Internodal sodium channels ensure active processes under myelin manifesting in depolarizing afterpotentials

Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit

The propagation of electrical impulses along axons is highly accelerated by the myelin sheath and produces saltating or "jumping" action potentials across internodes, from one node of Ranvier to the next. The underlying electrical circuit, as well as the existence and role of submyelin conduction in saltatory conduction remain, however, elusive. Here, we made patch-clamp and high-speed voltage-calibrated optical recordings of potentials across the nodal and internodal axolemma of myelinated neocortical pyramidal axons combined with electron microscopy and experimentally constrained cable modeling. Our results reveal a nanoscale yet conductive periaxonal space, incompletely sealed at the paranodes, which separates the potentials across the low-capacitance myelin sheath and internodal axolemma. The emerging double-cable model reproduces the recorded evolution of voltage waveforms across nodes and internodes, including rapid nodal potentials traveling in advance of attenuated waves in the internodal axolemma, revealing a mechanism for saltation across time and space.

Mechanisms of sodium channel clustering and its influence on axonal impulse conduction

The efficient propagation of action potentials along nervous fibers is necessary for animals to interact with the environment with timeliness and precision. Myelination of axons is an essential step to ensure fast action potential propagation by saltatory conduction, a process that requires highly concentrated voltage-gated sodium channels at the nodes of Ranvier. Recent studies suggest that the clustering of sodium channels can influence axonal impulse conduction in both myelinated and unmyelinated fibers, which could have major implications in disease, particularly demyelinating pathology. This comprehensive review summarizes the mechanisms governing the clustering of sodium channels at the peripheral and central nervous system nodes and the specific roles of their clustering in influencing action potential conduction. We further highlight the classical biophysical parameters implicated in conduction timing, followed by a detailed discussion on how sodium channel clustering along unmyelinated axons can impact axonal impulse conduction in both physiological and pathological contexts.

Internodal mechanism of pathological afterdischarges in myelinated axons

INTRODUCTION

Recent optical recordings of transmembrane potentials in the axons of pyramidal neurons have shown that the internodal action potentials (APs) predicted in our previous studies do exist. These novel processes are not well understood. In this study we aim to clarify electrical phenomena in peripheral myelinated axons (MAs).

METHODS

We used a multi-cable Hodgkin-Huxley-type model to simulate MAs with potassium channels that were either normal or inhibited along a short region of the internodal membrane. A brief stimulus was applied to the first node.

RESULTS

We demonstrated peculiarities in the internodal APs induced by a saltatory AP: They existed across internodal membranes, were detectable in periaxonal space but not in intracellular space, propagated continuously, collided near the mid-internodes, and produced internodal sources of afterdischarges.

CONCLUSIONS

These results highlight the importance of the MA internodal regions as new therapeutic targets for avoiding afterdischarges provoked by reduced axonal fast potassium channel expression.

Simulation analysis of intermodal sodium channel function

Although most sodium ion channels clustered in nodes of Ranvier provide the physiological basis for saltatory conduction, sodium ion channels cannot be excluded from internodal regions completely. The density of internodal sodium ion channels is of the order of 10/microm2. The function of internodal sodium ion channels has been neglected for a long time; however, experimental and theoretical results show that internodal sodium ion channels play an important role in action potential propagation. In this paper, based on the compartment model, we investigate the function of internodal sodium ion channels. We find that internodal sodium ion channels can promote action potential propagation, enlarge the maximal internodal distance guaranteeing stable action potential propagation, and increase the propagation speed of action potentials. In this paper, we find an optimal conductance of internodal sodium ion channels (4-5 mS/cm2), which accords with the active internodal sodium ion conductance in a real myelinated axon. With the optimal conductance, the average sodium ion channel conductance of the axon is minimal, and the metabolic energy consumption due to ion channels is also minimal.

Simulating mild systematic and focal demyelinating neuropathies: membrane property abnormalities

