Latent addition in human motor and sensory axons: Different site-dependent changes across the carpal tunnel related to persistent Na+ currents

The I/T curve coefficient for evaluating changes in neuromuscular excitability after polarized light irradiation - a placebo-controlled randomized trial

Introduction: PILER light affects the sensory and motor excitability of the tissue, and these changes may depend on the color of the filter used in the irradiations. Objective: To evaluate changes in neuromuscular excitability after PILER irradiation with different filters. To evaluate the usefulness of the I/T curve coefficient in neuromuscular excitability test. Material and methods: 60 healthy volunteers assigned to four groups: group v - without filter (n=15), group x - red filter (n=15), group y - blue filter (n=15), group z - placebo (n=15) had biceps brachii irradiated with PILER light. Outcome Measures: I/T curve coefficient for rectangular (■I/T coeff) and triangular (▲I/T coeff) pulses for sensory and motor excitability and the pressure pain threshold (PPT). Results: ■I/T coeff (p=0.0013) and ▲I/T coeff (p=0.0011) for sensory excitability increased significantly in the irradiated group. ■I/T coeff (p=0.0356) and ▲I/T coeff (p=0.0022) increased significantly after blue light irradiation. A significant increase in the▲I/T coeff (p=0.0439) in motor excitability was observed in the irradiated group. ■I/T coeff (p=0.0309) and ▲I/T coeff (p=0.0064) increased significantly after blue light irradiation. Conclusion: PILER light may reduce muscle excitability. Using a blue filter may increase the sensory threshold, and myorelaxation. Further experiments are necessary to confirm the usefulness of the I/T curve coefficient.

Measurement of axonal excitability: Consensus guidelines

Measurement of axonal excitability provides an in vivo indication of the properties of the nerve membrane and of the ion channels expressed on these axons. Axonal excitability techniques have been utilised to investigate the pathophysiological mechanisms underlying neurological diseases. This document presents guidelines derived for such studies, based on a consensus of international experts, and highlights the potential difficulties when interpreting abnormalities in diseased axons. The present manuscript provides a state-of-the-art review of the findings of axonal excitability studies and their interpretation, in addition to suggesting guidelines for the optimal performance of excitability studies.

Antidromic vs orthodromic sensory median nerve conduction studies

Objective

Median sensory nerve conduction studies are arguably the most often performed electrodiagnostic tests worldwide. Routine tests in clinical practice are done using either antidromic or orthodromic techniques type of stimulation, with no universal agreement on the use of one or the other technique.

Methods

We review the advantages and drawbacks of antidromic and orthodromic as well as their particularities for clinical application and research.

Results

The two techniques differ on how physical and physiological changes affect the action potential. Near-nerve recording is better suited for the orthodromic than for the antidromic technique, while studies of nerve excitability are better suited for the antidromic than for the orthodromic technique.

Conclusion

Both techniques are equally suitable for routine tests but research studies may specifically demand one or the other.

Coupled left-shift of Nav channels: modeling the Na⁺-loading and dysfunctional excitability of damaged axons

Injury to neural tissue renders voltage-gated Na⁺ (Nav) channels leaky. Even mild axonal trauma initiates Na⁺-loading, leading to secondary Ca²⁺-loading and white matter degeneration. The nodal isoform is Nav1.6 and for Nav1.6-expressing HEK-cells, traumatic whole cell stretch causes an immediate tetrodotoxin-sensitive Na⁺-leak. In stretch-damaged oocyte patches, Nav1.6 current undergoes damage-intensity dependent hyperpolarizing- (left-) shifts, but whether left-shift underlies injured-axon Nav-leak is uncertain. Nav1.6 inactivation (availability) is kinetically limited by (coupled to) Nav activation, yielding coupled left-shift (CLS) of the two processes: CLS should move the steady-state Nav1.6 "window conductance" closer to typical firing thresholds. Here we simulated excitability and ion homeostasis in free-running nodes of Ranvier to assess if hallmark injured-axon behaviors--Na⁺-loading, ectopic excitation, propagation block--would occur with Nav-CLS. Intact/traumatized axolemma ratios were varied, and for some simulations Na/K pumps were included, with varied in/outside volumes. We simulated saltatory propagation with one mid-axon node variously traumatized. While dissipating the [Na⁺] gradient and hyperactivating the Na/K pump, Nav-CLS generated neuropathic pain-like ectopic bursts. Depending on CLS magnitude, fraction of Nav channels affected, and pump intensity, tonic or burst firing or nodal inexcitability occurred, with [Na⁺] and [K⁺] fluctuating. Severe CLS-induced inexcitability did not preclude Na⁺-loading; in fact, the steady-state Na⁺-leaks elicited large pump currents. At a mid-axon node, mild CLS perturbed normal anterograde propagation, and severe CLS blocked saltatory propagation. These results suggest that in damaged excitable cells, Nav-CLS could initiate cellular deterioration with attendant hyper- or hypo-excitability. Healthy-cell versions of Nav-CLS, however, could contribute to physiological rhythmic firing.

