Conduction In DeMyelinated Axons

The prominent symptoms associated with Central DeMyelinating Diseases such as Multiple Sclerosis (MS) are primarily caused by Conduction Deficits in affected Axons.

The symptoms may go into remission, but the mechanisms underlying remissions are uncertain. One factor that could be important is the restoration of Conduction to affected Axons.

But, it is not known whether DeMyelinated Central Axons resemble their Peripheral counterparts in being able to conduct, in the absence of repair by ReMyelination.

In the present study we have made Intra-Axonal recordings from Central Axons affected by a DeMyelinating lesion.

And then the Axons have been labeled IonoPhoretically, to permit their subsequent identification.

UltraStructural examination of 23 labeled preparations has established that some Segmentally DeMyelinated Central Axons can conduct.

And, that they can do so over continuous lengths of DeMyelination exceeding several InterNodes (2500 micron).

Such segmentally DeMyelinated Central Axons were found to conduct with the anticipated reduction in velocity and a Refractory Period of Transmission (RPT).

As much as 34 times the value obtained from the NonDeMyelinated portion of the same Axon; the RPT was typically prolonged to 2-5 times the normal value.
(View: Image)

We conclude that some Segmentally DeMyelinated Central Axons can conduct.

And, we propose that the restoration of Conduction to such Axons is likely to contribute to the remissions commonly observed in Diseases such as MS.

Voltage-gated Sodium Channels are largely localized to the Nodes of Ranvier in Myelinated Axons, providing the physiological basis for Saltatory Conduction.

Studies using AntiSodium Channel AntiBodies have shown that along DeMyelinated Axons, Sodium Channels form new distributions.

The nature of this changed distribution appears to vary with the time course and mechanism of DeMyelination.

In chronic DeMyelination, Sodium Channels increase in number and redistribute along previously InterNodal Axon segments.

In chronic DeMyelination produced by Doxorubicin, the increase in Sodium Channels appeared independently of Schwann Cells, suggesting increased Neuronal synthesis.

In acute DeMyelination produced by LysoLecithin new clusters of Sodium Channels developed.

But, only in association with the edges of ReMyelinating Schwann Cells, which appeared to control the distribution and mobility of the Channels.

These findings affirm the plasticity of Sodium Channels in DeMyelinated Axons and are relevant to understanding how these Axons recover Conduction.

4-AminoPyridine (4AP) and TetraEthylAmmonium Ions (TEA), which block voltage-dependent Potassium Channels in other Nerve membranes.

Have been used, to study nerve Conduction in Fibers of normal rat Spinal Roots and those DeMyelinated with Diphtheria Toxin.

The pharmacological actions have been compared with those of Temperature.

Both TEA and 4AP increased the amplitude and duration of the monophasically recorded compound Action Potentials of Non-Myelinated Fibers in normal rat Dorsal Roots.

Enhancement of the Action Potential amplitude by 4AP was maximal near 1 mM and was not readily reversed by washing. At concentrations up to 50 mM the action of TEA was weaker and reversible.

In normal Dorsal and Ventral Roots TEA (20 mM) and 4AP (5 mM) had only a mildly depressant action on the compound Action Potentials of Myelinated Fibers.

Whereas the slight reduction in peak amplitude and increase in width was also found in a single Fibers treated with TEA.

None was discerned in single Fibers exposed to 4AP over a wide temperature range.

It is concluded that voltage-dependent Potassium Channels occur in significant numbers in mammalian Non-Myelinated Fibers, but not at Nodes of Ranvier.

Spinal Roots previously treated with Diphtheria Toxin to cause DeMyelination were studied by longitudinal current analysis.

Fibers affected by Diphtheria Toxin had a late phase of outward current.

Either restricted to Nodes or, in the case of continuous Conduction, distributed along InterNodes, and this outward current was specifically blocked by 4AP.

Both 4AP and TEA increased the temperature at which Conduction Block occurred in most single DeMyelinated Fibers.

So that in some cases, Fibers blocked at physiological temperatures were enabled to conduct. 4AP was more potent than TEA, but less consistent in its effect.

It is concluded that Potassium Channels are present at widened Nodes and in InterNodal Axolemma exposed by DeMyelination.

Their presence enables TEA and 4AP to overcome Conduction Block in some DeMyelinated Nerve Fibers.

Elevations in temperature may produce Conduction Block in DeMyelinated Neurons.

A well-described phenomenon in Multiple Sclerosis, it has also been reported in some patients with Inflammatory DeMyelinating PolyNeuropathies.

We used Carpal Tunnel Syndrome (CTS) as a model to study the effect of Heat on Nerves with Focal DeMyelination Secondary to Chronic Compression.

Compound Motor and Sensory responses were measured in 12 CTS patients and 12 normal subjects at 32 degrees C and with heating to 42 degrees C.

Changes in relative Motor response amplitude and area were similar for both normal subjects and CTS patients.

In CTS patients, however, Sensory response amplitude and area decreased 34.3% and 48.9%.

Significantly more than the 25.2% and 39.1% reductions in normal subjects (P=0.021 and P=0.018 respectively).

We hypothesize that these reductions in response amplitude are secondary to the occurrence of heat-induced Conduction Block in DeMyelinated Sensory Neurons.

