James Howells

Assessment of cortical excitability using threshold tracking techniques.

Conventional paired‐pulse transcranial magnetic stimulation (TMS) techniques of assessing cortical excitability are limited by fluctuations in the motor evoked potential (MEP) amplitude. The aim of the present study was to determine the feasibility of threshold tracking TMS for assessing cortical excitability in a clinical setting and to establish normative data. Studies were undertaken in 26 healthy controls, tracking the MEP response from abductor pollicis brevis. Short‐interval intracortical inhibition (SICI) occurred up to an interstimulus interval (ISI) of 7–10 ms, with two distinct peaks evident, at ISIs of ≤1 and 3 ms, followed by intracortical facilitation to an ISI of 30 ms. Long‐interval intracortical inhibition (LICI) occurred at ISIs of 50–300 ms, peaking at 150 ms. The present study has confirmed the effectiveness of the threshold tracking TMS technique in reliably and reproducibly measuring cortical excitability. Simultaneous assessment of upper and lower motor neuronal function with threshold tracking techniques may help to determine the site of disease onset and patterns of progression in neurodegenerative diseases. Muscle Nerve, 2005

The voltage dependence of Ih in human myelinated axons

Key points  •  Pacemaking in cardiac and neuronal cells is primarily controlled by the interaction between different voltage gated ion channels, and in particular the hyperpolarization‐activated cyclic‐nucleotide gated (HCN) family of channels. •  HCN channels are activated by membrane hyperpolarization and play a key role in the determination of resting membrane potential. •  We provide evidence suggesting that differences in (i) the modulation and expression of HCN channels, (ii) the expression of slow K+ channels; and (iii) the resultant changes in resting membrane potential are the major determinants of the functional differences between human motor and sensory axons. •  Contrary to current wisdom, this study supports the view that the greater persistent Na+ current observed in sensory axons is not due to greater expression of persistent Na+ channels but instead to the relatively depolarized membrane potential driving greater resting activation.

Sensitivity and specificity of threshold tracking transcranial magnetic stimulation for diagnosis of amyotrophic lateral sclerosis: a prospective study

BACKGROUND Diagnosis of amyotrophic lateral sclerosis (ALS) remains problematic, with substantial diagnostic delays. We assessed the sensitivity and specificity of a threshold tracking transcranial magnetic stimulation (TMS) technique, which might allow early detection of upper motor neuron dysfunction, for the diagnosis of the disorder. METHODS We did a prospective study of patients referred to three neuromuscular centres in Sydney, Australia, in accordance with the Standards for Reporting of Diagnostic Accuracy. Participants had definite, probable, or possible ALS, as defined by the Awaji criteria; or pure motor disorder with clinical features of upper and lower motor neuron dysfunction in at least one body region, progressing over a 6 month follow-up period; or muscle wasting and weakness for at least 6 months. All patients underwent threshold tracking TMS at recruitment (index test), with application of the reference standard, the Awaji criteria, to differentiate patients with ALS from those with non-ALS disorders. The investigators who did the index test were masked to the results of the reference test and all other investigations. The primary outcome measures were the sensitivity and specificity of TMS in differentiating ALS from non-ALS disorders; these measures were derived from receiver operator curve analysis. FINDINGS Between Jan 1, 2010, and March 1, 2014, we screened 333 patients; 281 met our inclusion criteria. We eventually diagnosed 209 patients with ALS and 68 with non-ALS disorders; the diagnosis of four patients was inconclusive. The threshold tracking TMS technique differentiated ALS from non-ALS disorders with a sensitivity of 73·21% (95% CI 66·66-79·08) and specificity of 80·88% (69·53-89·40) at an early stage in the disease. All patients tolerated the study well, and we did not record any adverse events from performance of the index test. INTERPRETATION The threshold tracking TMS technique reliably distinguishes ALS from non-ALS disorders and, if these findings are replicated in larger studies, could represent a useful diagnostic investigation when combined with the Awaji criteria to prove upper motor neuron dysfunction at early stages of ALS. FUNDING Motor Neuron Disease Research Institute of Australia, National Health and Medical Research Council of Australia, and Pfizer.