Left-shifted nav channels in injured bilayer: primary targets for neuroprotective nav antagonists?

Mechanical, ischemic, and inflammatory injuries to voltage-gated sodium channel (Nav)-rich membranes of axon initial segments and nodes of Ranvier render Nav channels dangerously leaky. By what means? The behavior of recombinant Nav1.6 (Wang et al., 2009) leads us to postulate that, in neuropathologic conditions, structural degradation of axolemmal bilayer fosters chronically left-shifted Nav channel operation, resulting in E(Na) rundown. This "sick excitable cell Nav-leak" would encompass left-shifted fast- and slow-mode based persistent I(Na) (i.e., I(window) and slow-inactivating I(Na)). Bilayer-damage-induced electrophysiological dysfunctions of native-Nav channels, and effects on inhibitors on those channels, should, we suggest, be studied in myelinated axons, exploiting I(Na)(V,t) hysteresis data from sawtooth ramp clamp. We hypothesize that (like dihydropyridines for Ca channels), protective lipophilic Nav antagonists would partition more avidly into disorderly bilayers than into the well-packed bilayers characteristic of undamaged, healthy plasma membrane. Whereas inhibitors using aqueous routes would access all Navs equally, differential partitioning into "sick bilayer" would co-localize lipophilic antagonists with "sick-Nav channels," allowing for more specific targeting of impaired cells. Molecular fine-tuning of Nav antagonists to favor more avid partitioning into damaged than into intact bilayers could reduce side effects. In potentially salvageable neurons of traumatic and/or ischemic penumbras, in inflammatory neuropathies, in muscular dystrophy, in myocytes of cardiac infarct borders, Nav-leak driven excitotoxicity overwhelms cellular repair mechanisms. Precision-tuning of a lipophilic Nav antagonist for greatest efficacy in mildly damaged membranes could render it suitable for the prolonged continuous administration needed to allow for the remodeling of the excitable membranes, and thus functional recovery.

Differences in excitability properties of FDI and ADM motor axons

The first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles are innervated by the same ulnar nerve, but studies have shown that the former is much more severely affected in amyotrophic lateral sclerosis. In this study, threshold tracking was used to investigate whether membrane properties differ between FDI and ADM motor axons. In 12 normal subjects, compound muscle action potentials were recorded from FDI and ADM after ulnar nerve stimulation at the wrist. The strength-duration time constant was significantly longer in the FDI axons than in the ADM axons, and latent addition studies showed greater threshold changes at the conditioning-test stimulus of 0.2 ms in FDI than in ADM axons. These findings suggest that nodal persistent sodium conductances are more prominent in FDI axons than in ADM axons, and therefore excitability is physiologically higher in FDI axons. Even in the same nerve at the same sites, membrane properties of FDI and ADM motor axons differ significantly, and thus their axonal/neuronal responses to disease may also differ.

Change in excitability of motor axons modifies statistical MUNE results

Motor unit number estimation (MUNE) techniques--whether they reflect a true motor unit count or some related index--should not be confounded by changes in the neuromuscular system other than a decline in the number of functional motor units. In neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS), there is evidence of changes in the excitability of motor axons. If changes in axon excitability confound a particular MUNE technique, this would influence the use of that technique in ALS patients. We hypothesized on the basis of computational models that changes in axon membrane excitability would change the outcome of a statistical MUNE test, even though the true number of motor units remained unchanged. To test the validity of the model predictions we induced changes in axon excitability of healthy control subjects by applying a polarizing current while simultaneously carrying out a statistical MUNE test. In a group of 7 subjects we found a significant difference in MUNE as a result of the change in axon excitability produced by the polarizing current (paired t-test, P < 0.05). We conclude that the statistical MUNE method is confounded by changes in axon excitability. Since increasing evidence shows that axon excitability is altered as part of the pathophysiological process underlying ALS, clinical researchers should be cautious when using statistical MUNE with this patient population.

Effects of age on excitability properties in human motor axons

OBJECTIVE

The threshold tracking technique is a new approach to non-invasively assess biophysical properties of axonal membrane in human subjects. The aim of this study was to evaluate the effects of age and gender on excitability properties of human motor axons.

METHODS

Computerized threshold tracking was used to measure multiple excitability indices in median motor axons of 93 normal subjects (50 men; age, 20-86 years).

