Reduced corticomotoneuronal excitatory postsynaptic potentials (EPSPs) with normal Ia afferent EPSPs in amyotrophic lateral sclerosis

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.

Neurophysiological measures in amyotrophic lateral sclerosis: Markers of progression in clinical trials

In this review we evaluate clinical neurophysiological methods, originally described for use in diagnosis that can be applied to measurement of change during the progress of amyotrophic lateral sclerosis (ALS). Such measurements are potentially important in clinical trials, and also in clinical practice. We have assessed methods for lower and upper motor neuron function, including conventional EMG, nerve conduction and F-wave studies, the derived Neurophysiological Index, motor unit counting methods (MUNE), and transcranial magnetic motor cortex stimulation. We have also addressed the validity of measurements of electromechanical coupling. Methods for measuring muscle strength are beyond the scope of this review. We conclude that MUNE, M-wave amplitude and the Neurophysiological Index are sufficiently reliable, sensitive, and relevant to the clinical problem of ALS, to be used in clinical trials in the disease. Transcranial magnetic stimulation is of limited value, but a combination of the measurements made as part of this technique may also be useful. We conclude that clinical neurophysiological techniques should now be used in measuring change in clinical trials in ALS.

Changes in cortically induced inhibition in amyotrophic lateral sclerosis with time

Changes in intracortical inhibition have been detected by means of paired-pulse transcranial magnetic stimulation (TMS) based on electromyographic recordings in many neurological or psychiatric disorders, including amyotrophic lateral sclerosis (ALS). By contrast, inhibitory responses have been generally overlooked in single motor unit (MU) studies in patients with ALS. The aim of this study was to investigate the TMS-induced inhibitory responses of single MUs in peristimulus time histograms and the changes observed with time. For this purpose, 263 MUs were tested in 10 ALS patients in two to four recording sessions. Upon subdividing the data into epochs corresponding to mean disease durations of 12, 20, 32, 43, and 168 months, we found that inhibitory responses occurred more frequently than normal throughout the course of the disease and were stronger than normal during the first year after disease onset. This finding argues against the hypothesis that loss of inhibition may be part of the pathogenic process in ALS.

Corticomotoneuronal dysfunction in ALS patients with different SOD1 mutations

OBJECTIVE

To examine corticomotoneuronal function in amyotrophic lateral sclerosis (ALS) patients carrying superoxide dismutase 1 (SOD1) mutations using peristimulus time histograms (PSTH).

METHODS

Six I113T, 3 A4V, one G41D and one G114A patient were studied along with 21 healthy control subjects. Analyses included comparison with previously reported data from 8 D90A homozygous and 12 sporadic ALS (SALS) patients examined by the authors using identical methodology.

RESULTS

Cortical threshold was significantly reduced in A4V patients (41.3%) compared to I113T (58%), SALS (57%) and D90A (71%) patients, as well as healthy controls (49.7%). Estimated excitatory postsynaptic potentials (EPSPs) were significantly larger in A4V patients (4.39 mV) compared to healthy controls (2.95 mV), I113T (2.71 mV) and SALS (2.39 mV) patients. Clinical features and PSTH parameters in I113T were similar to SALS, however, PSTH primary peaks (PP) were significantly more dispersed, 9.5 ms compared to 4ms in SALS. PSTHs from single G41D and G114A patients were unremarkable, apart from large EPSP amplitudes in the G114A patient.

CONCLUSIONS

ALS patients with A4V and I113T SOD1 mutations have distinctive corticomotoneuronal changes that are different from those in D90A homozygous and SALS patients.

SIGNIFICANCE

PSTH studies should be considered for future in vivo studies of SOD1 pathophysiology in ALS.

Cortical versus spinal dysfunction in amyotrophic lateral sclerosis

Little is known about the possible link between cortical and spinal motor neuron dysfunction in amyotrophic lateral sclerosis (ALS). We correlated the characteristics of the responses to transcranial magnetic stimulation (TMS) with the electromechanical properties and firing pattern of single motor units (MUs) tested in nine ALS patients, three patients with Kennedy's disease, and 15 healthy subjects. In Kennedy's disease, 19 of 22 MUs were markedly enlarged with good electromechanical coupling and discharged with great variability. Their excitatory responses increased with MU size. In ALS, 17 of 34 MUs with excitatory responses behaved as in Kennedy's disease. By contrast, 28 MUs with nonsignificant responses showed poor electromechanical coupling and high firing rates, whereas 28 MUs with inhibitory responses showed moderate functional alterations. This result indicates that in ALS as in Kennedy's disease, sprouting of corticospinal axons may occur on surviving motoneurons. A clear relationship exists between the responsiveness of MUs to TMS and their functional state.

