Motoneurone dysfunction in patients with hemiplegic atrophy

Adaptation in the spinal cord after stroke: Implications for restoring cortical control over the final common pathway

Control of voluntary movement is predicated on integration between circuits in the brain and spinal cord. Although damage is often restricted to supraspinal or spinal circuits in cases of neurological injury, both spinal motor neurons and axons linking these cells to the cortical origins of descending motor commands begin showing changes soon after the brain is injured by stroke. The concept of 'transneuronal degeneration' is not new and has been documented in histological, imaging and electrophysiological studies dating back over a century. Taken together, evidence from these studies comports more with a system attempting to survive rather than one passively surrendering to degeneration. There tends to be at least some preservation of fibres at the brainstem origin and along the spinal course of the descending white matter tracts, even in severe cases. Myelin-associated proteins are observed in the spinal cord years after stroke onset. Spinal motor neurons remain morphometrically unaltered. Skeletal muscle fibres once innervated by neurons that lose their source of trophic input receive collaterals from adjacent neurons, causing spinal motor units to consolidate and increase in size. Although some level of excitability within the distributed brain network mediating voluntary movement is needed to facilitate recovery, minimal structural connectivity between cortical and spinal motor neurons can support meaningful distal limb function. Restoring access to the final common pathway via the descending input that remains in the spinal cord therefore represents a viable target for directed plasticity, particularly in light of recent advances in rehabilitation medicine.

Gluteus Maximus Muscle Activation Characteristics During a Chair-Rise in Adults With Chronic Stroke

BACKGROUND AND PURPOSE

A successful chair-rise is an important indicator of functional independence post-stroke. Lower extremity electromyographic analyses provide a basis for muscle activation from which clinical intervention protocols may be derived. Gluteus maximus activation during the chair-rise has not been thoroughly researched in the chronic stroke population. This study investigated the magnitude and onset of gluteus maximus activation during the chair-rise comparing adults post-stroke with healthy controls.

METHODS

In this cross-sectional study, adults with chronic stroke (n = 12) and healthy controls (n = 12) completed 4 natural-speed chair-rise trials. Magnitude and onset of bilateral gluteus maximus activation were measured during the movement with secondary comparative data from biceps femoris and vastus lateralis muscles. Kinetic and kinematic measurements were used to quantify chair-rise phases and movement cycle duration.

RESULTS

Significant decreases in paretic ( P = 0.002), and nonparetic ( P = 0.001) gluteus maximus magnitudes were noted post-stroke compared with ipsilateral extremities of healthy adults. Significant gluteus maximus onset delays were noted in paretic extremities compared with nonparetic extremities post-stroke ( P = 0.009) that were not apparent in comparative muscles. Similar onset times were noted when comparing the paretic extremity post-stroke to the ipsilateral extremity of healthy controls ( P = 0.714) despite prolonged movement cycle durations in those with chronic stroke ( P = 0.001). No onset delays were evident in the biceps femoris ( P = 0.72) or vastus lateralis ( P = 0.338) muscles.

DISCUSSION AND CONCLUSIONS

Despite apparent unilateral muscle weakness post-stroke, bilateral decreases in gluteus maximus activation magnitudes and compounding onset deficits of the paretic extremity were observed during chair-rising. Further research is needed to determine whether interventions maximizing bilateral activation magnitudes and improving temporal activation congruency during chair-rising will carry over to functional gainsVideo Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A387 ).

Corticospinal recruitment of spinal motor neurons in human stroke survivors

KEY POINTS

Muscle weakness after stroke results from damage to corticospinal fibres that structurally and functionally connect cerebral cortex to the spinal cord. Here, we show an asymmetry in corticospinal recruitment of spinal motor neurons that is linked to maximal voluntary output of hand muscles weakened by stroke. Spike timing-dependent plasticity of synapses between corticospinal and spinal motor neurons transiently reversed recruitment failures in some survivors. These modulatory effects were strongly associated with recruitment asymmetry and hand impairment. Our findings highlight the functional relevance of spinal motor neuron recruitment by corticospinal inputs and the viability of corticospinal motor neuronal synapses for restoring activation of lower motor neurons after stroke.

