter initial delay of outgrowth. The enhanced outgrowth following sialectomy was specific to the nerve containing the majority of axons projecting to the salivary glands, suggesting that the conditioning lesion effect is restricted to previously injured neurons. Deletion of the gene for leukemia inhibitory factor (LIF), a gene induced by axotomy, did not abolish the conditioning lesion effect in SCG explants or dissociated cell cultures. In conclusion, sympathetic neurons are capable of responding to a conditioning lesion with increased neurite outgrowth. The hypothesis that the neuronal cell body response to axotomy plays an important role in the conditioning lesion response is discussed.     

Rank-ordered regulation of motor units

Myoelectric signals were detected from the tibialis anterior muscle of 5 subjects with a quadrifilar needle electrode while the subjects generated isometric forces that increased linearly with time (10% of maximal voluntary contraction/s) up to maximal voluntary level. Motor unit firing rates were studied as a function of force throughout the full range of muscle force output. The relationship between force and firing rate was found to contain three distinct regions. At recruitment and near maximal force levels, firing rates increased more rapidly with force than in the intermediate region. Furthermore, in the regions with rapid increases, the rate of change of firing rate was correlated to the recruitment threshold, with higher recruitment threshold motor units displaying greater rates of change. In the intermediate region, all motor units had similar rates of change of firing rate. A weak positive correlation was found between initial firing rate and recruitment threshold. Firing rates of motor units at any instant were found to be ordered according to the recruitment order: at any given time in the contraction motor units with lower recruitment thresholds had higher firing rates than units with higher recruitment thresholds. Firing rates of all motor units were observed to converge to the same value at maximal forces. Mechanisms underlying motor unit recruitment and firing rate modulation are discussed in the context of a conceptual model. © 1996 John Wiley & Sons, Inc.     

Efficient testing of motor function in spinal cord injured rats

In experimental spinal cord injury studies, animal models are widely used to examine anatomical and functional changes after different treatments and lesion types. A variety of behavioral paradigms exists in the literature, but definitions and criteria for motor performance vary considerably. In this study, we examined the outcome and relation of tests such as the BBB open field locomotion score, footprint analysis, kinematic analysis, placing response, grid walk and narrow beam crossing following two different lesion types. The information obtained was used to design an efficient and reliable testing strategy, which includes a broad spectrum of parameters to enhance sensitivity. This approach should help to standardize modular testing procedures across different laboratories working on spinal cord injury.     

Central motor conduction in cerebrovascular disease and motor neuron disease

Conduction in the central motor pathways was studied in 9 patients with cerebrovascular disease (CVD), 13 with amyotrophic lateral sclerosis (ALS) and 3 with spinal progressive muscular atrophy (SPMA). Motor responses evoked in the limb by cortical, cervical and lumbar stimulations were recorded. The central conduction time (CCT) was calculated for each muscle. In patients with CVD, responses to cortical stimulation were unobtainable or delayed in the paretic limb muscles. In patients with ALS the abnormality of central motor conduction had significant correlation with the extensor plantar response. The CCTs were normal in patients with SPMA. This technique demonstrated a subclinical lesion in some patients. We conclude that the new technique of examining motor conduction along the corticospinal tract may be useful to detect a subclinical lesion in the corticospinal tract.     

A novel method to attain sinusoidal mechanical responses from single motor units

Introduction: The relationship between output force and motor command depends on the intrinsic dynamic responses of motor units (MUs), which can be characterized by evoking accurate sinusoidal force responses at different frequencies. In this study we sought to determine whether sinusoidal modulation of the stimulation rate of single MUs results in reliable sinusoidal force changes. Methods: Single axons of rat ventral roots were stimulated electrically by changing the pulse rate sinusoidally at different frequency modulation (0.4–1.0–2.0–4.0 Hz for slow, 1.0–2.0–4.0–7.0 Hz for fast MUs). The twitching sinusoidal force signal was interpolated. We calculated harmonic distortion (HD) and the correlation coefficient (r) between theoretical sines and interpolated signals. Results: HD was always <5%, and r was always >0.97. Conclusions: The HD and r‐values obtained indicate highly reliable sinusoidal responses, which supports the potential use of this method to further characterize the dynamic behavior of single MUs. Muscle Nerve 51 : 134–136, 2015     

The pain-induced decrease in low-threshold motor unit discharge rate is not associated with the amount of increase in spike-triggered average torque.

OBJECTIVE: Activation of nociceptive afferents decreases motor unit discharge rates in static contractions. There is also evidence that during experimental muscle pain the motor unit twitch force increases, which has been hypothesized to compensate for the decrease in discharge rate to maintain constant force. This study examined whether there is an association between the magnitude of change in motor unit discharge rate and the amount of increase in the spike-triggered average torque during experimental muscle pain. METHODS: Sixteen subjects performed three constant-torque isometric ankle dorsi-flexions at 10% of the maximal force (MVC) alternated with two contractions at constant discharge rate of a target motor unit, before and following injection of 0.5 ml of hypertonic (painful) or isotonic (control) saline into the tibialis anterior muscle. RESULTS: The discharge rate of the target unit at 10% MVC decreased following injection of hypertonic saline (P<0.05; mean+/-SD, before: 9.9+/-1.3 pulses per second, pps; after injection: 8.9+/-1.0 pps). The peak of the spike-triggered average torque increased with pain (P<0.05; before: 0.56+/-0.55 mNm; during pain: 0.95+/-1.02 mNm) but the increase was not correlated with the decrease in discharge rate (R=0.08). Propagation velocity and action potential peak-to-peak amplitude did not change with pain. CONCLUSIONS: The pain-induced modifications in the estimated motor unit twitch torque (1) were not caused by changes in muscle fiber action potential, and (2) were not associated with the decrease in discharge rate. SIGNIFICANCE: Maintenance of constant force during static painful contractions is not explained by a matching between changes in contractile and control motor unit properties.     

Evaluation of epileptogenic networks in children with tuberous sclerosis complex using EEG-fMRI

PURPOSE: Ninety percent of patients with tuberous sclerosis complex (TSC) have epilepsy. Identification of epileptogenic areas can be difficult and studies are needed to characterize the epileptogenic network in more detail. METHODS: Five children with TSC and focal epilepsy were studied using simultaneous EEG and functional MRI recordings. Tubers were marked by a neuroradiologist on the anatomical MRI. Spike-associated BOLD (blood oxygenation level-dependent) responses were superimposed with lesions. RESULTS: Thirteen different types of interictal epileptiform discharges (IED) were analyzed with 12 showing a BOLD response, all involving more than one tuber. Five studies had tubers with activations exclusively within the lesion, three studies had lesional activations extending to perilesional areas, and two studies had activations involving exclusively perilesional areas of at least one tuber. Deactivations exclusively within a tuber were found in six studies, lesional deactivations extending to perilesional areas were found in four studies, and tubers with exclusively perilesional deactivations were found in five studies. A BOLD response was found in at least one tuber in the lobe of IED generation and presumed seizure onset (according to telemetry) in all patients. In four patients, the same tubers were involved following different IED localizations. The observed changes were always multifocal, sometimes involving tubers distant from the IED field. DISCUSSION: These findings suggest extended epileptogenic networks in patients with TSC, which exceed networks described in PET and SPECT studies. It was possible to identify specific interictally active tubers. EEG-fMRI provides a noninvasive method to select tubers and areas at their borders for further presurgical investigations.