Parallel neuronal mechanisms underlying physiological force tremor in steady muscle contractions of humans

We present results from a study of the 6-to 12-Hz force tremor in relation to motor unit (MU) firing synchrony. Our experimental observations from 32 subjects, 321 contractions, and 427 recorded MUs reveal that tremor is accompanied by corresponding, in-phase MU rhythms that are additional to the ones at the MU intrinsic firing rates. This rhythmical synchrony is widespread and has a uniform strength that ranges from near zero to very large (MU/MU coherence > 0.50) in different contractions. Both the synchrony and the tremor are suppressed during ischemia, and this strongly suggests an involvement of spindle feedback in their generation. Furthermore, in the presence of substantial synchrony, the tremor enhancement, relative to the minimal tremor of ischemia, reflects the strength of the synchrony. Theoretical considerations based on these observations indicate that the muscle force signal is expected to show 1) frequency components in the band of the firing rates of the last-recruited, large MUs, and 2) because of the synchronized MU rhythms, an additional, distinct component with a size reflecting the strength of synchrony. Furthermore, synchronized MU rhythms, with frequencies in the 6- to 12-Hz range, are expected to arise from self-oscillations in the monosynaptic stretch reflex loop, due primarily to the associated muscle delay (several tens of milliseconds). Our results therefore reveal the parallel action of two tremor mechanisms, one of which involves MU synchrony probably caused by loop action. Clearly, the results on the synchrony and its impact also apply to other possible generators of tremor synchrony, including supraspinal ones.

Estimated magnitude and interactions of cortico-motoneuronal and Ia afferent input to spinal motoneurones of the human hand

Magnitude and interactions of cortico-motoneuronal (CM) and Ia afferent input to spinal alpha-motoneurones (MNs) of the human hand are largely unknown. This is, however, an important question, which bears on the cortical versus peripheral-segmental 'interest' in controlling alpha-MN excitation. Alpha-MN excitation can be quantified by estimating the amplitude of alpha-MN compound excitatory post-synaptic potentials (cEPSPs) from single motor unit (SMU) recordings, if certain assumptions about the membrane trajectory are made [Exp. Brain Res. 47 (1982) 33]. Here we recorded 29 SMUs from three different hand muscles (FDI, first dorsal interosseous; ADM, abductor digiti minimi; APB, abductor pollicis brevis) of healthy subjects. Each SMU was tested for CM input by transcranial magnetic stimulation of the contralateral motor cortex, for Ia afferent input by electrical peripheral nerve stimulation, and for the interaction between inputs by paired stimulation timed to arrive coincidently at the alpha-MN. Mean cEPSP amplitude elicited by CM input was larger in the alpha-MNs of the FDI than in those of the ADM or APB, whereas mean cEPSP amplitude elicited by Ia input was larger in the alpha-MNs of the APB than in those of the FDI. Generally, cEPSP amplitude evoked by paired input closely matched the arithmetic sum of the cEPSP amplitudes evoked by the single inputs. In conclusion, alpha-MNs of the human hand can be viewed as linear integrators of CM and Ia excitatory inputs. The weights of these inputs may relate to the specific functions of the different intrinsic hand muscles in skilled finger movements.

Different mechanisms contribute to motor cortex hyperexcitability in amyotrophic lateral sclerosis

OBJECTIVES

Different physiological approaches demonstrated motor system hyperexcitability in amyotrophic lateral sclerosis (ALS), probably reflecting excitotoxic mechanisms. Transcranial magnetic stimulation (TMS) showed that both increased excitability of corticomotoneurons and reduced intracortical inhibition (ICI) contribute to motor cortex hyperexcitability, but the importance of these factors in inducing this cortical dysfunction is unknown. The aim of the study was to establish how different mechanisms interact to promote motor system hyperexcitability in ALS in relation to clinical features.

