Impairment of sensory-motor integration at spinal level in amyotrophic lateral sclerosis.

OBJECTIVE: Subclinical sensory defect can be detected early in ALS. Its impact on spinal excitability was assessed by testing the effects produced by intrinsic hand muscle afferents in triceps brachii motoneurons of patients with distal motor weakness. METHODS: TMS was applied over the motor cortex to produce MEP in contralateral triceps during tonic contraction. The intensity varied to compare the full MEP recruitment curve in ALS patients and controls. Then, median and ulnar nerve stimulations at wrist level were combined to TMS to compare the resulting changes in MEP size in both groups. RESULTS: MEP recruitment curves were similar in both groups but MEP threshold was significantly higher in ALS. At sub-threshold intensity for MEP, TMS depressed more EMG activity in ALS than in controls. Nerve stimuli increased MEP size in both groups with similar temporal characteristics but the level of facilitation was stronger in ALS. CONCLUSION: Cortical hypo-excitability in ALS was accompanied with stronger intra-cortical inhibition in triceps area. While the corticospinal and peripheral inputs were likely depressed, spinal motoneuron response to combined inputs was particularly enhanced in ALS. SIGNIFICANCE: Spinal network properties likely compensate for depression of afferent inputs leading to motoneuron hyper-excitability, which may contribute to excito-toxicity.

Electrophysiological and spinal imaging evidences for sensory dysfunction in amyotrophic lateral sclerosis.

OBJECTIVES: The prevalence of sensory impairment at an early stage of amyotrophic lateral sclerosis (ALS) is still debated. The study aim was to investigate the anatomofunctional properties of sensory pathways in patients with ALS, combining spinal diffusion tensor imaging (DTI) and somatosensory evoked potentials (SEPs). DESIGN: Case-control study. SETTINGS: ALS referral centre and laboratory of biomedical imaging (Paris, France). PARTICIPANTS: Well-characterised group of 21 patients with ALS with moderate disability (mean amyotrophic lateral sclerosis Functional Rating Scale (ALSFRS) score 39.3±1.0) and no clinical sensory signs and control group of 21 gender and age-matched healthy subjects. OUTCOME MEASURES: Fractional anisotropy and diffusivity of the dorsal columns at C5-T1 levels (DTI metrics) and SEPs after median and ulnar nerve stimulations (latency and amplitude of N9 and N20 components). RESULTS: Abnormal DTI metrics indicated anatomical damages of ascending sensory fibres in ∼60% of patients (p<0.05). Raw SEPs (µV) were smaller in ∼40% of patients but the difference with healthy subjects was not significant (p>0.16). Their normalisation to prestimulus activity strengthened the difference between groups (p<0.05) and allowed identification of ∼60% of patients with abnormal values. According to N9 latency, the peripheral conduction time was normal in patients (p>0.32) but based on N20 latency, the central conduction time (between spinal cord and parietal cortex) was found to be slower (p<0.05). Significant correlation was found between DTI metrics and N9 amplitude (p<0.05). Altered SEPs were also correlated with the disease duration (p<0.05). Taken together, spinal imaging and electrophysiology helped to identify ∼85% of patients with subclinical sensory defect while separated methods revealed abnormal values in ∼60%. CONCLUSIONS: Sensory impairments have been underestimated at early stages of ALS. These results show for the first time the interest to combine electrophysiology and imaging to assess non-motor system involvement in ALS. TRIAL REGISTRATION NUMBER: IDRCB2012-A00016-37.

Multi-parametric spinal cord MRI as potential progression marker in amyotrophic lateral sclerosis.