Plasticity of inwardly rectifying conductances following a corticospinal lesion in human subjects

This study investigated whether there are changes in the excitability of motor axons in peripheral nerves of patients with corticospinal lesions, reflecting plasticity of the motoneuron due to altered descending drives and/or changes in afferent feedback. The excitability of motor and sensory axons in peripheral nerves of the affected limb of 11 patients with unilateral hemiparesis due to stroke was compared with that for the unaffected limbs and with data for 12 age‐matched controls. There was significantly less accommodation to hyperpolarizing currents in motor axons on the affected side. There were small differences between the data for the unaffected side and that of the control subjects but these were not statistically significant. Other findings indicate that there was no change in resting membrane potential. There was no comparable alteration in the excitability of sensory axons. The changes in response of motor axons to hyperpolarizing currents could be reproduced in a computer model of the human motor axon by reducing the hyperpolarization‐activated conductance, IH, by 30% and the quantitatively small leak conductance by 77%. The data for the uninvolved side matched the data for control subjects best when IH was increased. These findings are consistent with modulation of IH by activity. They demonstrate a change in the biophysical properties of motor axons not directly affected by the pathology and synaptically remote from the lesion, and have implications for ‘trans‐synaptic’ changes in central nervous system pathways. In human subjects studies of motor axon properties may allow insight into processes affecting the motoneuron.

The voltage dependence of I h in human myelinated axons

Key points  •  Pacemaking in cardiac and neuronal cells is primarily controlled by the interaction between different voltage gated ion channels, and in particular the hyperpolarization‐activated cyclic‐nucleotide gated (HCN) family of channels. •  HCN channels are activated by membrane hyperpolarization and play a key role in the determination of resting membrane potential. •  We provide evidence suggesting that differences in (i) the modulation and expression of HCN channels, (ii) the expression of slow K+ channels; and (iii) the resultant changes in resting membrane potential are the major determinants of the functional differences between human motor and sensory axons. •  Contrary to current wisdom, this study supports the view that the greater persistent Na+ current observed in sensory axons is not due to greater expression of persistent Na+ channels but instead to the relatively depolarized membrane potential driving greater resting activation.

Properties of low-threshold motor axons in the human median nerve

This study investigated the excitability and accommodative properties of low‐threshold human motor axons to test whether these motor axons have greater expression of the persistent Na+ conductance, INaP. Computer‐controlled threshold tracking was used to study 22 single motor units and the data were compared with compound motor potentials of various amplitudes recorded in the same experimental session. Detailed comparisons were made between the single units and compound potentials that were 40% or 5% of maximal amplitude, the former because this is the compound potential size used in most threshold tracking studies of axonal excitability, the latter because this is the compound potential most likely to be composed entirely of motor axons with low thresholds to electrical recruitment. Measurements were made of the strength–duration relationship, threshold electrotonus, current–voltage relationship, recovery cycle and latent addition. The findings did not support a difference in INaP. Instead they pointed to greater activity of the hyperpolarization‐activated inwardly rectifying current (Ih) as the basis for low threshold to electrical recruitment in human motor axons. Computer modelling confirmed this finding, with a doubling of the hyperpolarization‐activated conductance proving the best single parameter adjustment to fit the experimental data. We suggest that the hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channel(s) expressed on human motor axons may be active at rest and contribute to resting membrane potential.

Cortical Function in Asymptomatic Carriers and Patients With C9orf72 Amyotrophic Lateral Sclerosis

IMPORTANCE The identification of the chromosome 9 open reading frame 72 (c9orf72) gene hexanucleotide repeat expansion represents a major advance in the understanding of amyotrophic lateral sclerosis (ALS) pathogenesis. The pathophysiological mechanism by which the c9orf72 gene expansion leads to neurodegeneration is not yet elucidated. Cortical hyperexcitability is potentially an important pathophysiological process in sporadic ALS and familial ALS (FALS). OBJECTIVE To investigate whether cortical hyperexcitability forms the pathophysiological basis of c9orf72 FALS using the threshold-tracking transcranial magnetic stimulation technique. DESIGN, SETTING, AND PARTICIPANTS Prospective case-control single-center study that took place at hospitals and outpatient clinics from January 1, 2013, to January 1, 2015. Clinical and functional assessments along with transcranial magnetic stimulation studies were taken on 15 patients with c9orf72 FALS and 11 asymptomatic expansion carriers of c9orf72 who were longitudinally followed up for 3 years. Results were compared with 73 patients with sporadic ALS and 74 healthy control participants. MAIN OUTCOMES AND MEASURES Cortical excitability variables, including short-interval intracortical inhibition, were measured in patients with c9orf72 FALS and results were compared with asymptomatic c9orf72 carriers, patients with sporadic ALS, and healthy control participants. RESULTS Mean (SD) short-interval intracortical inhibition was significantly reduced in patients with c9orf72 FALS (1.2% [1.8%]) and sporadic ALS (1.6% [1.2%]) compared with asymptomatic c9orf72 expansion carriers (10.2% [1.8%]; F = 16.1; P < .001) and healthy control participants (11.8% [1.0%]; F = 16.1; P < .001). The reduction of short-interval intracortical inhibition was accompanied by an increase in intracortical facilitation (P < .01) and motor-evoked potential amplitude (P < .05) as well as a reduction in the resting motor threshold (P < .05) and cortical silent period duration (P < .001). CONCLUSIONS AND RELEVANCE This study establishes cortical hyperexcitability as an intrinsic feature of symptomatic c9orf72 expansion-related ALS but not asymptomatic expansion carriers.