ABSTRACT

Corticospinal input to spinal motor neurons is structurally and functionally altered by hemiparetic stroke. The pattern and extent to which corticospinal recruitment of spinal motor neurons is reorganized and whether such changes are linked to the severity of motor impairments is not well understood. Here, we performed experiments using the triple stimulation technique to quantify corticospinal recruitment of spinal motor neurons serving paretic and non-paretic intrinsic hand muscles of humans with longstanding motor impairment secondary to stroke (n = 13). We also examined whether recruitment failures could be transiently reversed by strengthening corticospinal-motoneuronal synaptic connectivity via targeted, temporally controlled non-invasive stimulation to elicit spike timing-dependent plasticity (STDP). Asymmetries were detected in corticospinal recruitment of spinal motor neurons, central conduction time and motor-evoked potential (MEP) latency. However, only recruitment asymmetry correlated with maximal voluntary motor output from the paretic hand. STDP-like effects were observed as an increase in spinal motor neuron recruitment. Control experiments to isolate the locus of plasticity demonstrated a modulation in MEPs elicited by electrical stimulation of primary motor cortex but not F-wave size or persistence, suggesting that plasticity was mediated through enhanced efficacy of residual corticospinal-motor neuronal synapses. The modulation in recruitment was strongly associated with baseline recruitment asymmetry and impairment severity. Our findings demonstrate that asymmetry in corticospinal recruitment of spinal motor neurons is directly related to impairments experienced by stroke survivors. These recruitment deficits may be partially and transiently reversed by spike timing-dependent plasticity of synapses between upper and lower motor neurons in the spinal cord, downstream of supraspinal circuits damaged by stroke.

Peripheral axonal excitability in hemiplegia related to subacute stroke

Background/aim

This study aims to investigate peripheral nerve excitability in patients with subacute stroke.

Materials and methods

The study was performed in 29 stroke patients within the subacute period and 29 healthy controls using QTRAC software and TRONDNF protocol. The threshold electrotonus, recovery cycle, stimulus-response, strength-duration, and current-threshold relationships were recorded.

Results

The membrane was more hyperpolarized, and excitability was decreased in the hemiplegic side. The impairment of inward rectifying channel function, degree of hyperpolarization, and decrease of excitability were directly related to the Brunnstrom stages, which were more pronounced in lower stages.

Conclusion

The lower motor neurons were affected at the level of axonal channels as a result of upper motor neuron lesions. It can be due to dying back neuropathy, homeostasis, and neurovascular regulation changes in the axonal environment, activity-dependent plastic changes, loss of drive coming from the central nervous system, or a combination of these factors.

Motor Unit Properties of the First Dorsal Interosseous in Chronic Stroke Subjects: Concentric Needle and Single Fiber EMG Analysis

The purpose of this study was to better understand changes in motor unit electrophysiological properties in people with chronic stroke based on concentric needle electromyography (EMG) and single fiber EMG recordings. The first dorsal interosseous (FDI) muscle was studied bilaterally in eleven hemiparetic stroke subjects. A significant increase in mean fiber density (FD) was found in the paretic muscle compared with the contralateral side based on single fiber EMG (1.6 ± 0.2 vs. 1.3 ± 0.1, respectively, P = 0.003). There was no statistically significant difference between the paretic and contralateral sides in most concentric needle motor unit action potential (MUAP) parameters, such as amplitude (768.7 ± 441.7 vs. 855.0 ± 289.9 μV), duration (8.9 ± 1.8 vs. 8.68 ± 0.9 ms) and size index (1.2 ± 0.5 vs. 1.1 ± 0.3) (P > 0.18), nor was there a significant difference in single fiber EMG recorded jitter (37.0 ± 9.6 vs. 39.9 ± 10.6 μs, P = 0.45). The increase in FD suggests motor units of the paretic FDI have enlarged due to collateral reinnervation. However, sprouting might be insufficient to result in a statistically significant change in the concentric needle MUAP parameters. Single fiber EMG appears more sensitive than concentric needle EMG to reflect electrophysiological changes in motor units after stroke. Both single fiber and concentric needle EMG recordings may be necessary to better understand muscle changes after stroke, which is important for development of appropriate rehabilitation strategies. The results provide further evidence that motor units are remodeled after stroke, possibly in response to a loss of motoneurons.