OBJECTIVE: To evaluate multimodal MRI of the spinal cord in predicting disease progression and one-year clinical status in amyotrophic lateral sclerosis (ALS) patients. MATERIALS AND METHODS: After a first MRI (MRI1), 29 ALS patients were clinically followed during 12 months; 14/29 patients underwent a second MRI (MRI2) at 11±3 months. Cross-sectional area (CSA) that has been shown to be a marker of lower motor neuron degeneration was measured in cervical and upper thoracic spinal cord from T2-weighted images. Fractional anisotropy (FA), axial/radial/mean diffusivities (λ⊥, λ//, MD) and magnetization transfer ratio (MTR) were measured within the lateral corticospinal tract in the cervical region. Imaging metrics were compared with clinical scales: Revised ALS Functional Rating Scale (ALSFRS-R) and manual muscle testing (MMT) score. RESULTS: At MRI1, CSA correlated significantly (P<0.05) with MMT and arm ALSFRS-R scores. FA correlated significantly with leg ALFSRS-R scores. One year after MRI1, CSA predicted (P<0.01) arm ALSFSR-R subscore and FA predicted (P<0.01) leg ALSFRS-R subscore. From MRI1 to MRI2, significant changes (P<0.01) were detected for CSA and MTR. CSA rate of change (i.e. atrophy) highly correlated (P<0.01) with arm ALSFRS-R and arm MMT subscores rate of change. CONCLUSION: Atrophy and DTI metrics predicted ALS disease progression. Cord atrophy was a better biomarker of disease progression than diffusion and MTR. Our study suggests that multimodal MRI could provide surrogate markers of ALS that may help monitoring the effect of disease-modifying drugs.

Adult spinal motoneurones are not hyperexcitable in a mouse model of inherited amyotrophic lateral sclerosis.

In amyotrophic lateral sclerosis (ALS), an adult onset disease in which there is progressive degeneration of motoneurones, it has been suggested that an intrinsic hyperexcitability of motoneurones (i.e. an increase in their firing rates), contributes to excitotoxicity and to disease onset. Here we show that there is no such intrinsic hyperexcitability in spinal motoneurones. Our studies were carried out in an adult mouse model of ALS with a mutated form of superoxide dismutase 1 around the time of the first muscle fibre denervations. We showed that the recruitment current, the voltage threshold for spiking and the frequency-intensity gain in the primary range are all unchanged in most spinal motoneurones, despite an increased input conductance. On its own, increased input conductance would decrease excitability, but the homeostasis for excitability is maintained due to an upregulation of a depolarizing current that is activated just below the spiking threshold. However, this homeostasis failed in a substantial fraction of motoneurones, which became hypoexcitable and unable to produce sustained firing in response to ramps of current. We found similar results both in lumbar motoneurones recorded in anaesthetized mice, and in sacrocaudal motoneurones recorded in vitro, indicating that the lack of hyperexcitability is not caused by anaesthetics. Our results suggest that, if excitotoxicity is indeed a mechanism leading to degeneration in ALS, it is not caused by the intrinsic electrical properties of motoneurones but by extrinsic factors such as excessive synaptic excitation.

Weak motor cortex contribution to the quadriceps activity during human walking.

Cortical and sub-cortical contribution to the basic locomotor rhythm is still unclear in humans. While motor cortex is involved in the ankle muscle activity during walking, recent findings suggest lesser contribution to that of knee extensors. This was further tested during treadmill walking (3-4 km/h; end swing and early stance) using transcranial magnetic stimulation (TMS). Sub-threshold TMS successively suppressed and increased Vastus Lateralis (VL) EMG activity during tonic contraction while standing, and both responses were significantly depressed during walking. Paired pulse TMS produced weak intra-cortical inhibition during tonic VL contraction, which did not change during walking. Lastly, sub-threshold TMS did not produce any change in VL H-reflex during walking. It is shown that the excitability of pathways, mediating short intra-cortical inhibition and facilitation in VL motor area, is particularly depressed during walking compared to tonic contraction. The present study thus reveals different modulation in VL than that reported in ankle muscles, suggesting lesser cortical contribution to its activity during walking.

Mixed mode oscillations in mouse spinal motoneurons arise from a low excitability state.

