Perceptual threshold to cutaneous electrical stimulation in patients with spinal cord injury.

STUDY DESIGN: Prospective experimental. OBJECTIVES: The aim of this study was to develop a quantitative sensory test (QST) that could be used for assessing the level and the density (degree of impairment) of spinal cord injury (SCI) and for monitoring neurological changes in patients with SCI. SETTING: National Spinal Injuries Centre, Stoke Mandeville Hospital, Buckinghamshire Hospitals NHS Trust, UK. METHODS: Perceptual threshold to 3 Hz cutaneous electrical stimulation was measured in 30 control subjects and in 45 patients with SCI at American Spinal Injuries Association (ASIA) sensory key points for selected dermatomes between C3 and S2 bilaterally. Electrical perceptual threshold (EPT) was recorded as the lowest ascending stimulus intensity out of three tests at which the subject reported sensation. The level of SCI according to EPT results was established for right and left sides as the most caudal spinal segment at which patient's EPT was within the control range (mean +/- 2 standard deviation (SD)). The level of SCI, according to EPT, was then compared with clinical sensory level derived according to ASIA classification. RESULTS: In the control group, EPT depended on the dermatome tested and was lowest for T1 (1.01 +/- 0.23 mA, mean +/- SD) and highest for L5 (3.32 +/- 1.14 mA). There was strong correlation between corresponding right and left dermatomes and between repeated assessments. In the SCI group, the level of lesion according to EPT and clinical testing was the same in 43 of the 90 tests (48%). In 37 cases (41%), the EPT level was higher than the clinical level, and in 10 cases (11%), it was lower. Below the level of lesion in incomplete SCI and in the zone of partial preservation in complete SCI, the EPT values in most dermatomes were raised compared with the control group. CONCLUSIONS: EPT is a simple, reproducible QST that can assess both the level and the density of SCI. It seems to add sensitivity and resolution to the standard clinical testing and could be a useful adjunct in longitudinal monitoring of patients with SCI for research purposes during natural recovery and therapeutic interventions. SPONSORSHIP: International Spinal Research Trust (ISRT), UK, Grant CLI001.

Motor neurone excitability in back muscles assessed using mechanically evoked reflexes in spinal cord injured patients.

OBJECTIVE: The clinical and functional assessment of back muscles in human spinal cord injury (SCI) has received little attention. The aim of this study was to develop a method to assess the level of a thoracic spinal cord lesion based on the reflex activation of back muscles. METHODS: In 11 control subjects and in 12 subjects with clinically complete thoracic SCI (T2-T12), either a spinous process or an erector spinae muscle was prodded to elicit short latency reflexes recorded electromyographically at the spinal level of stimulation. An electromagnetic servo, attached to a blunt probe, applied stimuli at a frequency of 1 Hz and amplitude of 3 mm. Two trials of 50 mechanical prods were conducted at each site. RESULTS: Reflexes were evoked in control subjects in 82% of trials when the spinous process was prodded, and in 80% of trials when the muscle was prodded. In contrast, reflexes in SCI subjects could be elicited in 90-100% of trials two segments either above or below the lesion. Reflex responses in control subjects had a mean (SEM) latency of 5.72 (0.53) ms when the spinous process was prodded, and 5.42 (0.42) ms when the muscle was prodded. In the SCI subjects, responses had slightly (but insignificantly) longer latencies both above and below the lesion to either stimulus. The amplitude of reflex responses, expressed as a percentage of the background EMG, was on average 2-3 times larger at the three vertebral levels spanning the lesion in SCI subjects than at sites above or below the lesion or at any level in control subjects. CONCLUSION: We propose that the size of these mechanically evoked reflexes may be useful in determining the level of thoracic SCI. Furthermore, the reflexes might provide a valuable tool with which to monitor recovery after an intervention to repair or improve function of a damaged spinal cord.

The effect of an energy drink containing glucose and caffeine on human corticospinal excitability.

