Simulating focal demyelinating neuropathies: membrane property abnormalities

Sensory root demyelination: Transforming touch into pain

The normal feeling of touch is vital for nearly every aspect of our daily life. However, touching is not always felt as touch, but also abnormally as pain under numerous diseased conditions. For either mechanistic understanding of the faithful feeling of touch or clinical management of chronic pain, there is an essential need to thoroughly dissect the neuropathological changes that lead to painful touch or tactile allodynia and their corresponding cellular and molecular underpinnings. In recent years, we have seen remarkable progress in our understanding of the neural circuits for painful touch, with an increasing emphasis on the upstream roles of non-neuronal cells. As a highly specialized form of axon ensheathment by glial cells in jawed vertebrates, myelin sheaths not only mediate their outstanding neural functions via saltatory impulse propagation of temporal and spatial precision, but also support long-term neuronal/axonal integrity via metabolic and neurotrophic coupling. Therefore, myelinopathies have been implicated in diverse neuropsychiatric diseases, which are traditionally recognized as a result of the dysfunctions of neural circuits. However, whether myelinopathies can transform touch into pain remains a long-standing question. By summarizing and reframing the fragmentary but accumulating evidence so far, the present review indicates that sensory root demyelination represents a hitherto underappreciated neuropathological change for most neuropathic conditions of painful touch and offers an insightful window into faithful tactile sensation as well as a potential therapeutic target for intractable painful touch.

Simulating perinodal changes observed in immune-mediated neuropathies: impact on conduction in a model of myelinated motor and sensory axons

Immune-mediated neuropathies affect myelinated axons, resulting in conduction slowing or block that may affect motor and sensory axons differently. The underlying mechanisms of these neuropathies are not well understood. Using a myelinated axon model, we studied the impact of perinodal changes on conduction. We extended a longitudinal axon model (41 nodes of Ranvier) with biophysical properties unique to human myelinated motor and sensory axons. We simulated effects of temperature and axonal diameter on conduction and strength-duration properties. We then studied effects of impaired nodal sodium channel conductance and paranodal myelin detachment by reducing periaxonal resistance, as well as their interaction, on conduction in the 9 middle nodes and enclosed paranodes. Finally, we assessed the impact of reducing the affected region (5 nodes) and adding nodal widening. Physiological motor and sensory conduction velocities and changes to axonal diameter and temperature were observed. The sensory axon had a longer strength-duration time constant. Reducing sodium channel conductance and paranodal periaxonal resistance induced progressive conduction slowing. In motor axons, conduction block occurred with a 4-fold drop in sodium channel conductance or a 7.7-fold drop in periaxonal resistance. In sensory axons, block arose with a 4.8-fold drop in sodium channel conductance or a 9-fold drop in periaxonal resistance. This indicated that motor axons are more vulnerable to developing block. A boundary of block emerged when the two mechanisms interacted. This boundary shifted in opposite directions for a smaller affected region and nodal widening. These differences may contribute to the predominance of motor deficits observed in some immune-mediated neuropathies.NEW & NOTEWORTHY Immune-mediated neuropathies may affect myelinated motor and sensory axons differently. By the development of a computational model, we quantitatively studied the impact of perinodal changes on conduction in motor and sensory axons. Simulations of increasing nodal sodium channel dysfunction and paranodal myelin detachment induced progressive conduction slowing. Sensory axons were more resistant to block than motor axons. This could explain the greater predisposition of motor axons to functional deficits observed in some immune-mediated neuropathies.

Physiological Dynamics in Demyelinating Diseases: Unraveling Complex Relationships through Computer Modeling

Despite intense research, few treatments are available for most neurological disorders. Demyelinating diseases are no exception. This is perhaps not surprising considering the multifactorial nature of these diseases, which involve complex interactions between immune system cells, glia and neurons. In the case of multiple sclerosis, for example, there is no unanimity among researchers about the cause or even which system or cell type could be ground zero. This situation precludes the development and strategic application of mechanism-based therapies. We will discuss how computational modeling applied to questions at different biological levels can help link together disparate observations and decipher complex mechanisms whose solutions are not amenable to simple reductionism. By making testable predictions and revealing critical gaps in existing knowledge, such models can help direct research and will provide a rigorous framework in which to integrate new data as they are collected. Nowadays, there is no shortage of data; the challenge is to make sense of it all. In that respect, computational modeling is an invaluable tool that could, ultimately, transform how we understand, diagnose, and treat demyelinating diseases.

Differences between the channels, currents and mechanisms of conduction slowing/block and accommodative processes in simulated cases of focal demyelinating neuropathies

To clarify the differences between the mechanisms of conduction slowing/block and accommodative processes in focal demyelinating neuropathies, this computational study presents the kinetics of the ionic, transaxonal and transmyelin currents defining the intracellular and electrotonic potentials in different segments of human motor nerve fibres. The computations use our previous double cable model of the fibres. The simulated fibres have focal demyelination of internodes, paranodes or both together. The intracellular potentials are defined mainly by the Na+ current, as the contribution of the K+ fast and K+ slow currents to the total nodal ionic current is negligible. The paranodal demyelinations cause an increase in the transaxonal current and a decrease in the transmyelin current at the paranodal segments. However, there is an inverse relationship between the transaxonal and transmyelin currents at the same segments in the cases of internodal demyelination. The internodal ionic channels beneath the myelin sheath do not contribute to the intracellular potentials, but they show a high sensitivity to long-lasting pulses. The slow components of the electrotonic potentials depend on the activation of the channel types in the nodal or internodal axolemma, whereas the fast components of the potentials are determined mainly by the passive cable responses. However, the current kinetics changes (defining the investigated electrotonic changes) are relatively weak. The study summarizes the results from these modelling investigations on the mechanisms underlying the conduction slowing/block and accommodative processes in focal demyelinating neuropathies such as Guillain-Barré syndrome and multifocal motor neuropathy.

Membrane property abnormalities in simulated cases of mild systematic and severe focal demyelinating neuropathies

The investigation of multiple nerve membrane properties by mathematical models has become a new tool to study peripheral neuropathies. In demyelinating neuropathies, the membrane properties such as potentials (intracellular, extracellular, electrotonic) and indices of axonal excitability (strength-duration time constants, rheobases and recovery cycles) can now be measured at the peripheral nerves. This study provides numerical simulations of the membrane properties of human motor nerve fibre in cases of internodal, paranodal and simultaneously of paranodal internodal demyelinations, each of them mild systematic or severe focal. The computations use our previous multi-layered model of the fibre. The results show that the abnormally greater increase of the hyperpolarizing electrotonus, shorter strength-duration time constants and greater axonal superexcitability in the recovery cycles are the characteristic features of the mildly systematically demyelinated cases. The small decrease of the polarizing electrotonic responses in the demyelinated zone in turn leads to a compensatory small increase of these responses outside the demyelinated zone of all severely focally demyelinated cases. The paper summarizes the insights gained from these modeling studies on the membrane property abnormalities underlying the variation in clinical symptoms of demyelination in Charcot-Marie-Tooth disease type 1A, chronic inflammatory demyelinating polyneuropathy, Guillain-Barré syndrome and multifocal motor neuropathy. The model used provides an objective study of the mechanisms of these diseases which up till now have not been sufficiently well understood, because quite different assumptions have been given in the literature for the interpretation of the membrane property abnormalities obtained in hereditary, chronic and acquired demyelinating neuropathies.