Analysis of linear electrode array EMG for assessment of hemiparetic biceps brachii muscles

This study presents a frequency analysis of surface electromyogram (EMG) signals acquired by a linear electrode array from the biceps brachii muscles bilaterally in 14 hemiparetic stroke subjects. For different levels of isometric contraction ranging from 10 to 80% of the maximum voluntary contraction (MVC), the power spectra of 19 bipolar surface EMG channels arranged proximally to distally along the muscle fibers were examined in both paretic and contralateral muscles. It was found that across all stroke subjects, the median frequency (MF) and the mean power frequency (MPF), averaged from different surface EMG channels, were significantly smaller in the paretic muscle compared to the contralateral muscle at each of the matched percent MVC contractions. The muscle fiber conduction velocity (MFCV) was significantly slower in the paretic muscle than in the contralateral muscle. No significant correlation between the averaged MF, MPF, or MFCV vs. torque was found in both paretic and contralateral muscles. However, there was a significant positive correlation between the global MFCV and MF. Examination of individual EMG channels showed that electrodes closest to the estimated muscle innervation zones produced surface EMG signals with significantly higher MF and MPF than more proximal or distal locations in both paretic and contralateral sides. These findings suggest complex central and peripheral neuromuscular alterations (such as selective loss of large motor units, disordered control of motor units, increased motor unit synchronization, and atrophy of muscle fibers, etc.) which can collectively influence the surface EMG signals. The frequency difference with regard to the innervation zone also confirms the relevance of electrode position in surface EMG analysis.

Application of the F-Response for Estimating Motor Unit Number and Amplitude Distribution in Hand Muscles of Stroke Survivors

The F-response was used in this study to assess changes in the first dorsal interosseous (FDI) muscle after a hemispheric stroke. The number of motor units and their sizes were estimated bilaterally in 12 stroke survivors by recording both the compound muscle action potential (CMAP) and F wave responses. These F waves were induced by applying a large number of electrical stimuli to the ulnar nerve. The amplitude distribution of individual motor unit action potentials (MUAPs) was also compared between paretic and contralateral muscles. When averaged across all the subjects, a significantly lower motor unit number estimate was obtained for the paretic FDI muscle ( 88 ±13) compared with the contralateral side ( 139 ±11) ( ). Pooled surface MUAP amplitude analysis demonstrated a right-skewed distribution for both paretic (kurtosis 3.0) and contralateral (kurtosis 8.52) muscles. When normalized to each individual muscle's CMAP, the surface MUAP amplitude ranged from 0.22% to 4.94% (median 1.17%) of CMAP amplitude for the paretic muscle, and from 0.13% to 3.2% (median 0.62%) of CMAP amplitude for the contralateral muscle. A significant difference in MUAP outliers was also observed between the paretic and contralateral muscles. The findings of this study suggest significant motor unit loss and muscle structural reorganization after stroke.