Glucose- and caffeine-containing energy drinks are said to influence the cognitive and cellular function within the brain. In this study, we have used the size of motor-evoked potentials (MEPs) produced in response to transcranial magnetic stimulation (TMS) of the motor cortex as an index of corticospinal excitability after ingestion of Lucozade and control drinks of glucose-containing or caffeine-containing carbonated water or carbonated water alone. With local ethical approval and informed consent, 10 healthy volunteers took part; surface electromyographic (EMG) recordings were taken from the thenar muscles of the dominant hand. In each assessment, 15 TMS stimuli were delivered over the motor cortex at an intensity of 1.1 T. Six subjects ingested a 380-ml bottle of carbonated Lucozade drink containing 68 g of glucose and 46 mg caffeine. Four subjects took part in three control trials drinking: (A) carbonated water with caffeine, (B) carbonated water with glucose and (C) carbonated water alone. Assessments were made before and at 30-min intervals after each drink. Mean fasting blood glucose concentrations and mean areas of MEPs rose after the Lucozade, remaining elevated for 90 min. Similar rises in MEP areas were seen in trials after drinking carbonated water with caffeine or with glucose, but not after drinking carbonated water alone. No change was seen in the M-wave evoked by electrical stimulation of the ulnar nerve. We conclude that Lucozade can affect the size of MEPs to activation of the motor cortex with fixed-intensity TMS. The underlying mechanism is likely to relate to the combined effects of caffeine and glucose on the brain.

Magnetic brain stimulation can improve clinical outcome in incomplete spinal cord injured patients.

STUDY DESIGN: Preliminary longitudinal clinical trial. OBJECTIVES: To test the efficacy of repetitive transcranial magnetic stimulation (rTMS) in modulating corticospinal inhibition and improving recovery in stable incomplete spinal cord injury (iSCI). SETTING: National Spinal Injuries Centre, Stoke Mandeville Hospital, Bucks, UK and Division of Neuroscience, Imperial College Faculty of Medicine, Charing Cross Hospital, London, UK. METHODS: Four stable iSCI patients were treated with rTMS over the occipital cortex (sham treatment) and then over the motor cortex (real treatment). Patients were assessed using electrophysiological, clinical and functional measures before treatment, during sham treatment, during the therapeutic treatment and during a 3-week follow-up period. RESULTS: Cortical inhibition was reduced during the treatment week. Perceptual threshold to electrical stimulation of the skin, ASIA clinical measures of motor and sensory function and time to complete a peg-board improved and remained improved into the follow-up period. CONCLUSION: In this preliminary trial, rTMS has been shown to alter cortical inhibition in iSCI and improve the clinical and functional outcome. SPONSORSHIP: This work was supported by the International Spinal Research Trust.

Specificity and functional impact of post-exercise depression of cortically evoked motor potentials in man.

Magnetic stimulation of the motor cortex with electromyographic recordings from exercising muscles has shown corticospinal excitability to be depressed following exercise. We now investigate whether this depression spreads to non-exercising muscles and its influence on performance. Healthy volunteers made unilateral biceps curls to exhaustion and, in another later session, for 25% of the time to exhaustion. Bilateral motor-evoked potentials (MEPs) in biceps brachii and first dorsal interosseus muscles were measured at 2-min intervals before and after exercise. In another experiment, subjects performed exhaustive curls and, in addition to MEP areas, force production in biceps, hand-grip force, simple reaction times and movement times were measured bilaterally. MEPs were depressed after exhaustive exercise in the exercising biceps for over 60 min; depression was also observed 10-15 min after exercise in the non-exercising biceps but not in the first dorsal interosseus of either hand. The shorter exercise period produced depression of MEPs only in the exercising muscle. After exhaustive exercise maximum voluntary contraction fell in the exercising biceps and this correlated with MEP areas. No reduction in force was seen in the non-exercising biceps but hand-grip force fell slightly in both arms. There was no change in reaction times or movement times. Depression of MEPs can occur in non-exercising homonymous muscles but not in heteronymous muscles and only when exercise levels are high. There was no measurable functional deficit in the non-exercising limb, so we conclude that the reduced corticospinal excitability observed in this limb has little or no consequence on the performance parameters measured.