We explain the mechanism that elicits the mixed mode oscillations (MMOs) and the subprimary firing range that we recently discovered in mouse spinal motoneurons. In this firing regime, high-frequency subthreshold oscillations appear a few millivolts below the spike voltage threshold and precede the firing of a full blown spike. By combining intracellular recordings in vivo (including dynamic clamp experiments) in mouse spinal motoneurons and modeling, we show that the subthreshold oscillations are due to the spike currents and that MMOs appear each time the membrane is in a low excitability state. Slow kinetic processes largely contribute to this low excitability. The clockwise hysteresis in the I-F relationship, frequently observed in mouse motoneurons, is mainly due to a substantial slow inactivation of the sodium current. As a consequence, less sodium current is available for spiking. This explains why a large subprimary range with numerous oscillations is present in motoneurons displaying a clockwise hysteresis. In motoneurons whose I-F curve exhibits a counterclockwise hysteresis, it is likely that the slow inactivation operates on a shorter time scale and is substantially reduced by the de-inactivating effect of the afterhyperpolarization (AHP) current, thus resulting in a more excitable state. This accounts for the short subprimary firing range with only a few MMOs seen in these motoneurons. Our study reveals a new role for the AHP current that sets the membrane excitability level by counteracting the slow inactivation of the sodium current and allows or precludes the appearance of MMOs.

Fast kinetics, high-frequency oscillations, and subprimary firing range in adult mouse spinal motoneurons.

The fast contraction time of mouse motor units creates a unique situation in which motoneurons have to fire at low frequencies to produce small forces but also at very high frequency (much higher than in cat or rat motoneurons) to reach the fusion frequency of their motor units. To understand how this problem is solved, we performed intracellular recordings of adult mouse spinal motoneurons and investigated systematically their subthreshold properties and their discharge pattern. We show that mouse motoneurons have a much wider range of firing frequencies than cat and rat motoneurons because of three salient features. First, they have a short membrane time constant. This results in a higher cutoff frequency and a higher resonance frequency, which allow mouse motoneurons to integrate inputs at higher frequencies. Second, their afterhyperpolarization (AHP) is faster, allowing the motoneurons to discharge at a higher rate. Third, motoneurons display high-frequency (100-150 Hz) subthreshold oscillations during the interspike intervals. The fast membrane kinetics greatly favors the appearance of these oscillations, creating a "subprimary range" of firing. In this range, which has never been reported in cat and in rat spinal motoneurons, the oscillations follow the AHP and trigger spiking after a variable delay, allowing a discharge at low frequency but at the expense of an irregular rate.

Corticospinal inhibition of transmission in propriospinal-like neurones during human walking.

It is crucial for human walking that muscles acting at different joints are optimally coordinated in relation to each other. This is ensured by interaction between spinal neuronal networks, sensory feedback and supraspinal control. Here we investigated the cortical control of spinal excitation from ankle dorsiflexor afferents to quadriceps motoneurones mediated by propriospinal-like interneurones. During walking and tonic contraction of ankle dorsiflexors and knee extensors while standing [at matched electromyography (EMG) levels], the effect of common peroneal nerve (CPN) stimulation on quadriceps motoneurones was tested by assessing averaged and rectified EMG activity, H-reflexes [evoked by femoral nerve (FN) stimulation] and motor evoked potentials (MEPs) produced by transcranial magnetic stimulation (TMS). The biphasic EMG facilitation (CPQ-reflex) produced by isolated CPN stimulation was enhanced during walking, and when CPN stimulation was combined with FN or TMS, the resulting H-reflexes and MEPs were inhibited. The CPQ-reflex was also depressed when CPN stimulation was combined with subthreshold TMS. The peripheral (in CPN and FN) and corticospinal volleys may activate inhibitory non-reciprocal group I interneurones, masking spinal excitations to quadriceps motoneurones mediated by propriospinal-like interneurones. It is proposed that the enhanced CPQ-reflex produced by isolated CPN stimulation during walking cannot be fully explained by an increase in corticospinal and peripheral inputs, but is more likely caused by central facilitation of the propriospinal-like interneurones from other sources. The corticospinal control of non-reciprocal group I interneurones may be of importance for reducing reflex activity between ankle dorsiflexors and quadriceps during walking when not functionally relevant.

Modulation of recurrent inhibition from knee extensors to ankle motoneurones during human walking.