This study provides numerical simulations of some of the abnormalities in the potentials and axonal excitability indices of human motor nerve fibers in simulated cases of internodal, paranodal and simultaneously of paranodal internodal demyelinations, each of them systematic or focal. A 70% reduction of the myelin lamellae (defining internodal demyelination), or of the paranodal seal resistance (defining paranodal demyelination), or simultaneously both of them (defining paranodal internodal demyelination) was uniform along the fiber length for the systematically demyelinated subtypes. These permutations were termed internodal systematic demyelination (ISD), paranodal systematic demyelination (PSD) and paranodal internodal systematic demyelination (PISD). In other tests, the same reductions of the myelin sheath parameters were used but restricted to only three (8th, 9th and 10th) consecutive internodes. Such fiber demyelinations were termed internodal focal demyelination (IFD), paranodal focal demyelination (PFD) and paranodal internodal focal demyelination (PIFD). The computations used our previous double cable model of the fibers. The axon model was comprised of 30 nodes and 29 internodes. The 70% reduction value was not sufficient to develop conduction block in all investigated demyelinations, which were regarded as mild. The membrane property abnormalities obtained in the ISD, PSD and PISD cases were quite different and abnormally greater than those in the IFD, PFD and PIFD cases. The changes in the excitability indices such as strength-duration time constants, rheobasic currents and recovery cycles in the focally demyelinated subtypes were so slight as to be essentially indistinguishable from normal values. Consequently, the excitability based approaches that have shown strong potential as diagnostic tools in systematically demyelinated conditions may not be useful in detecting mild focal demyelinations. The membrane property changes simulated in the systematically demyelinated subtypes are in good accordance with the data from patients with Charcot-Marie-Tooth disease type 1A (CMT1A) and chronic inflammatory demyelinating polyneuropathy (CIDP). The excitability abnormalities obtained in each focally demyelinated subtype match those observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome (GBS). The results indicate that the model that was used is a rather promising tool in studying the membrane property abnormalities of hereditary, chronic and acquired demyelinating neuropathies, which up till now, have not been sufficiently well understood.

Simulating focal demyelinating neuropathies: membrane property abnormalities

Membrane properties such as potentials (intracellular, extracellular, electrotonic) and axonal excitability indices (strength-duration and charge-duration curves, strength-duration time constants, rheobasic currents, recovery cycles) can now be measured in healthy subjects and patients with demyelinating neuropathies. They are regarded here in two cases of simultaneously reduced paranodal seal resistance and myelin lamellae in one to three consecutive internodes of human motor nerve fiber. The investigations are performed for 70 and 96% myelin reduction values. The first value is not sufficient to develop a conduction block, but the second leads to a block and the corresponding demyelinations are regarded as mild and severe. For both the mild and severe demyelinations, the paranodally internodally focally demyelinated cases (termed as PIFD1, PIFD2, and PIFD3, respectively, with one, two, and three demyelinated internodes) are simulated using our previous double-cable model of the fiber. The axon model consists of 30 nodes and 29 internodes. The membrane property abnormalities obtained can be observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome (GBS) and multifocal motor neuropathy (MMN). The study confirms that focal demyelinations are specific indicators for acquired demyelinating neuropathies. Moreover, the following changes have been calculated in our previous papers: (1) uniform reduction of myelin thickness in all internodes (Stephanova et al. in Clin Neurophysiol 116: 1153-1158, 2005); (2) demyelination of all paranodal regions (Stephanova and Daskalova in Clin Neurophysiol 116: 1159-1166, 2005a); (3) simultaneous reduction of myelin thickness and paranodal demyelination in all internodes (Stephanova and Daskalova in Clin Neurophysiol 116: 2334-2341, 2005b); and (4) reduction of myelin thickness of up to three internodes (Stephanova et al., in J Biol Phys, 2006a,b, DOI: 10.1007/s10867-005-9001-9; DOI: 10.1007/s10867-006-9008-x). The membrane property abnormalities obtained in the homogeneously demyelinated cases are quite different and abnormally greater than those in the case investigated here of simultaneous reduction in myelin thickness and paranodal demyelination of up to three internodes. Our previous and present results show that unless focal demyelination is severe enough to cause outright conduction block, changes are so slight as to be essentially indistinguishable from normal values. Consequently, the excitability-based approaches that have shown strong potential as diagnostic tools in systematically demyelinated conditions may not be useful in detecting mild focal demyelinations, independently of whether they are internodal, paranodal, or paranodal internodal.

Differences in membrane properties in simulated cases of demyelinating neuropathies: internodal focal demyelinations with conduction block

The aim of this study is to investigate the membrane properties (potentials and axonal excitability indices) in the case of myelin wrap reduction (96%) in one, two and three consecutive internodes along the length of human motor nerve fibre. The internodally focally demyelinated cases (termed as IFD1, IFD2 and IFD3, respectively, with one, two and three demyelinated internodes are simulated using our previous double cable model of the fibre. The progressively greater increase of focal loss of myelin lamellae blocks the invasion of the intracellular potentials into the demyelinated zones. For all investigated cases, the radial decline of the extracellular potential amplitudes increases with the increase of the radial distance and demyelination, whereas the electrotonic potentials show a decrease in the slow part of the depolarizing and hyperpolarizing responses. The time constants are shorter and the rheobases higher for the IFD2 and IFD3 cases than for the normal case. In the recovery cycles, the same cases have less refractoriness, greater supernormality and less late subnormality than the normal case. The simulated membrane abnormalities can be observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome. The study provides new information about the pathophysiology of acquired demyelinating neuropathies.