METHODS

The resting motor threshold (RMT), the motor evoked potential (MEP) recruitment curve and the cortical silent period (CSP) to single-pulse TMS were evaluated in 35 patients with ALS. Early ICI and intracortical facilitation (ICF) and late ICI were evaluated by paired TMS.

RESULTS

The main abnormal TMS findings were: (a) a steeper MEP recruitment curve associated with a lowering of the RMT; (b) reduced or even absent early and late ICI; (c) reduced CSP lengthening with increasing TMS intensity. ICF was not affected. RMT increased and the MEP recruitment curve became less steep with longer disease duration, but they did not correlate with the motor deficit, the type of motoneuron affection and the decrease of ICI. Impairment of early and late ICI were significantly correlated to each other, to disease severity and to clinical evidence of upper motor neuron involvement.

CONCLUSIONS

Different and partially independent mechanisms contribute to motor cortex hyperexcitability in ALS. The increased gain in MEP recruitment with a lowering of the RMT appears to be a primary event reflecting an increase in the strength of corticospinal projections, probably related to changes in the ion-channel permeability of the neuronal membrane. On the other hand, inhibitory functions linked to multiple neurotransmitter systems decline with disease progression. Both depletion of specific subpopulations of intracortical GABAergic neurons and mechanisms involved in motor cortex reorganization following progressive neuronal loss have been considered to account for the impaired inhibition. The clarification of the importance of these factors in the pathogenesis of ALS may have diagnostic and therapeutic implications.

Amyotrophic lateral sclerosis: objective upper motor neuron markers

The diagnosis of amyotrophic lateral sclerosis (ALS) remains a clinical diagnosis. It is based on the combination of both upper and lower motor neuron signs in the neurologic examination. With several new therapeutic agents on the horizon, effective and objective disease markers for diagnosis and surrogate outcome measures in clinical trials are crucial. Whereas the presence of lower motor neuron signs on neurologic examination can be ascertained by electromyography, there is no widely accepted marker for upper motor neuron involvement. Neuroimaging changes of the corticospinal tract in ALS patients have been studied using magnetic resonance (MR) imaging, but appear to lack sensitivity and specificity. MR spectroscopy, a technique that allows one to evaluate biochemical tissue composition in vivo, has been widely used to establish the progressive decrease in N-acetylaspartate, a marker of neuronal integrity, in the course of ALS. More recently, diffusion tensor imaging, a newer MR technique, has demonstrated changes in diffusivity along the corticospinal tract in ALS patients. Metabolic aspects in the brains of ALS patients have been evaluated using positron emission tomography. Transcranial magnetic stimulation is a more established technique that evaluates the neurophysiologic integrity of upper motor neurons in ALS. This article reviews the progress that has been made over the past two decades towards establishing valid diagnostic and natural history markers of upper motor neuron involvement in ALS.

The motor cortex and amyotrophic lateral sclerosis

On theoretical grounds, abnormalities of the motor cortex in patients with amyotrophic lateral sclerosis (ALS) could lead to anterograde ("dying-forward") transneuronal degeneration of the anterior horn cells as suggested by Charcot. Conversely, retrograde ("dying-back") degeneration of the corticospinal tracts could affect the motor cortex. Evidence derived from clinical, neuropathological, static, and functional imaging, and physiological studies, favors the occurrence of anterograde degeneration. It is hypothesized that transneuronal degeneration in ALS is an active excitotoxic process in which live but dysfunctional corticomotoneurons, originating in the primary motor cortex, drive the anterior horn cell into metabolic deficit. When this is marked, it will result in more rapid and widespread loss of lower motor neurons. In contrast, slow loss of corticomotoneurons, as occurs in primary lateral sclerosis (PLS), precludes excitotoxic drive and is incompatible with anterograde degeneration. Preservation of slow-conducting non-M1 direct pathways in PLS is not associated with excitotoxicity, and anterior horn cells survive for long periods of time.

