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 < or =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.

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.Abstract HCN channels are responsible for I h , a voltage-gated inwardly rectifying current activated by hyperpolarization. This current appears to be more active in human sensory axons than motor and may play a role in the determination of threshold. Differences in I h are likely to be responsible for the high variability in accommodation to hyperpolarization seen in different subjects. The aim of this study was to characterise this current in human axons, both motor and sensory. Recordings of multiple axonal excitability properties were performed in 10 subjects, with a focus on the changes in threshold evoked by longer and stronger hyperpolarizing currents than normally studied. The findings confirm that accommodation to hyperpolarization is greater in sensory than motor axons in all subjects, but the variability between subjects was greater than the modality difference. An existing model of motor axons was modified to take into account the behaviour seen with longer and stronger hyperpolarization, and a mathematical model of human sensory axons was developed based on the data collected. The differences in behaviour of sensory and motor axons and the differences between different subjects are best explained by modulation of the voltage dependence, along with a modest increase of expression of the underlying conductance of I h . Accommodation to hyperpolarization for the mean sensory data is fitted well with a value of −94.2 mV for the mid-point of activation (V 0.5 ) of I h as compared to −107.3 mV for the mean motor data. The variation in response to hyperpolarization between subjects is accounted for by varying this parameter for each modality (sensory: −89.2 to −104.2 mV; motor −87.3 to −127.3 mV). These voltage differences are within the range that has been described for physiological modulation of I h function. The presence of slowly activated I h isoforms on both motor and sensory axons was suggested by modelling a large internodal leak current and a masking of the Na + /K + -ATPase pump activity by a tonic depolarization. In addition to an increased activation of I...

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.

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Key points• In six healthy subjects, the excitability of both motor and sensory axons was altered during hyperthermia, lowering their safety margin.• The results suggest that slow K + channels play a significant role in these changes in axonal excitability during hyperthermia.• Inward rectification was reduced during hyperthermia, and the modelling suggests that the hyperpolarization-activated cation current, I h , was reduced, thus hampering its ability to counter activity-dependent hyperpolarization.• Hyperthermia lowers the safety margin for action potential generation and propagation.Differences in their responses to hyperthermia suggest that motor axons undergo conduction block more readily than sensory axons during fever, particularly when the safety margin is already impaired.Abstract Hyperthermia challenges the nervous system's ability to transmit action potentials faithfully. Neuromuscular diseases, particularly those involving demyelination have an impaired safety margin for action potential generation and propagation, and symptoms are commonly accentuated by increases in temperature. The aim of this study was to examine the mechanisms responsible for reduced excitability during hyperthermia. Additionally, we sought to determine if motor and sensory axons differ in their propensity for conduction block during hyperthermia. Recordings of axonal excitability were performed at normal temperatures and during focal hyperthermia for motor and sensory axons in six healthy subjects. There were clear changes in excitability during hyperthermia, with reduced superexcitability following an action potential, faster accommodation to long-lasting depolarization and reduced accommodation to hyperpolarization. A verified model of human motor and sensory axons was used to clarify the effects of hyperthermia. The hyperthermia-induced changes in excitability could be accounted for by increasing the modelled temperature by 6• C (and adjusting the maximum conductances and activation kinetics according to their Q 10 values; producing a 2 mV hyperpolarization of resting membrane potential), further hyperpolarizing the voltage dependence of I h (motor, 11 mV; sensory, 7 mV) and adding a small depolarizing current at the internode (motor, 20 pA; sensory, 30 pA). The modelling suggested that slow K + channels play a significant role in reducing axonal excitability during hyperthermia. The further hyperpolarization of the activation of I h would limit its ability to counter the hyperpolarization produced by activity, thereby allowing conduction block to occur during hyperthermia.J. Howells and D. Czesnik have contributed equally to this work as joint first authors.

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, I NaP . 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 I NaP . Instead they pointed to greater activity of the hyperpolarization-activated inwardly rectifying current (I h ) 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.

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.

A transient decrease in excitability occurs regularly during the S1 phase of threshold electrotonus to depolarizing conditioning stimuli for sensory and, less frequently, motor axons. This has been attributed to the outwardly rectifying action of fast K + channels, at least in patients with demyelinating diseases. This study investigates the genesis of this notch in healthy axons. Threshold electrotonus was recorded for sensory and motor axons in the median nerve at the wrist in response to test stimuli of different width. The notch occurred more frequently the briefer the test stimulus, and more frequently in sensory studies. In studies on motor axons, the notch decreased in latency and increased in amplitude as the conditioning stimulus increased or the limb was cooled. Low-threshold axons displayed profound changes in strength-duration time constant even though the threshold electrotonus curves contained no detectable notch. When a 1.0 ms current was added to subthreshold conditioning stimuli to trigger EMG, the notch varied with the timing and intensity of the brief current pulse. This study finds no evidence for an outwardly rectifying deflection due to K + channels, other than the slow accommodation attributable to slow K + currents. In normal motor axons, a depolarization-induced notch during the S1 phase of threshold electrotonus is the result of the conditioning stimulus exceeding threshold for some axons. The notch is more apparent in sensory axons probably because of the lower slope of the stimulus-response curve and their longer strength-duration time constant rather than a difference in K + conductances. This may also explain the notch in demyelinating diseases.