RESULTS

Regression analyses showed that the higher age was associated with longer strength-duration time constant (p=0.03), smaller threshold changes in depolarizing threshold electrotonus (p=0.02), smaller supernormality (p=0.01), and steeper slope of the current-threshold relationship for hyperpolarizing currents (p<0.001). There were slight sex differences in rheobase, threshold electrotonus, supernormality, late subnormality, and current-threshold slope, though they were significant only in the subgroup with age <50 years.

CONCLUSIONS

Aging may increase persistent sodium currents, inward rectification, and possibly, outward potassium currents. The combination of changes raises the possibility of slight membrane depolarization in elderly people. For the sex-related differences, further studies will be required with the evaluation of sex hormonal effects.

SIGNIFICANCE

Age-related effects on excitability properties are subtle, but should be taken into consideration in the clinical application of nerve excitability testing, particularly in elderly subjects.

Changes in Na(+) channel expression and nodal persistent Na(+) currents associated with peripheral nerve regeneration in mice

Patients with peripheral neuropathy frequently suffer from positive sensory (pain and paresthesias) and motor (muscle cramping) symptoms even in the recovery phase of the disease. To investigate the pathophysiology of increased axonal excitability in peripheral nerve regeneration, we assessed the temporal and spatial expression of voltage-gated Na(+) channels as well as nodal persistent Na(+) currents in a mouse model of Wallerian degeneration. Crushed sciatic nerves of 8-week-old C57/BL6J male mice underwent complete Wallerian degeneration at 1 week. Two weeks after crush, there was a prominent increase in the number of Na(+) channel clusters per unit area, and binary or broad Na(+) channel clusters were frequently found. Excess Na(+) channel clusters were retained up to 20 weeks post-injury. Excitability testing using latent addition suggested that nodal persistent Na(+) currents markedly increased beginning at week 3, and remained through week 10. These results suggest that axonal regeneration is associated with persistently increased axonal excitability resulting from increases in the number and conductance of Na(+) channels.

Sympathetic sweat responses and skin vasomotor reflexes in carpal tunnel syndrome

OBJECTIVE

To investigate cutaneous sympathetic functions in carpal tunnel syndrome (CTS) using sympathetic sweat responses (SSwRs) and skin vasomotor reflexes (SVmRs).

METHODS

In 29 hands (20 patients) with idiopathic CTS, SSwRs were recorded with a sudorometer from the thenar eminence, and SVmRs were used to measure cutaneous blood flow using a Doppler flowmeter placed on the index finger tip. Normal data were obtained from 15 volunteers of similar age.

RESULTS

SSwRs or SVmRs were abnormal in 23 (80%) hands; SSwRs were absent in 38%, whereas SVmRs were abnormally decreased in 59%. Autonomic symptoms were present in 18 (62%) hands; finger edema (38%) and dry hand (35%) were frequent symptoms. Autonomic symptoms, and abnormal SSwRs and SVmRs did not correlate with results of nerve conduction studies.

CONCLUSIONS

Skin sudomotor or vasomotor sympathetic function is frequently impaired in CTS. Susceptibility to compression ischemia may be different in sympathetic unmyelinated and large myelinated fibers.

Increased nodal persistent Na+ currents in human neuropathy and motor neuron disease estimated by latent addition

OBJECTIVE

To investigate the changes in nodal persistent Na(+) currents in human neuropathy and motor neuron disease. In human motor axons, approximately 1.0% of total Na(+) channels are active at rest, termed "persistent" Na(+) channels, and the conductance can be non-invasively estimated by the technique of latent addition in vivo.

METHODS

Latent addition was performed in median motor axons of 93 patients with axonal neuropathy (n=38), lower motor neuron disorder (LMND; n=19) or amyotrophic lateral sclerosis (ALS; n=36) and in 27 age-matched normal subjects. Brief hyperpolarizing conditioning current pulses were delivered, and threshold change at the conditioning-test interval of 0.2 ms was measured as an estimator of the magnitude of persistent Na(+) currents. Threshold electrotonus and supernormality were also measured as indicators of resting membrane potential.

RESULTS

Threshold changes at 0.2 ms were significantly greater in patients with neuropathy or LMND (p<0.05), and tended to be greater in ALS patients (p=0.075) than in normal controls. Threshold electrotonus and supernormality did not differ in each patient group and normal controls, suggesting that membrane potential is not altered in patients. In the recovery phase of axonal neuropathy, the threshold changes increased in parallel with an increase in amplitudes of compound muscle action potential.

CONCLUSIONS

Persistent Na(+) currents appear to increase commonly in disorders involving lower motor neurons, possibly associated with axonal regeneration or collateral sprouting or changes in Na(+) channel gating.

SIGNIFICANCE

The increased axonal excitability could partly be responsible for positive motor symptoms such as muscle cramping frequently seen in lower motor neuron disorders.