As Oligodendrocytes wrap Axons of the Central Nervous System (CNS) with insulating Myelin sheaths, Sodium Channels that are initially continuously distributed along Axons, become segregated into regularly spaced gaps in the Myelin called Nodes of Ranvier.

It is not known whether the regular spacing of Nodes results from regularly spaced Glial contacts or is instead intrinsically specified by the Axonal CytoSkeleton.

Contact with Schwann Cells induces clustering of Sodium Channels along the Axons of Peripheral Neurons in vitro and in vivo.

Similarly, it has been suggested that Astrocytes contact induces clustering of Sodium Channels along CNS Axons.

Here we show that Oligodendrocytes are necessary for clustering of Sodium Channels in vitro and in vivo.

The induction, but not the maintenance, of Sodium-Channel clustering along the Axons of highly purified rat Retinal Ganglion Cells in culture depends on a protein secreted by Oligodendrocytes.

Surprisingly, the Oligodendrocyte-induced clusters are regularly spaced at the predicted interval in the absence of Glial-Axonal contact.

Mutant rats that are deficient in Oligodendrocytes develop few Axonal Sodium Channel clusters in vivo. These results demonstrate a crucial role for Oligodendrocytes in inducing clustering of Sodium Channels.

Two major Glial Cell types, the Oligodendrocyte (which produces Myelin) and the PeriNodal Astrocyte (which contacts the Sodium Channel-rich Axon membrane at the Nodes of Ranvier), collaborate with the Axon in forming the central Myelinated Fiber.

Astrocytes have been demonstrated to synthesize Voltage-sensitive Sodium Channels, and in some Astrocytes these Channels have Neuronal properties.

A variety of lines of evidence suggest that Astrocytes play an important role in the reorganization of the Axon membrane following DeMyelination. Possible functions of Astrocytes include:

1. The targeting, or anchoring, of Ion Channels at specific sites within the Axon membrane

2. The specification of Channel characteristics as a result of production of ExtraCellular Matrix molecules
   • - that interact with the glycosylated dome of Sodium Channels

3. The synthesis of Channels that are subsequently transferred to Axons

4. It is also possible that Astrocytes participate in Ionic homeostasis at the Nodes of Ranvier

A second major Glial Cell type, the Oligodendrocyte, produces Myelin during normal development.

Morphologic studies have demonstrated that transplantation of Myelin-forming Cells, or their Precursors, can result in the production of Myelin with normal structure in the Myelin-deprived CNS.

In recent physiological studies, the function of Axons following Myelination by exogenous, transplanted Glial Cells has been examined.

In regions of Glial Cell transplantation, there are significant increases in Conduction velocities that approach normal values.

Both of these lines of investigation, on Astrocytes and Myelin-forming Oligodendrocytes/Oligodendrocyte Precursors.

Suggest, that manipulation of Glial Cell properties may emerge as a viable strategy for promoting the recovery of Conduction in CNS White Matter Axons following DeMyelination.

Until now, no direct ElectroPhysiological information has been available on the molecular basis of human Nerve excitability.

We here report patch-clamp recordings, both single- and multiChannel, from acutely dissociated human Axons.

Voltage-dependent Sodium Channels with a Conductance gamma = 13 pS (measured in Ringer at room temperature) are found in the Nodal area.

Types of Voltage-dependent Potassium Channels:
1. I Channels (gamma = 34 pS)
2. F Channels (gamma = 50 pS)
3. Channels with small Conductance measured in high Potassium solution (gamma = 7-9 pS)

Most of them are closely similar to those already reported in xenopus and rat Axons; in addition a 200-pS, Calcium-dependent Potassium Channel, similar to that in xenopus, is present.

Differences between the electrical behavior of human Axons and those of other species are probably not due to the presence of fundamentally different Channel types.

But, may be due to differences in Channel density or distribution.

As well as increasing our understanding of the basis of excitability in human Nerve, this method may prove useful in the investigation of inherited and other human Neuropathies.

Redistribution of Sodium Channels along DeMyelinated pathways in Multiple Sclerosis (MS) could be an important event in restoring Conduction prior to other reparative mechanisms such as ReMyelination.

Sodium Channels in human Multiple Sclerosis lesions were identified by quantitative light microscopic AutoRadiography using tritiated SaxiToxin (STX), a highly specific Sodium Channel Ligand.

DeMyelinated areas in various Central Nervous System regions containing denuded but vital Axons exhibited a high increase of STX-binding sites by up to a factor of 4 as compared to normal human White Matter.

This important finding could explain aspects of fast clinical remissions and 'silent' MS lesions on functional and morphological properties.

DeMyelinated Axons may functionally reorganize their membranes and adapt properties, similar to those of slow conducting UnMyelinated Nerve Fibers which have a higher amount and a more diffuse distribution of STX binding sites.

This report is the first description of an altered distribution of Voltage-sensitive Sodium Channels in human Multiple Sclerosis lesions.

Data are herein reviewed which show that there are structural differences between Nodal and InterNodal membrane in normal Myelinated fibers.

It appears likely that in most normal Myelinated fibers, Ionic Channel densities in the InterNodal Axon membrane are lower than those at the Nodes of Ranvier.

Conduction through DeMyelinated fibers may require structural reorganization, for example, redistribution, or production of new Channels in the InterNodal membrane.

Impedance mismatch may also block Conduction at sites of DeMyelination.