Temporal dispersion of cortically evoked single motor unit potentials in ALS

Peristimulus time histograms (PSTHs) can be used to investigate corticomotoneuronal dysfunction in amyotrophic lateral sclerosis (ALS). The most characteristic change is temporal dispersion of the primary peak. We recorded PSTHs in the extensor digitorum communis with voluntary motor units activation (standard PSTHs) or at rest (non-activated PSTHs). Standard PSTHs were recorded in 29 motor units of 12 healthy control subjects and 12 sporadic ALS patients. Double primary peaks were seen in three motor units of two healthy control subjects and 10 motor units from five ALS patients. The number of subpeaks was up to three in most of the normal motor units as well as in the earlier component of double primary peaks. The subpeaks were smaller and less discernible in the later component of double primary peaks. Non-activated PSTHs of ALS patients demonstrated similar decomposition of subpeaks in the motor units with significantly increased variability of latency. Similar findings in the standard PSTHs and non-activated PSTHs suggest that the abnormalities seen in ALS are independent of the membrane potential of the spinal motoneuron and therefore supraspinal in origin. The decomposed additional later component may indicate activation of slow conducting corticospinal tracts.

Further evidence for corticomotor hyperexcitability in amyotrophic lateral sclerosis

A collision experiment has been used to investigate repetitive firing of first dorsal interosseous motoneurons following a single transcranial magnetic stimulus (TMS) in healthy subjects and patients with amyotrophic lateral sclerosis (ALS). An appropriately timed supramaximal peripheral nerve shock blocks the first descending impulses in the motor axons and allows the response due to repetitive firing to be quantified. Multiple firing of motoneurons in healthy subjects increases as TMS intensity rises and saturates at about 1.25 times threshold. Increasing background force also augments repetitive firing and saturates at force levels above 50% maximum. The ratio of the area of the response attributed to repetitive motoneuron firing to the area of the initial direct response to TMS was compared in 10 ALS patients and 10 healthy controls. In ALS patients, the ratio was significantly higher (P = 0. 0005), indicating a greater degree of repetitive firing of motoneurons. This suggests that, in ALS, there is corticomotor hyperexcitability either at the spinal motoneuron or motor cortex.

Peristimulus time histograms (PSTHs)--a marker for upper motor neuron involvement in ALS?

Upper motor neuron involvement in ALS has been widely studied by means of transcranial magnetic stimulation and various imaging methods, such as magnetic resonance imaging, proton emission tomography, single photon emission computer tomography and magnetic resonance spectroscopy. Although almost all of these methods have detected 'abnormalities' of the motor cortex, no method has yet proven to be capable of monitoring disease progression. We and others have used peristimulus time histograms (PSTH) to evaluate corticomotoneuronal dysfunction. The primary peak (PP) in the PSTH reflects the initial rising phase of the excitatory post-synaptic potential evoked at the anterior horn cell by converging corticomotoneurons. In ALS, the PP is delayed in onset, increased in duration, and desynchronized. These abnormalities become more pronounced over time. Delayed PPs also occur more frequently in non-hereditary ALS (nHALS) with disease progression and are a hallmark of the autosomal recessively inherited D90A Cu-Zn SOD mutation. Evidence indicates that in this particular mutation, as well as in nHALS, the fast-conducting monosynaptic pathway originating from large pyramidal Betz cells is preferentially lost, whereas a slow-conducting pathway seems to be spared. This is most likely due to the enormous metabolic demand of the large pyramidal cells, which may explain the selective vulnerability of the most recent phylogenetic system to develop in the human nervous system.

Corticomotoneuronal activity in ALS: changes in the peristimulus time histogram over time

OBJECTIVE

The primary peak in the peristimulus time histogram (PSTH) reflects the initial rising phase of the excitatory post-synaptic potential (EPSP) evoked at the anterior horn cell. In ALS the primary peak is delayed in onset. increased in duration and desynchronized. abnormalities reflecting dysfunction of the corticomotoneurons. It is not known whether these abnormalities change over time in amyotrophic lateral sclerosis (ALS).

METHODS

PSTHs were constructed from changes in the firing probability of single, voluntarily activated motor units subjected to subthreshold transcranial magnetic stimuli. We studied 58 motor units in 12 patients with ALS on two separate occasions (mean time interval of 10.6 +/- 1.6 months). Results were compared with 49 motor units in 11 age matched controls.

