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Node of Ranvier as an Array of Bio-Nanoantennas for Infrared Communication in Nerve Tissue

Article

Abstract

Electromagnetic radiation, in the visible and infrared spectrum, is increasingly being investigated for its possible role in the most evolved brain capabilities. Beside experimental evidence of electromagnetic cellular interactions, the possibility of light propagation in the axon has been recently demonstrated using computational modelling, although an explanation of its source is still not completely understood. We studied electromagnetic radiation onset and propagation at optical frequencies in myelinated axons, under the assumption that ion channel currents in the node of Ranvier behave like an array of nanoantennas emitting in the wavelength range from 300 to 2500 nm. Our results suggest that the wavelengths below 1600 nm are most likely to propagate throughout myelinated segments. Therefore, a broad wavelength window exists where both generation and propagation could happen, which in turn raises the possibility that such a radiation may play some role in neurotransmission.

Introduction

The intriguingly complex nature of the brain has always encouraged extensive studies on neuronal communication, aiming to understand signaling mechanisms and their integration into neural functions of the highest level. New perspectives have been revealed by approaching different biophysical mechanisms, which may coexist with the established chemical and electrical properties of cellular membranes1. In this context, previous studies on the electromagnetic properties of neurons gained increasing interest, resulting in further achievements and new open questions2,3,4. It seems therefore appropriate to explore the possible implications, which may add further knowledge to the current theoretical and experimental work in this direction.

Since early decisive studies, electrochemical phenomena have been shown to be predominant in the generation and traveling of information5.

The conduction of signals within neurons is sustained by a propagating phenomenon known as action potential (AP), which is a sharp change in the electrical potential across the cell membrane, in which different ionic species are involved. Once triggered, this process travels down the whole axon towards synapses. Some axons are coated with myelin, a multilayered lipid envelope, provided by surrounding glial cells and interrupted at regular distances. These gaps are called nodes of Ranvier (NR)6. In myelinated fibers the AP is triggered in the axon initial segment (AIS) and in the NR, where ion channels are concentrated, and leaps from node to node at a rate significantly higher than in unmyelinated axons. This process is known as “saltatory conduction”7.

The original Hodgkin–Huxley (HH) theory models each component of an excitable cell as an electrical element, taking into account the concentration of the main ionic species involved5. The transmission of APs in myelinated fibers has been described borrowing some concepts of the cable theory to simulate impulse initiation and saltatory propagation8.

Beside the fundamental mechanisms of neuronal membrane excitability described by the HH model, a number of other biophysical phenomena are associated with neuronal activity1.

Different physical approaches to these processes, which take into account mechanical forces, thermodynamics and electromagnetism, drew growing interest from researchers and may provide further understanding of the mechanisms underlying neuronal signaling and encoding of information2,9.

We focused our attention on the possible electromagnetic (EM) aspects of axonal impulse conduction, which have been investigated so far. Optical propagation of photons through myelinic waveguides has been recently shown to be possible by detailed modeling, and therefore raising the question of what could be the source of such radiation4.

Like any other cellular process, axonal activity involves energy generation and exchange. Since early investigations on neuronal function, measurements during action potential revealed the production of heat10, while infrared radiation transfer between nerve ends, following stimulation, has been experimentally detected11.

Beyond these reports, many researchers have been considering a possible role of EM radiation, either of the infrared or visible spectrum, in neural excitability and signaling, resulting in theoretical work on what has been referred to as an electromagnetic theory of neural communication2.

Actually, the existence and transport of infrared and visible light have been recently demonstrated in different tissues and even in nerves3,12,13.

Next to the studies on the existence of photon emissions as possible carriers of cellular information, different hypotheses of EM propagation through membranes or axonal structures have been advanced14, until recently, when a comprehensive model described the possible propagation of EM waves through optical communication pathways in the axon4. Alongside a growing interest in the interaction between EM radiations and biological tissues for its diagnostic and therapeutic implications, some evidence of axonal response to infrared and visible light has been observed, adding a further step towards an EM interpretation of neuronal signaling15.

However, as suggested by the aforementioned study on optical communication pathways in the axon, the possible sources of EM waves in certain cellular compartments have not yet been explored beyond speculative hypotheses and need further investigation4.

If we suppose that the propagation of action potentials in myelinated fibers occurs along axonal structures in the form of electromagnetic optical waves, then an explanation of their origin is needed. This should take place at the NR, where the action potential is regenerated. In particular, since electromagnetic radiation can be generated by charge movements, it could be hypothesized that its source relies on the flux of ions through channels in the time frame where action potential takes place.

Here we describe a model of generation of electromagnetic waves by active sodium channels at the sites where action potential is initiated or regenerated, as in the axon initial segment and in the NR of myelinated axons. A simulation of their propagation through axonal pathways behaving as waveguides is also provided, with findings in full agreement with those recently reported4.

Schematic representation of ion channels in a Node of Ranvier behaving as nanoantenna array generating EM radiation.

[neu2neuro-blog1]

Neuroscientists have made a discovery that turns 160 years of neuroanatomy on its head. Myelin, the electrical insulating material long known to be essential for the fast transmission of impulses along the axons of nerve cells, is not as ubiquitous as thought, according to a new work. “The fact that it is the most evolved neurons, the ones that have expanded dramatically in humans, suggests that what we’re seeing might be the “future.” As neuronal diversity increases and the brain needs to process more and more complex information, neurons change the way they use myelin to “achieve” more,” says the main researcher.

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Nodes Of Ranvier