Towards improved clinical and physiological assessments of recovery in spinal cord injury: a clinical initiative.

Clinical practice and scientific research may soon lead to treatments designed to repair spinal cord injury. Repair is likely to be partial in the first trials, extending only one or two segments below the original injury. Furthermore, treatments that are becoming available are likely to be applied to the thoracic spinal cord to minimise loss of function resulting from damage to surviving connections. These provisos have prompted research into the improvement of clinical and physiological tests designed (1) to determine the level and density of a spinal cord injury, (2) to provide reliable monitoring of recovery over one or two spinal cord segments, and (3) to provide indices of function provided by thoracic spinal root innervation, presently largely ignored in assessment of spinal cord injury. This article reviews progress of the Clinical Initiative, sponsored by the International Spinal Research Trust, to advance the clinical and physiological tests of sensory, motor and autonomic function needed to achieve these aims.

Excitability of the motor cortex in schizophrenia following typical and atypical antipsychotics: two serial case reports.

A previous electrophysiological investigation of schizophrenia using transcranial magnetic stimulation has shown altered corticospinal inhibition in medicated patients compared with drug-naive patients. Based on this work, we have carried out the first serial electrophysiological case studies of the human corticospinal system in schizophrenia in which a typical antipsychotic was compared with an atypical antipsychotic. We used transcranial magnetic stimulation-based techniques to study the time course of drug action in two antipsychotic drug-naive patients with schizophrenia; one patient treated with a typical antipsychotic (haloperidol) and the other with an atypical antipsychotic (risperidone). The typical antipsychotic was associated with an increase in corticospinal excitability which we found to occur 4-5 weeks after beginning medication. In contrast, the atypical antipsychotic was associated with a decrease in corticospinal excitability, occurring 3-4 weeks after starting pharmacotherapy. We conclude that these opposite neurophysiological effects are related to the relative dopaminergic and serotonergic actions of the two classes of drug.

Deficit in motor performance correlates with changed corticospinal excitability in patients with chronic fatigue syndrome.

Chronic fatigue syndrome (CFS) is characterised by fatigue and musculosketetal pain, the severity of which is variable. Simple reaction times (SRTs) and movement times (SMTs) are slowed in CFS. Our objective is to correlate the day-to-day changes in symptomatology with any change in SRT, SMT or corticospinal excitability. Ten CFS patients were tested on two occasions up to two years apart. Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) of the motor cortex were recorded from the thenar muscles. Threshold TMS strength to evoke MEPs was measured to index corticospinal excitability. SRTs and SMTs were measured. The percentage change in both SRTs and SMTs between the two test sessions correlated with the percentage change in corticospinal excitability assessed according to threshold TMS intensity required to produce MEPs. This study provides evidence that changing motor deficits in CFS have a neurophysiological basis. The slowness of SRTs supports the notion of a deficit in motor preparatory areas of the brain.

Relative increase in choline in the occipital cortex in chronic fatigue syndrome.

OBJECTIVE: To test the hypothesis that chronic fatigue syndrome (CFS) is associated with altered cerebral metabolites in the frontal and occipital cortices. METHOD: Cerebral proton magnetic resonance spectroscopy (1H MRS) was carried out in eight CFS patients and eight age- and sex-matched healthy control subjects. Spectra were obtained from 20 x 20 x 20 mm3 voxels in the dominant motor and occipital cortices using a point-resolved spectroscopy pulse sequence. RESULTS: The mean ratio of choline (Cho) to creatine (Cr) in the occipital cortex in CFS (0.97) was significantly higher than in the controls (0.76; P=0.008). No other metabolite ratios were significantly different between the two groups in either the frontal or occipital cortex. In addition, there was a loss of the normal spatial variation of Cho in CFS. CONCLUSION: Our results suggest that there may be an abnormality of phospholipid metabolism in the brain in CFS.