RESULTS

All the parameters except the amplitude differed significantly between normals and patients. In general the primary peak in ALS was complex, desynchronized and occasionally consisted of a double peak. The abnormalities persisted or were accentuated at tile follow up visit. This was reflected by an increase in the number of excess bins, longer duration and latency and decrease of synchrony.

CONCLUSIONS

Increasing desynchronization of the primary peak over time in ALS reflects dysfunction of the monosynaptic corticomotoneuronal pathway and may also reflect activation of additional slow conducting and/or polysynaptic corticomotoneuronal connections.

An electrophysiological study of the corticospinal projections in amyotrophic lateral sclerosis

OBJECTIVE

To elucidate the pattern of corticospinal tract involvement in patients with amyotrophic lateral sclerosis (ALS), we analyzed motor evoked potential (MEP) waveforms and their relationship to the behaviour of single motor units using the peristimulus time histogram (PSTH) technique.

METHODS

Abnormality of the corticospinal pathways was studied in 35 ALS patients using MEPs. PSTHs were also constructed to assess the effect of magnetic cortical stimulation on the discharge pattern of a voluntarily activated motor unit.

RESULTS

MEPs showed a complex waveform in 10 out of 18 (56%) ALS patients with upper motor neuron signs (UMN). PSTHs revealed double primary peaks (PPs), PP1 and PP2, in 6 out of 16 motor units (38%) in ALS with UMN, as compared to only 2 out of 16 (13%) motor units in multiple sclerosis or cerebrovascular disease with UMN. None of the patients with lower motor neuron diseases or ALS without UMN had these abnormalities. The late component of complex MEPs showed a good correlation to PP2 (P < 0.0001), both probably being mediated by relatively preserved slower conducting corticospinal volleys.

CONCLUSIONS

These findings suggest preferential involvement of the fast conducting direct corticospinal tracts, sparing the slower or polysynaptic projections in ALS.

Electromechanical coupling and synchronous firing of single wrist extensor motor units in sporadic amyotrophic lateral sclerosis

Electrical and contractile properties of motor units (MU) were studied in the extensor carpi radialis muscles during voluntary contraction. The discharge of 234 single MUs was recorded in 11 patients with sporadic amyotrophic lateral sclerosis (ALS) and compared with that of the 260 MUs recorded in 12 healthy control subjects. Characteristics of the MU twitches and of the macro-potentials, the electromechanical coupling and the synchronization of the motor neurone discharges, were compared. In 5 patients (population ALS1), the twitch contraction force and macro-MUP area values were much larger than those of the controls. In the 6 other patients (population ALS2), the twitch force was considerably depressed, whereas the macro-MUP area was slightly, but significantly, increased. In ALS1, as well as in ALS2, the electromechanical coupling was much weaker than in the controls, and the fast-contracting MUs were more severely affected than the slowly contracting MUs. The motoneuronal synchronization was assessed by performing cross-correlation analysis on MUs discharges, and was used as an index to the strength of the common motoneuronal inputs. The rate of occurrence of synchronous firing was conspicuously lower in both populations of patients than in the control group. This might reflect the loss of corticospinal projections that occurs in ALS. The data are discussed in terms of the time course of motor neurone axonal sprouting, and in terms of the neuronal and muscular dysfunction possibly involved in ALS disease.

Assessment of upper and lower motor neurons in Kennedy's disease: implications for corticomotoneuronal PSTH studies

We used peristimulus time histograms (PSTHs) to estimate characteristics of the composite excitatory postsynaptic potentials (EPSPs) generated at the anterior horn cell by a descending cortical volley induced by subthreshold transcranial magnetic stimuli in 11 normal subjects, 9 patients with DNA-confirmed Kennedy's disease, and 10 patients with amyotrophic lateral sclerosis (ALS). In Kennedy's disease the mean of the estimated EPSP amplitude (3.5 +/- 1.6 mV), as well as the means of duration (4.1 +/- 1.3 ms) and onset latency (20.4 +/- 2.7 ms) of the primary peak of 35 different motor units were not significantly different from normal subjects, in whom corresponding values of 49 motor units measured 2.9 +/- 1.6 mV (EPSP amplitude), 3.5 +/- 1.4 ms (duration) and 19.3 +/- 2.8 ms (onset latency). In 48 motor units of patients with ALS the mean of the estimated EPSP amplitude, duration, and latency measured 2.7 +/- 1.8 mV, 7.8 +/- 5.7 ms, and 24.0 +/- 6.5 ms, respectively, differing significantly from normal subjects and patients with Kennedy's disease. The normal cortically induced EPSP in Kennedy's disease is very different from the range of EPSP abnormalities in ALS, indicating that lower motor neuron disease per se does not result in an abnormal cortically induced EPSP derived from PSTHs. The normal findings in Kennedy's disease support our previous suggestions that the EPSP abnormalities in ALS are supraspinal in origin.