Examination of Poststroke Alteration in Motor Unit Firing Behavior Using High-Density Surface EMG Decomposition

Recent advances in high-density surface electromyogram (EMG) decomposition have made it a feasible task to discriminate single motor unit activity from surface EMG interference patterns, thus providing a noninvasive approach for examination of motor unit control properties. In the current study, we applied high-density surface EMG recording and decomposition techniques to assess motor unit firing behavior alterations poststroke. Surface EMG signals were collected using a 64-channel 2-D electrode array from the paretic and contralateral first dorsal interosseous (FDI) muscles of nine hemiparetic stroke subjects at different isometric discrete contraction levels between 2 to 10 N with a 2 N increment step. Motor unit firing rates were extracted through decomposition of the high-density surface EMG signals and compared between paretic and contralateral muscles. Across the nine tested subjects, paretic FDI muscles showed decreased motor unit firing rates compared with contralateral muscles at different contraction levels. Regression analysis indicated a linear relation between the mean motor unit firing rate and the muscle contraction level for both paretic and contralateral muscles (p < 0.001), with the former demonstrating a lower increment rate (0.32 pulses per second (pps)/N) compared with the latter (0.67 pps/N). The coefficient of variation (averaged over the contraction levels) of the motor unit firing rates for the paretic muscles (0.21 ± 0.012) was significantly higher than for the contralateral muscles (0.17 ± 0.014) (p < 0.05). This study provides direct evidence of motor unit firing behavior alterations poststroke using surface EMG, which can be an important factor contributing to hemiparetic muscle weakness.

Motor unit rate coding is severely impaired during forceful and fast muscular contractions in individuals post stroke

Information regarding how motor units are controlled to produce forces in individuals with stroke and the mechanisms behind muscle weakness and movement slowness can potentially inform rehabilitation strategies. The purpose of this study was to describe the rate coding mechanism in individuals poststroke during both constant (n = 8) and rapid (n = 4) force production tasks. Isometric ankle dorsiflexion force, motor unit action potentials, and surface electromyography were recorded from the paretic and nonparetic tibialis anterior. In the paretic limb, strength was 38% less and the rate of force development was 63% slower. Linear regression was used to describe and compare the relationships between motor unit and electromyogram (EMG) measures and force. During constant force contractions up to 80% maximal voluntary contraction (MVC), rate coding was compressed and discharge rates were lower in the paretic limb. During rapid muscle contractions up to 90% MVC, the first interspike interval was prolonged and the rate of EMG rise was less in the paretic limb. Future rehabilitation strategies for individuals with stroke could focus on regaining these specific aspects of motor unit rate coding and neuromuscular activation.

Reduction in the motor unit number estimate (MUNE) after cerebral infarction

We examined the relationship between the degree to which motor unit number estimates (MUNEs) decrease in association with the clinical features of patients with the infarction. Using a multiple-point stimulation technique, we obtained the MUNE of the hypothenar muscle group in 13 age-matched control subjects and 30 patients with cerebral infarction. In all patients, we obtained the Japan Stroke Scale (JSS) and head MR images. In 8 patients with acute cerebral infarction, admitted within 24 h after onset, we also obtained head MR angiograms and single-photon emission CT. There was a decrease in the MUNE of the hypothenar muscle group on the affected side of 24 patients with cerebral infarction and hand weakness. The decrease in the MUNE started from 4 to 30 h after the infarction, when T1-weighted MR images of the brain involved were normal. The degree to which the MUNE decreased correlated with the part of the JSS showing the upper extremity weakness. A decrease in the MUNE of the hypothenar muscle group within 30 h after cerebral infarction may be due to transsynaptic inhibition of the spinal alpha motor neurons innervating this muscle.

The correlation between F-wave motor unit number estimation (F-MUNE) and functional recovery in stroke patients