After-effects of near-threshold stimulation in single human motor axons

Subthreshold electrical stimuli can generate a long‐lasting increase in axonal excitability, superficially resembling the phase of superexcitability that follows a conditioning nerve impulse. This phenomenon of ‘subthreshold superexcitability’ has been investigated in single motor axons in six healthy human subjects, by tracking the excitability changes produced by conditioning stimuli of different amplitudes and waveforms. Near‐threshold 1 ms stimuli caused a mean decrease in threshold at 5 ms of 22.1 ± 6.0% (mean ±s.d.) if excitation occurred, or 6.9 ± 2.6% if excitation did not occur. The subthreshold superexcitability was maximal at an interval of about 5 ms, and fell to zero at 30 ms. It appeared to be made up of two components: a passive component linearly related to conditioning stimulus amplitude, and a non‐linear active component. The active component appeared when conditioning stimuli exceeded 60% of threshold, and accounted for a maximal threshold decrease of 2.6 ± 1.3%. The passive component was directly proportional to stimulus charge, when conditioning stimulus duration was varied between 0.2 and 2 ms, and could be eliminated by using triphasic stimuli with zero net charge. This change in stimulus waveform had little effect on the active component of subthreshold superexcitability or on the ‘suprathreshold superexcitability’ that followed excitation. It is concluded that subthreshold superexcitability in human motor axons is mainly due to the passive electrotonic effects of the stimulating current, but this is supplemented by an active component (about 12% of suprathreshold superexcitability), due to a local response of voltage‐dependent sodium channels.

Up-regulation of slow K+ channels in peripheral motor axons: a transcriptional channelopathy in multiple sclerosis

Spinal lesions produce plastic changes in motoneuron properties. We have documented the excitability of motor axons in the median nerve of 12 patients with multiple sclerosis and 50 normal subjects, hypothesizing that plastic changes in the properties of spinal motoneurons might be reflected in the properties of peripheral motor axons and be demonstrable in vivo. In the patients, there were changes in physiological measures of axonal excitability attributable to increased slow K(+) channel activity. Other measures were within control limits. These changes could be modelled by an 11% increase in slow K(+) current, with compensatory changes in membrane potential, suggesting increased expression of the responsible channels. The changes cannot be explained solely by changes in membrane potential and are not those expected if peripheral nerve axons were involved in the inflammatory process of multiple sclerosis. They probably represent a transcriptional channelopathy, due to up-regulation of channel expression. The abnormalities do not imply that peripheral nerve function has been significantly compromised, but they do suggest that the properties of the parent motoneurons have changed. This study thus provides evidence for plasticity in motoneuronal properties at a molecular level, the first such evidence for intact human subjects.

Threshold behaviour of human axons explored using subthreshold perturbations to membrane potential.

The present study explores the threshold behaviour of human axons and the mechanisms contributing to this behaviour. The changes in excitability of cutaneous afferents in the median nerve at the wrist were recorded to a long‐lasting subthreshold conditioning stimulus, with a waveform designed to maximize the contribution of currents active in the just‐subthreshold region. The conditioning stimulus produced a decrease in threshold that developed over 3–5 ms following the end of the depolarization and then decayed slowly, in a pattern similar to the recovery of axonal excitability following a discharge. To ensure that the conditioning stimulus did not activate low‐threshold axons, similar recordings were then made from single motor axons in the ulnar nerve at the elbow. The findings were comparable, and behaviour with the same pattern and time course could be reproduced by subthreshold stimuli in a model of the human axon. In motor axons, subthreshold depolarizing stimuli, 1 ms long, produced a similar increase in excitability, but the late hyperpolarizing deflection was less prominent. This behaviour was again reproduced by the model axon and could be explained by the passive properties of the nodal membrane and conventional Na+ and K+ currents. The modelling studies emphasized the importance of leak current through the Barrett–Barrett resistance, even in the subthreshold region, and suggested a significant contribution of K+ currents to the threshold behaviour of axons. While the gating of slow K+ channels is slow, the resultant current may not be slow if there are substantial changes in membrane potential. By extrapolation, we suggest that, when human axons discharge, nodal slow K+ currents will be activated sufficiently early to contribute to the early changes in excitability following the action potential.