Corticomotorneuronal hyper-excitability in amyotrophic lateral sclerosis

We have analysed how the behaviour of a voluntarily activated motor unit changes when subjected to 100-150 threshold cortical stimuli using peristimulus time histograms (PSTHs). This is a measure of the integrity of the corticomotoneuronal core innervating a single anterior horn cell. One hundred and thirty units in 29 patients with ALS and 35 units in eight age-matched normal controls were studied. PSTHs were constructed using 1-ms bins of stimulus triggered sweeps with a total analysis time of 250 ms (50 ms before and 200 ms after the stimulus). In ALS the primary peak of the PSTH was delayed in onset and prolonged in duration. The primary peak was further analysed by finer 0.2-ms bins, which showed in ALS there were more sub-components than normally occur. Additional sub-components in the PSTH primary peak implies a hyper-excitable corticomotoneuron that fires excessively. Excitability could be glutamate induced and/or due to failure of GABA inhibitory mechanisms. Some glutamate antagonsits may be therapeutic in ALS because of their anticonvulsant or GABergic properties rather than their anti-glutamate properties. GABA(B) agonists might have a role in future therapeutic combined therapies for ALS.

Comparison of corticomotoneuronal EPSPs and macro-MUPs in amyotrophic lateral sclerosis

We correlated the size of the corticomotoneuronal excitatory postsynaptic potential (CM-EPSP) arising in a single spinal motor neuron with the function of the target motor unit as measured by conventional and macro EMG in early amyotrophic lateral sclerosis (ALS). Macro motor unit potentials (macro-MUPs) were recorded from a surface electrode after spike-triggered averaging in the extensor digitorum communis muscle. The size of the CM-EPSP projecting to the same motor unit was measured from changes in the firing probability of single motor units induced by transcranial magnetic stimulation using peristimulus time histograms. In controls, the amplitudes of CM-EPSPs and macro-MUPs correlated inversely, probably reflecting a lower input resistance of larger spinal motoneurons. In ALS the amplitude of macro-MUPs did not correlate with that of CM-EPSPs and one third of normal ALS motor units had a reduced or temporally dispersed CM-EPSP. The findings indicate primary dysfunction of the corticomotoneuronal projection system in ALS that is independent of functional changes of spinal motoneurons.

Diverse abnormalities of corticomotoneuronal projections in individual patients with amyotrophic lateral sclerosis

Using peristimulus time histograms (PSTHs), abnormalities of composite excitatory postsynaptic potentials (EPSPs) induced by transcranial magnetic stimulation were studied in multiple motor units from individuals with amyotrophic lateral sclerosis (ALS) and normal subjects. We studied 97 motor units in the extensor digitorum communis muscle of 22 patients with sporadic ALS and 47 motor units of 10 healthy control subjects. Four or five motor units were studied in each patient and normal subject. For each unit, macro motor unit potentials (Macro-MUPs) were simultaneously recorded from a surface electrode after spike-triggered averaging. The composite EPSPs in ALS showed a generally bi-directional deviation from the normal curve, with small EPSPs at one end, and larger amplitude EPSPs with a prolonged rise time at the other end. The variability of EPSPs from adjacent motor units in the same individual was significantly larger in ALS than in controls. In normal subjects there is a significant negative correlation between the amplitude of composite EPSPs and the Macro-MUPs. In ALS, the trend is reversed (positive) suggesting that the abnormalities of composite EPSPs are supraspinal in origin. A combination of partial attrition of the corticomotoneuronal core and hyper-excitability of surviving corticomotoneurons projecting to a given spinal motoneuron pool best explains the diversity of the composite EPSP in individuals with ALS.