The aim of this study was to follow up the changes in the number of motor units according to the Brunnstrom stage through a motor unit number estimation of the Fwave (F-MUNE) after a stroke, and to identify the functional significance of F-MUNE. Twenty-five patients (15 men, 10 women) with a first unilateral stroke were recruited. The maximal M-potential was evoked by the supramaximal stimulation of the median nerve at the wrist, and the maximal stimulation intensity was determined on both hemiplegic and unaffected hands. The reproducible all-or-none F-wave was evoked in 30% of the maximal stimulation intensity and was constantly stimulated at that level. The prototypes of the F-wave were chosen, and the values of F-MUNE were calculated by dividing the amplitude of the maximal M-potential by the mean amplitude of the F-prototype. The changes in F-MUNE were compared according to the progression of the Brunnstrom stage and correlated with those of the functional scales. The mean motor unit numbers decreased significantly in the hemiplegic side compared with the unaffected side. According to the progression of the Brunnstrom stage, the values of F-MUNE were reduced significantly by increasing the amplitude and recruitment of the F-prototype, and the functional scores also improved. These results show that the F-MUNE equation did not show a functional recovery related increase in stroke patients.

Reduction in the motor unit number estimate (MUNE) after cerebral infarction

BACKGROUND

The mechanism of the decrease in motor unit number estimates (MUNEs) after cerebral infarction has not been studied systematically. We examined the relationship between the degree to which MUNEs decreased and the other clinical features of patients with the infarction.

METHODS

Using a multiple point stimulation technique, we obtained the MUNE of the hypothenar muscle group in 13 age-matched control subjects and 30 patients with cerebral infarction. In all patients, we obtained the Japan Stroke Scale (JSS) and head MR images. In eight patients with acute cerebral infarction, admitted within 24 h after onset, we also obtained head MR angiograms and single-photon emission CT.

FINDINGS

There was a decrease in the MUNE of the hypothenar muscle group on the affected side of 24 patients with cerebral infarction and hand weakness. The decrease in the MUNE started from 4 to 30 h after the infarction, when T1-weighted MR images of the brain involved were normal. The degree to which the MUNE decreased correlated with the part of the JSS showing the upper extremity weakness.

INTERPRETATIONS

A decrease in the MUNE of the hypothenar muscle group within 30 h after cerebral infarction may be due to trans-synaptic inhibition of the spinal alpha motor neurons innervating this muscle.

The physiological functional loss of single thenar motor units in the stroke patients: when does it occur? Does it progress?

OBJECTIVE

We examined the time at which loss of functioning motor units occurs on the hemiparetic side, the relationship between that loss and hemiparetic severity, and how long that loss continues.

METHODS

Sample surface motor unit action potentials (S-MUAPs) were evoked in F-waves. They entirely represent the activity of the relative numbers of different shape S-MUAPs for each abductor pollicis brevis muscle. S-MUAPs from selected population of F-waves were averaged after aligning onset latency. Motor unit number was obtained by dividing the maximum M-potential negative peak amplitude by the averaged S-MUAP one.

RESULTS

The motor unit number on the hemiparetic side was significantly lower than that on the unaffected side in stroke patients who had suffered hemiparesis for more than 9 days. This motor unit loss was greater in patients with severe hemiparesis. One year after onset, the chronic stroke patients showed the same motor unit loss on hemiparetic side as they had 3-4 months after onset.

CONCLUSIONS

Motor unit loss on the hemiparetic side is present as early as the second week after onset and is correlated with hemiparesis severity, and this loss continues out to 1 year. This may be due to trans-synaptic degeneration that occurs secondarily to upper motor neuron lesion.

Physiologic decrease of single thenar motor units in the F-response in stroke patients

OBJECTIVE

To investigate the left-right difference and the reproducibility by the F-wave motor unit number estimation and to compare the motor unit number between the hemiplegic and unaffected side in stroke patients.

SETTING

A referral center and institutional practice providing outpatient care.

SUBJECTS

Seven healthy volunteers and 15 consecutive stroke patients.

DESIGN

Diagnostic statistical test and correlational study.

METHOD

Submaximal stimuli were used to evoke a sample of surface motor unit action potentials (S-MUAPs) in the F-waves that are entirely representative of the relative numbers of detected S-MUAPs of different sizes. The average S-MUAP amplitude was calculated from a selected population of F-wave responses for each abductor pollicis brevis (APB) muscle. The motor unit number was calculated by dividing the maximum M-potential negative peak amplitude by the average S-MUAP negative peak amplitude.