Corticospinal inhibition appears normal in patients with chronic fatigue syndrome.

The pathogenesis of chronic fatigue syndrome (CFS) remains unknown. Thresholds and latencies of motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) are normal but intracortical inhibition has not been investigated. Eleven patients with CFS were compared with 11 control subjects. Each patient completed a questionnaire using visual analogue indices of pain, fatigue, anxiety and depression. Subjects released a button to initiate simple (SRTs) and choice reaction time (CRTs) tasks; for each task, movement times were measured between release of the initiation button and depression of a second button 15 cm away. Subjects held a 10 % maximum voluntary contraction in the thenar muscles of their dominant hand while TMS was applied to the motor cortex; the duration and extent of inhibition of surface electromyographic (EMG) activity were assessed at stimulus strengths above and below the threshold for MEPs. Patients had significantly (P < 0.05) higher mean indices of fatigue than of pain, anxiety or depression. Mean (+/- S.E.M.) SRTs (but not CRTs) were longer in patients (309 +/- 45 ms) than in controls (218 +/- 9 ms). Movement times were longer in patients for both SRTs and CRTs. TMS thresholds, expressed as a percentage of the maximum stimulator output, were not significantly (P > 0.05) different in both groups for both MEPs (patients, 34 +/- 3%; controls, 36 +/- 3%) and inhibition of voluntary contraction (patients, 29 +/- 2%; controls, 34 +/- 4%). The duration and extent of inhibition did not differ significantly between groups at any stimulus strength. The pattern of change in duration and extent of inhibition with increasing stimulus intensity was no different in the two groups. The duration and extent of corticospinal inhibition in patients with CFS did not differ from controls, adding further evidence to the notion that the feeling of fatigue and the slowness of movement seen in CFS is not manifest in corticospinal output pathways.

Corticospinal control of human erector spinae muscles.

We used transcranial magnetic stimulation (TMS) to study corticospinal excitability to erector spinae (ES) muscles during graded voluntary contractions in bilateral trunk extension (BTE) and forced expiratory breath holding (FEBH) in normal individuals. Motor evoked potentials (MEPs) could be produced in all subjects in the absence of voluntary activation. At maximum voluntary contraction, levels of surface electromyographic (EMG) activity were 4 times greater during BTE than FEBH. When EMG was normalized to maximum, MEP amplitudes increased in proportion to contraction in both tasks. MEPs in FEBH were compared with extrapolated values at similar EMG levels in BTE and were found to be larger. EMG and MEPs in left and right ES were symmetrical throughout the range of contractions in both tasks. ES muscles have a facilitation pattern similar to that previously shown in leg muscles, but subtle differences at low levels of EMG suggest that the facilitation is dependent on the task.

Somatopy of perceptual threshold to cutaneous electrical stimulation in man.

Neurological testing tools for measuring and monitoring somatosensory function lack resolution and are often dependent on the clinician testing. In this study we have measured perceptual threshold (PT) to electrical stimulation of the skin and compared it with two-point discriminative ability (TPDA) in 12 control subjects. Tests were made on both sides of the body at American Spinal Injury Association (ASIA) key points on seven spinal dermatomes (C3 (neck), C4 (shoulder), C5 (upper arm), C6 (thumb), T8 (abdomen), L3 (knee), L5 (foot)) and in the mandibular (chin) and maxillary (cheek) fields of the trigeminal (V) nerve. Electrical stimulation (0.5 ms pulse width; 3 Hz) was applied via a self-adhesive cathode and an anode strapped to the wrist or ankle. The stimulus intensity was adjusted and PT was recorded as the lowest current at which the subject reported sensation. Sites were tested in random order. Indices for both TPDA and PT differed according to the dermatome tested but there was no correlation between TPDA and PT for any dermatome. There was good correlation between results from equivalent dermatomes on left and right sides for both PT and TPDA. Women frequently had lower mean (+/- S.E.) PTs and better TPDA than men; differences were significant (P < 0.05) for PT on the knee (women, 1.31 +/- 0.15 mA; men, 2.05 +/- 0.26 mA) and the foot (women, 2.90 +/- 0.19 mA; men, 4.13 +/- 0.28 mA) and for TPDA on the thumb (women, 3.8 +/- 0.2 mm; men, 7.8 +/- 1.3 mm) and the knee (women, 17.8 +/- 1.6 mm; men, 27.1 +/- 4.0 mm). Four subjects repeated the experiment on another day and the results correlated well with the first test for PT (r2, 0.62) and TPDA (r2, 0.48). PT differs between dermatomes in a predictable way but does not relate to TPDA. PT is easy to measure and may be a useful assessment tool with which to monitor recovery or deterioration in neuropathies, neurotrauma or after surgery.