The neural control for muscle coordination during human locomotion involves spinal and supraspinal networks, but little is known about the exact mechanisms implicated. The present study focused on modulation of heteronymous recurrent inhibition from knee extensors to ankle motoneurones at different times in the gait cycle, when quadriceps (Quad) muscle activity overlaps that in tibialis anterior (TA) and soleus (Sol). The effects of femoral nerve stimulation on ankle motoneurones were investigated during treadmill walking and during tonic co-contraction of Quad and TA/Sol while standing. Recurrent inhibition of TA motoneurones depended on the level of background EMG, and was similar during walking and standing for matched background EMG levels. On the other hand, recurrent inhibition in Sol was reduced in early stance, with respect to standing, and enhanced in late stance. Reduced inhibition in Sol was also observed when Quad was coactivated with TA around the time of heel contact, compared to standing at matched background EMG levels in the two muscles. The modulation of recurrent inhibition of Sol during walking might reflect central and/or peripheral control of the Renshaw cells. These modulations could be implicated in the transition phases, from swing to stance to assist Sol activation during the stance phase, and from stance to swing, for its deactivation.

Speed-related spinal excitation from ankle dorsiflexors to knee extensors during human walking.

Automatic adjustments of muscle activity throughout the body are required for the maintenance of balance during human walking. One mechanism that is likely to contribute to this control is the heteronymous spinal excitation between human ankle dorsiflexors and knee extensors (CPQ-reflex). Here, we investigated the CPQ-reflex at different walking speeds (1-6 km/h) and stride frequencies (0.6-1.3 Hz) in healthy human subjects to provide further evidence of its modulation, and its role in ensuring postural stability during walking. The CPQ-reflex was small or absent at walking speeds below 2-3 km/h, then increased with walking speeds about 3-4 km/h, and reached a plateau without any further change at walking speeds from 4 to 6 km/h. The reflex showed no modulation when the stride cycle was varied at constant speed (4 km/h; short steps versus long steps). These changes were unlikely to be only caused by changes in the background EMG activity and modifications in peripheral input, and likely reflected central modulation of transmission in the involved reflex pathways as well. It is suggested that the purpose of the reflex is to ensure knee stability at moderate-to-high walking speeds.

Properties of human spinal interneurones: normal and dystonic control.

The muscles that control wrist posture receive large inputs from reflexes driven by hand afferents. In several studies, we have investigated these reflexes by electrical stimulation of cutaneous (median nerve) and proprioceptive (ulnar nerve) afferents from the hand. Median stimulation produced short latency inhibition in all motor nuclei investigated, possibly through inhibitory propriospinal-like interneurones. Ulnar stimulation produced similar inhibition but only in wrist extensors. In the other motor nuclei, ulnar stimulation produced short latency excitation mediated by group I motoneuronal drive through both monosynaptic and non-monosynaptic pathways involving excitatory propriospinal-like interneurones. This was followed by late excitations mediated through spinal group II and trans-cortical group I pathways. These results show that these pathways are concerned with the integration of afferent inputs, proprioceptive and cutaneous, to control of wrist posture during hand movements. Patients with focal hand dystonia exhibit abnormal postures. To investigate whether these spinal pathways contribute to these conditions, the effects of ulnar stimulation on wrist muscle activity during voluntary tonic contraction were examined in patients who suffer writer's cramp. Ulnar-induced inhibition of the wrist extensors was reduced on the dystonic side of patients compared with their normal side and controls. In patients who exhibited abnormal wrist posture, group II excitation of the wrist flexors was also modified on the dystonic side. Cutaneous stimuli, by contrast, increased wrist flexor EMG on both sides and only in patients who exhibited normal posture. We conclude that spinal interneurones have a significant role in integrating afferent inputs from the hand to control wrist posture during hand movements and that altered function in these spinal networks is associated with the complex pathophysiology of writer's cramp.

Task-related changes in propriospinal excitation from hand muscles to human flexor carpi radialis motoneurones.