RESULT

There was no statistical difference between motor unit numbers on either side and between test and retest in this motor unit number estimation method among normal subjects. The motor unit number on the hemiplegic side was significantly lower than on the unaffected side (p < .05, Mann-Whitney test) among stroke patients.

CONCLUSION

The motor unit could decrease in the hemiplegic side after a moderate-to-severe hemiplegic stroke and this decrement might be due to the transsynaptic degeneration secondary to an upper motor neuron lesion.

Upper motor neuron lesions in stroke patients do not induce anterograde transneuronal degeneration in spinal anterior horn cells

BACKGROUND AND PURPOSE

To determine whether upper motor neuron lesions in stroke can cause transneuronal degeneration of lower motor neurons, we assessed spinal anterior horn cells in patients dying with poststroke hemiplegia.

METHODS

Subjects were four stroke patients with severe left hemiplegia and four age-matched control subjects who died of nonneurological disease. After histological processing and staining, cytoarchitectonic assessment was made of all neurons in the ventral horns of the 4th lumbar segment of the spinal cord according to cell diameter and topography.

RESULTS

In the four stroke patients, no differences were seen in anterior horn cell populations or diameter and size distribution patterns between affected and unaffected sides or between these patients and the control subjects.

CONCLUSIONS

The present quantitative analysis provides no evidence of anterograde transneuronal degeneration of lower motor neurons after upper motor neuron damage in stroke patients.

Invited review: motor unit estimation: methods, results, and present status

The renewed interest in motor unit estimation (counting) has coincided with the introduction of computer-based methodology and with the application of the technique to proximal as well as distal muscles. The advantages and disadvantages of the different methods are considered, together with the assumptions inherent in this type of examination. In normal subjects, the extensor digitorum brevis (EDB) muscle has approximately 200 motor units while each of the intrinsic muscles of the hand has about 100 units; larger muscles in the limbs contain greater numbers of units. Beyond the age of 60 years, there is a decline in the number of functioning motor units in both proximal and distal muscles. In denervating disorders, motor unit estimation is useful for diagnosis and assessment; abnormal values may often be observed in muscles judged clinically to be unaffected. Serial studies have enabled the rate of motor unit loss to be determined in ALS and in spinal muscular atrophy. Depletion of motor units has also been found following upper motoneuron lesions caused by injury to the spinal cord or by cerebral hemorrhage; trans-synaptic dysfunction has been presumed responsible. Rather surprisingly, reduced numbers of motor units have been observed in a variety of myopathic disorders; of these, the most consistent abnormalities have been reported in myotonic muscular dystrophy.

An investigation of possible transynaptic neuronal degeneration in human spinal cord injury

Neurophysiological studies suggested that transynaptic neuronal degeneration of the anterior horn cells (AHC) may occur after an upper motoneuron lesion as the result of "deafferentation". To test this observation anatomically, patients with spinal cord injury (SCI) who had come to post mortem were investigated. Four patients with longstanding clinically and pathologically "complete" SCI were selected for comparison with 4 age-matched normal controls and with 2 patients who died of motoneuron disease (MND). The total number of AHCs in the L3 spinal cord segment was counted in each of the cases. The lesions in the traumatic group were all above the L3 segment. No significant differences in the number of AHC between the test cases and the normal controls was found. There was, as expected, a highly significant difference between the test cases and those with MND. The conclusion drawn from the study is that transynaptic neuronal degeneration of AHCs does not occur following complete transection of the human spinal cord. Thus the neurophysiological hypothesis is not supported anatomically.