Corticospinal facilitation studied during voluntary contraction of human abdominal muscles.

Transcranial magnetic stimulation (TMS) of the human motor cortex was used to study facilitation of motor-evoked potentials (MEPs) in the rectus abdominis (RA) muscle, a trunk flexor, during voluntary activation. MEPs could be produced in the relaxed RA muscles of all six normal subjects studied. The MEPs had short latencies (18-22 ms) which are consistent with other studies suggesting a fast corticospinal input to the trunk muscles. Marked facilitation was observed in the MEPs when subjects were asked to produce graded levels of voluntary contractions. The two tasks used to produce voluntary contractions were a forced expiration during a breath-holding task (FEBH) and bilateral trunk flexion (BTF). Maximal voluntary EMG activity during the BTF task produced around 4.2 times more integrated EMG than during the FEBH task. Similarly the MEP amplitude at MVC was 2.3 times greater during BTF than FEBH. The pattern of MEP facilitation with increasing voluntary EMG was not linear and a maximal MEP amplitude was observed at a level of voluntary contraction around 30 % MVC in both tasks. There were some subtle differences in the pattern of facilitation in the two tasks. When TMS was applied to the right cortex only, MEPs were seen in both left and right RA muscles suggesting some ipsilateral corticospinal innervation. The latency of the right (ipsilateral) response was approximately 2 ms longer than the left. Comparison with studies in hand and leg muscles suggests that the facilitation pattern in RA may reflect a substantial degree of corticospinal innervation. Experimental Physiology (2001) 86.1, 131-136.

Voluntary motor function in patients with chronic fatigue syndrome.

INTRODUCTION: The pathogenesis of chronic fatigue syndrome (CFS) remains unknown. In particular, little is known of the involvement of the motor cortex and corticospinal system. METHODS: Transcranial magnetic stimulation (TMS) was used to assess corticospinal function in terms of latency and threshold of motor-evoked potentials (MEPs) in thenar muscles. Reaction times and speed of movement were assessed using button presses in response to auditory tones. RESULTS: Patients had higher (P<.05) self-assessed indices of fatigue (7/10) than for pain (5/10), anxiety (4/10) or depression (3/10). Mean (+/-S.E.M.) simple reaction times (SRTs) were longer (P<.05) in the patients (275+/-19 ms) than in the controls (219+/-9 ms); choice reaction times (CRTs) were not significantly longer in the patients. Movement times, once a reaction task had been initiated, were longer (P<.05) in the patients in both SRTs (patients, 248+/-13 ms; controls, 174+/-9 ms) and CRTs (patients, 269+/-13 ms; controls, 206+/-12 ms). There was no difference (P>.05) in threshold or latency of MEPs in hand muscles between the patients (threshold, 54.5+/-2.2% maximum stimulator output [% MSO]; latency 22+/-0.3 ms) and controls (threshold 54.6+/-3.6% MSO; latency 22.9+/-0.5 ms). Regression analysis showed no correlation (P>.05) of SRTs with either threshold for MEPs or fatigue index. CONCLUSION: Corticospinal conduction times and excitability were within the normal range despite a slower performance time for motor tasks and an increased feeling of fatigue. This suggests that the feeling of fatigue and the slowness of movement seen in CFS are manifest outside the corticospinal system.