This study addresses whether there is excitation from human hand muscles to flexor carpi radialis (FCR) motoneurones mediated through propriospinal circuits and, if so, whether it is used in specific motor tasks. Electrical stimuli to the ulnar nerve at wrist level produced an excitation in FCR motoneurones with characteristics typical of a propriospinally mediated effect: low threshold (0.6 x motor threshold (MT)), a group I effect that was not reproduced by purely cutaneous stimuli, long central delay (4.1 +/- 0.4 ms in single units), suppression when the stimulus intensity was increased, and facilitation of the corticospinal excitation at the premotoneuronal level. Ulnar-induced propriospinally mediated excitation was compared during selective voluntary contractions of the FCR and, at equivalent level of FCR EMG, during tasks in which the FCR was activated automatically in postural contractions rather than voluntarily (grip, pinching and pointing). The excitation was significantly greater during grip (and pinching) than during voluntary FCR contractions and pointing, whether measured in single motor units or tonic EMG activity, or whether the response to motor cortex stimulation was assessed as the compound motor-evoked potential or the corticospinal peak in single units. The discrepancy between the tasks appeared with ulnar intensities above 0.8 x MT and was then present across a wide range of stimulus intensities. This suggests a reduction in the corticospinal control of 'feedback inhibitory interneurones' mediating peripheral inhibition to propriospinal neurones during grip and pinching. The resulting more effective background excitation of propriospinal neurones by the peripheral input from hand muscles could contribute to stabilizing the wrist during grip.

Mediation of late excitation from human hand muscles via parallel group II spinal and group I transcortical pathways.

This study addresses the question of the origin of the long-latency responses evoked in flexors in the forearm by afferents from human hand muscles. The effects of electrical stimuli to the ulnar nerve at wrist level were assessed in healthy subjects using post-stimulus time histograms for flexor digitorum superficialis and flexor carpi radialis (FCR) single motor units (eight subjects) and the modulation of the ongoing rectified FCR EMG (19 subjects). Ulnar stimulation evoked four successive peaks of heteronymous excitation that were not produced by purely cutaneous stimuli: a monosynaptic Ia excitation, a second group I excitation attributable to a propriospinally mediated effect, and two late peaks. The first long-latency excitation occurred 8-13 ms after monosynaptic latency and had a high-threshold (1.2-1.5 x motor threshold). When the conditioning stimulation was applied at a more distal site and when the ulnar nerve was cooled, the latency of this late excitation increased more than the latency of monosynaptic Ia excitation. This late response was not evoked in the contralateral FCR of one patient with bilateral corticospinal projections to FCR motoneurones. Finally, oral tizanidine suppressed the long-latency high-threshold excitation but not the early low-threshold group I responses. These results suggest that the late high-threshold response is mediated through a spinal pathway fed by muscle spindle group II afferents. The second long-latency excitation, less frequently observed (but probably underestimated), occurred 16-18 ms after monosynaptic latency, had a low threshold indicating a group I effect, and was not suppressed by tizanidine. It is suggested that this latest excitation involves a transcortical pathway.

Increase in group II excitation from ankle muscles to thigh motoneurones during human standing.

In standing subjects, we investigated the excitation of quadriceps (Q) motoneurones by muscle afferents from tibialis anterior (TA) and the excitation of semitendinosus (ST) motoneurones by muscle afferents from gastrocnemius medialis (GM). Standing with a backward lean stretches the anterior muscle pair (TA and Q) and they must be co-contracted to maintain balance. Equally, forward lean stretches the posterior muscle pair (GM and ST) and they must be co-contracted. We used these conditions of enhanced lean to increase the influence of gamma static motoneurones on muscle spindle afferents, which enhances the background input from these afferents to extrafusal motoneurones. The effects of the conditioning volleys on motoneurone excitability was estimated using the modulation of the on-going rectified EMG and of the H reflex. Stimulation of afferents from TA in the deep peroneal nerve at 1.5-2 x MT (motor threshold) evoked early group I and late group II excitation of Q motoneurones. Stimulation of afferents in the GM nerve at 1.3-1.8 MT evoked only late group II excitation of ST motoneurones. The late excitation produced by the group II afferents was significantly greater when subjects were standing and leaning than when they voluntarily co-contracted the same muscle pairs at the same levels of activation. The early effect produced by the group I afferents was unchanged. We propose that this increase in excitation by group II afferents reflects a posture-related withdrawal of a tonic inhibition that is exerted by descending noradrenergic control and is specific to the synaptic actions of group II afferents.