The Role and Importance of Myelin in Nervous System Functioning

Introduction

The myelin is a specialized multi-layer membrane that comprises of lipids and proteins, that is wrapped around the axons (Heath et al., 2018). Oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) form the myelin sheath. It is required for salutatory conduction of action potentials by ensuring the localization of voltage-gated sodium channels to the spaces between neighboring sheaths (nodes Ranveir). It also insulates the axons during signal transmission. Myelin decreases the axon’s membrane capacitance and it increases the resistance of the membrane thus displaying both passive and active properties (Rasband & Macklin, 2012).

Myelinated axons (as well as myelinated neutral circuits) conduct impulses (information) much faster than unmyelinated axons. When human beings are born they have a nearly unmyelinated CNS. However, after birth the oligodendrocyte number increases drastically and within the first few years of childhood widespread myelination can be observed (Wiliamson & Lyons, 2018). Consequently, myelin allows for a circuitry which is complex as well as compact.

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Historically, mapping of the cortical areas in human beings has been a challenge for neuroscience as it is an invasive process and was done post mortem. Of late, numerous in vivo MRI-based techniques have been developed to aid in the mapping of the cortical areas (i.e. the cerebral cortex) on the basis of the myeloarchitecture present. Myelin mapping in MRI uses approaches which identify the interactions between tissue macromolecules and water molecules. This technique uses the inverse of the longitudinal relaxation time-T1 (R1-longitudinal relaxation time and the inverse of the apparent transverse relaxation time-T2* (apparent transverse relation time- R2) (Shams et al., 2019).

Other non-invasive MRI-based techniques used are magnetization transfer imaging (MT) which is an observed attenuation of MR signal after treatment with radiofrequency irradiation. Magnetization transfer imaging ratio, where a single MT saturated pulse is applied after which it is normalized by an image that doesn’t have MT (Heath et al., 2018). Quantitative MT (QMT), myelin imaging relaxometry, steady-state multicomponent relaxometry (mcDESPOT), susceptibility weighted imaging (SWI) and Quantitative susceptibility mapping (QSM) are other MRI-based methods.

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SWI, uses the susceptibility difference between tissues to enhance the images contrast. This effect is created by examining the components of echo data, namely the gradient echo sequence and T2*- weighted magnitude image. SWI provides an additional enhancement to the tissues even though the T2*-weighted magnitude image offers some amount of susceptibility contrast. Different phase shift in MRI data is observed due to its sensitivity to magnetic and paramagnetic substances (Liu et al.,2015). Magnetic susceptibility is a quantity that can be measured by the degree in which a material is magnetized when placed in a magnetic field. Susceptibility can therefore be described as the response to the static field. Biological tissues can either be paramagnetic (have a positive susceptibility) or diamagnetic (have a negative susceptibility) which is dependent on the microstructure and the molecular content (Liu et al.,2015).

SWI images can provide a high resolution delineation of the cerebral venous as a minimum intensity projection. Therefore, it is widely used in the diagnosis of venous abnormalities. This technique is sensitive to intracranial mineral deposition and deoxygenated blood and can therefore be applied to numerous pathologies such as intracranial hemorrhage (Liu et al., 2015).

Due to the inability of the SWI to provide a quantitative measures of magnetic susceptibility there have been developments recently of quantitative susceptibility mapping (QSM). QMS calculates the underlying susceptibility of each of the voxels as a scalar quantity, unlike SWI which produces a contrast based on phase images (Liu et al., 2015).

QMS aims to quantify the susceptibility shift observed in the tissues. The unity of measurement used is Hertz (Hz) or parts per million (ppm). The susceptibility offset present in a particular voxel affects the whole image and therefore it is not possible for the image to be fitted on a voxelwise. The magnetization of an imagining voxel is treated as a magnetic dipole, where a magnetic field that extends beyond the voxel itself is produced by each dipole. The magnetic field at any particular dipole is therefore a superposition of all the dipole fields present (Liu et al., 2015).

Therefore, it becomes a matter of deconvolution (inversion) of the effect that the local susceptibility over the whole image phase. It is further complicated by the magnetic field that exists all over the imaged volume, however, it should be noted that the phase cane only be measured in areas where the tissue was able to produce a signal. Consequently, an inversion problem will arise that will require the filtration and regulation (Heath et al., 2018).

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References

Heath, F., Hurley, A. S., Johansen-Berg, H., & Sampaio-Baptista, S. (2018). Advances in noninvasive myelin imaging . WILEY DEVELOPMENTAL NEUROBIOLOGY, 136-151.

Liu, C., Li, W., Tong, K. A., Yeom, K. W., & Kuzminski, S. (2015). Susceptibility-Weighted Imaging and Quantitative Susceptibility Mapping in the Brain. J Magn Reson Imaging, 23-41.

Rasband, M. N., & Macklin, W. B. (2012). Chapter 10- Myelin Structure and Biochemistry . In S. T. Brady, R. W. Albers, G. J. Siegel, & D. L. Price, Basic Neurochemistry (pp. 180-199). Elsevier Inc.

Shams, Z., Norris, D. G., & Marques, J. P. (2019). A comparison on invivo MRI based cortical myelin mapping using T1w/T2w and R1 mapping at 3T. PLOS ONE.

Wiliamson, J. M., & Lyons, A. D. (2018). Myelin Dynamics Throughout Life: An Ever-Changing Landscape. Frontiers in Cellular Neurocience.

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