The fine specificity of human T cell lines towards myelin basic protein peptides in southern Italian multiple sclerosis patients.

We studied the relationship between the HLA specificities associated with multiple sclerosis (MS) susceptibility in southern Italy and the reactivity of the human myelin basic protein (hMBP) immunogenic peptides 84-98 and 143-168, using short-term T-cell lines established from 9 MS patients and from 8 healthy individuals. In our population, DR15 was significantly associated with MS (34.9% in MS versus 13.7% in healthy controls, P < 0.05). This result is in agreement with the association found in northern Europe, but not with data obtained in a population from the island of Sardinia (Italy). In MS patients the frequency of reactive T-cell lines (TCL), tested for fine specificity against the immunodominant hMBP peptides 84-98 and 143-168, was increased for the hMBP 143-168 peptide (P < 0.05) but not for the 84-98 peptide. Although this reactivity was higher in DR15+ MS patients than in DR 15- MS patients, it seemed not to be associated with DR15 specificity in the MS population. Furthermore, there were no significant differences in frequency of reactive TCL to hMBP peptide 84-98 in DR15-positive or DR15-negative MS patients. Consequently, it appears that peptide 84-98, considered as a relevant autoantigen, is not implicated in the pathogenesis of MS in our population from southern Italy.

Current orientation induced by magnetic stimulation influences a cognitive task.

The direction of the current induced by transcranial magnetic stimulation (TMS) over the motor cortex has been observed to influence the threshold and latency of evoked muscle responses. This study investigates the effect of TMS-induced current orientation (ICO) over the prefrontal cortex, on a specific cognitive task (memory-guided saccade). TMS was applied with a figure-of-eight coil, placed at one of eight different orientations over the prefrontal cortex. The most effective ICO was antero-lateral, which is a different optimal ICO from that seen over the hand area of the motor cortex. This demonstrates that ICO can alter the effect of TMS on cognitive functions and that ICO is an independent variable that should not be ignored when designing TMS studies.

Corticospinal function studied over time following incomplete spinal cord injury.

STUDY DESIGN: Longitudinal. OBJECTIVES: (1) To perform standard clinical neurological examinations and establish the pattern of clinical change with time following incomplete spinal cord injury (iSCI). (2) To establish the pattern of change in corticospinal electrophysiological function with time after iSCI. (3) To correlate clinical with electrophysiological findings. SETTING: The National Spinal Injuries Centre, Stoke Mandeville Hospital, Aylesbury, UK and Imperial College School of Medicine, Charing Cross Hospital, London, UK. METHODS: Neurological assessments and classification were performed according to American Spinal Injuries Association and International Medical Society of Paraplegia (ASIA/IMSOP) standards. Twenty-one patients (ages 18 - 72 years) with iSCI (level C2 - C7, ASIA impairment grades C - D) and 10 healthy control subjects (ages 27 - 57 years) were studied. Electrophysiological tests of corticospinal function were carried out using transcranial magnetic stimulation (TMS) of the motor cortex and electromyographic (EMG) recordings from thenar muscles. Both tests were performed on a number of occasions, beginning 19 - 384 days and ending 124 - 1109 days post-injury, and the group data were pooled into time epochs of 50 or 100 days post-injury for analysis. Seven of the patients were studied on seven or more occasions and were also assessed individually. RESULTS: Individual and pooled data indicated that neurological scores improved progressively and tended to stabilise by around 300 days post-injury. When the patients were first assessed, the mean latency for motor evoked potentials (MEPs) and inhibition of voluntary EMG were significantly different from control values. There was no significant change in latency on subsequent sessions for either the grouped or individual patient data. There was no correlation between clinical assessment and electrophysiological data. CONCLUSION: We conclude that the weakened inhibition seen following iSCI is established within a few days of the time of spinal cord trauma. We argue that reduced corticospinal inhibition may be a prerequisite for the recovery of useful motor function. SPONSORSHIP: The work was supported by a project grant from The Wellcome Trust.