Recent views on amyotrophic lateral sclerosis with emphasis on electrophysiological studies

Peripheral electrophysiological studies are of particular value of elucidating the anatomy and pathophysiology of neuromuscular diseases, but they can also help in providing clues to the etiology of the disease. Recent studies of the motor units in chronic denervating conditions including amyotrophic lateral sclerosis (ALS) are reviewed. These indicate that reinnervation is a relatively active process which compensates for the progressive loss of motoneurons in ALS until more than 50% of the motoneurons have died. There seems to be no predilection for death of motoneurons of any particular size in ALS. Fasciculations may arise both proximally and distally. The dying-back change is not a major feature of ALS. These and other data cast doubt on the etiological theories that ALS arises from premature aging of motoneurons, deficiency of motoneuron trophic factors, or an inhibitor of a motoneuronal sprouting factor, and point to the need to study metabolic changes intrinsic to the motoneuron in ALS.

Asymmetric function of peripheral nerves in children with cerebral palsy

Nerve conduction velocities (NCVs) in both motor and sensory nerves as well as nerve action potentials (NAPs) of sensory nerves were measured bilaterally in 24 children with cerebral palsy. The NAP amplitude and both sensory and motor NCV were on the average higher on the intact or less affected side. The NCV side difference was still present after the temperature asymmetry of the limbs had been taken into account by calculating the temperature-corrected NCV values, and was statistically significant for motor NCV in N. peroneus and for sensory NCV in N. suralis; this could neither be explained on the basis of spasticity nor by the length difference of the extremities. No correlation of NCV asymmetry with the degree of atrophy seemed to exist.

Muscle fiber type morphology and distribution in paraplegic patients with traumatic cord lesion. Histochemical and ultrastructural aspects of rectus femoris muscle

Biopsies of the rectus femoris muscle of 22 paraplegic patients with complete acute spinal cord transection due to trauma were taken for enzyme-histochemical and electron-microscopic studies in successive stages starting from occurrence of the accident (1-17 months). Ingravescent muscular atrophy was demonstrated with a progressive decrease in the fiber diameter and changes in the fiber type distribution with predominant type II atrophy in the first stage and type I atrophy in the later stage of the cord transection. Muscular "neurogenic" changes, such as angular dark atropic fibers, targetoid fibers, and type predominance are frequently observed. Myopathic alterations are observed in a low percentage in the later stages of the lesion. The ultrastructural findings are characterized by myofibrillar alterations and by dilatation and proliferative phenomena of the sarcoplasmic reticulum and T-system. There are ingravescent accumulation of lipid, interstitial fibrosis and microcirculatory alterations. The possible mechanism of "central" muscle atrophy is reviewed and discussed with reference to the morphological findings.

Axonal excitability and motor propagation velocity of peripheral nerves in patients with acute vascular lesions of the brain

From measurements of maximum and minimum motor nerve propagation velocity and neuronal excitability we conclude that there is a functional loss of motor units and distal nerve "dying back" in persons affected with unilateral acute cerebral vascular lesions. The study also suggests that transynaptic degeneration affects the lower motor neurone function on both sides.

A new method for the estimation of the number of motor units in a muscle. I. Control subjects and patients with myasthenia gravis

A new method, incorporating on-line computer analysis, is described for the estimation of the numbers of motor units in human muscle. The results obtained in the extensor digitorum brevis muscle in normal subjects and patients with myasthenia gravis are presented. These indicate that the numbers of motor units in that muscle in patients with myasthenia gravis are within the normal range, in contrast with the reduction in numbers reported by other workers using a different technique. Evidence is presented to suggest that the discrepancy in these results is due to increased sensitivity and discrimination of the computerized method. Several hypotheses on the aetiology of a number of neuromuscular diseases, based on the results of the other method, may require reevaluation.

Functional changes in motoneurones of hemiparetic patients

Forty-six patients have been studied after upper motor neurone lesions of cerebrovascular origin. The numbers of functioning motor units in extensor digitorum brevis muscles were reduced to approximately half between the second and sixth months after a hemiplegic episode. The surviving motor units tended to have slow twitches and appeared to increase their sizes after the lesions had been present for about 20 months. The findings are explained on the basis of transsynaptic changes in alpha-motoneurones after degeneration of corticospinal fibres.