Modulation of single motor unit discharges using magnetic stimulation of the motor cortex in incomplete spinal cord injury.

OBJECTIVES: Motor evoked potentials (MEPs) and inhibition of voluntary contraction to transcranial magnetic stimulation (TMS) of the motor cortex have longer latencies than normal in patients with incomplete spinal cord injury (iSCI) when assessed using surface EMG. This study now examines the modulation of single motor unit discharges to TMS with the aim of improving resolution of the excitatory and inhibitory responses seen previously in surface EMG recordings. METHODS: A group of five patients with iSCI (motor level C4-C7) was compared with a group of five healthy control subjects. Single motor unit discharges were recorded with concentric needle electrodes from the first dorsal interosseus muscle during weak voluntary contraction (2%-5% maximum). TMS was applied with a 9 cm circular stimulating coil centred over the vertex. Modulation of single motor unit discharges was assessed using peristimulus time histograms (PSTHs). RESULTS: Mean (SEM) threshold (expressed as percentage of maximum stimulator output (%MSO)) for the excitatory peak (excitation) or inhibitory trough (inhibition) in the PSTHs was higher (p<0.05) in the patients (excitation = 47.1 (5.9) %MSO; inhibition = 44.3 (3.2) %MSO) than in controls (excitation=31.6 (1.2) %MSO; inhibition = 27.4 (1.0) %MSO). Mean latencies of excitation and inhibition were longer (p<0.05) in the patients (excitation=35 (1.8) ms; inhibition = 47.1 (1.8) ms) than in the controls (excitation = 21.1 (1.6) ms; inhibition = 27 (0.4) ms). Furthermore, the latency difference (inhibition-excitation) was longer (p<0.05) in the patients (10.4 (2.1) ms) than in the controls (6.2 (0.6) ms). CONCLUSION: Increased thresholds and latencies of excitation and inhibition may reflect degraded corticospinal transmission in the spinal cord. However, the relatively greater increase in the latency of inhibition compared with excitation in the patients with iSCI may reflect a weak or absent early component of cortical inhibition. Such a change in cortical inhibition may relate to the restoration of useful motor function after iSCI.

Motor unit discharge characteristics during voluntary contraction in patients with incomplete spinal cord injury.

Synchronisation of motor unit discharges is commonly seen in hand muscles of normal man but is absent following neurologically complete spinal cord injury and reduced after stroke. These findings support the notion that some corticospinal inputs to motoneurones are shared and contribute to the observed synchrony of discharge. In this study we have examined motor unit discharge in hand muscles below the level of an incomplete spinal cord injury in an attempt to relate strength of synchrony to the integrity of the corticospinal tract. Eight patients with incomplete spinal cord injury (neurological level C3-C7) and eight control subjects took part in the study. The patients had sustained injury 14-191 weeks prior to the recordings and had since regained good motor function in their hands. Two concentric needle electrodes were inserted into the first dorsal interosseus muscle which subjects were instructed to contract weakly so that potentials from individual motor units could be reliably identified on both recordings. Synchrony was detected by constructing cross-correlograms between the discharges of pairs of individual motor units. The amount of synchronous firing was determined from the magnitude of any peak in the cross-correlogram, as the probability above chance (XP) of one motor unit firing with respect to the other and vice versa. The degree of synchrony was lower (P < 0.05) in the patient group (mean XP 0.06) than in the control group (mean XP 0.09). The incidence of significant synchrony was lower in the patient group (41.8 %) than in the control group (92.9 %). The mean (+/- S.E.M.) frequency of motor unit discharge was slightly lower (P < 0.05) in patients (9.7 +/- 0.4 impulses s-1) than controls (10.8 +/- 0.5 impulses s-1). The mean width of synchrony peaks was narrower (P < 0.05) in patients (11.4 +/- 1.1 ms) than controls (13.2 +/- 0.6 ms). We conclude that the weaker synchrony of motor unit discharge in incomplete spinal cord injury may reflect permanent damage to some corticospinal axons.