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What is myelin ?:

What is myelin ? Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. Myelin is produced by specialized cells, oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system.

Composition of myelin:

Composition of myelin Myelin made by different cell types varies in chemical composition and configuration, but performs the same insulating function. Myelinated axons are white in appearance, hence the "white matter" of the brain. Myelin is composed of about 80% lipid fat and about 20% protein. Some of the proteins that make up myelin are Myelin basic protein (MBP), Myelin oligodendrocyte glycoprotein (MOG) and Proteolipid protein (PLP). It has a characteristic protein-lipid-protein-lipid-protein structure. Myelin is made up primarily of a glycolipid called galactocerebroside . The intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.

Myelinisation :

Myelinisation The development of a myelin sheath around a nerve fiber. Myelin sheaths wrap themselves around axons. Each oligodendrocyte can myelinate several axons.(up to 40), so the destruction of even only a few oligodendrocytes can have an extensive demyelination effect.

Myelinisation :


Function of myelin layer:

Function of myelin layer The main consequence of a myelin layer (or sheath) is an increase in the speed at which impulses propagate along the myelinated fiber. Along unmyelinated fibers, impulses move continuously as waves, but, in myelinated fibers, they hop or "propagate by saltation". Myelin increases resistance across the cell membrane by a factor of 5,000 and decreases capacitance by a factor of 50. Myelination also helps prevent the electrical current from leaving the axon. When a peripheral fiber is severed, the myelin sheath provides a track along which regrowth can occur. Unmyelinated fibers and myelinated axons of the mammalian central nervous system do not regenerate.

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Axons with normal myelin Demyelinated axons: Nerve impulse conduction slows or stops completely


Myelinogenosis Myelinogenesis is the process of sequential myelination of the parts of the central nervous system whose order approximates the evolutionary development of the central nervous system. It takes place mostly between two months before and after birth. The first cortical region to myelinate is in the motor cortex (part of Brodmann's area 4), the second is the olfactory cortex and the third is part of the somatosensory cortex (BA 3,1,2). The last areas to myelinate are the anterior cingulate gyrus , the Inferior temporal lobe and the Dorsolateral prefrontal cortex .

Normal Myelination pattern of the pediatric brain :

Normal Myelination pattern of the pediatric brain Myelination causes changes in the signal intensity of the brain. Myelination starts during the 5th fetal month with myelination of the cranial nerves and continues throughout life. Myelination progress is from caudal to cephalad from dorsal to ventral and from central to periphery.

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The brain stem therefore, myelinates prior to the cerebellum and basal ganglia, and the cerebellum and basal ganglia myelinate prior to the cerebral hemispheres. Another generalization is that, with any particular portion of the brain, the posterior region tends to myelinate first. There fore , the dorsal brain stem , containing the medial lemiscus and medial longitudinal fasciculus, myelinates prior to the ventral brain stem, which contain the cortico spinal tracts. Like wise, the occipital lobes of the cerebral hemispheres myelinate early, whereas the frontal lobes myelinate late.

Imaging approaches:

Imaging approaches MR is the only imaging technique that assesses myelination. T1WI show myelination as increasing hyperintensity . T2WI show myelination as increasing hypointensity . Diffusion imaging shows myelnation as decreasing diffusivity. FA maps show myelination as increasing FA (fractional anisotropy) MT imaging shows myelination as increasing MT(magnetization transfer)

Assessment of myelination:

Assessment of myelination Many ways to assess myelination by MR Qualitative method: Assess milestones when changes of myelination appear on T1,T2 weighted images. Quantitative methods: Assess changes in diffusivity,FA,MT and compare with values of age-matched patients.

Basic principles of myelination on MRI:

Basic principles of myelination on MRI Myelinated WM appears hyper intense on T1W and hypointense on T2W images. Unmyelinated white matter appears hypointense on T1W and hyper intense on T2W images. Increase in signal intensity on T1W images precede the decrease in signal intensity on T2W images.

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White matter changes are seen best on T1W images during the first six to eight months of life and T2W images are best to evaluate myelination between six and eighteen months of life. Because T2 of the immature brain have relatively poor grey/white matter distinction due to high water content of the unmyelinated matter.

Normal Myelination:

Normal Myelination Birth (full term) Medulla Dorsal midbrain Inferior and superior cerebellar peduncles Posterior limb of internal capsule Ventrolateral thalamus Perirolandic region

One month :

One month Deep cerebellar white matter Corticospinal tracts Pre/ postcentral gyri Optic nerves, tracts

3 Months :

3 Months Brachium pontis , cerebellar follia Ventral brainstem Optic radiations Anterior limb of internal capsule Occipital subcortical U fibres Corpus callosum splenium

6 Months :

6 Months Corpus callosum genu Paracentral subcortical U fibres Centrum semiovale (partial)

8 Months :

8 Months Centrum semiovale (complete except for some fronto temporal areas) Subcortical U fibres (complete except for most rostral frontal areas)

12 Months :

12 Months Peripheral extension into the subcortical WM begins at about 1 year and is essentially complete by 22-24 months except in the “terminal zones”

18 Months:

18 Months Progressive myelination in the IC, CC, forceps minor, forceps major and central and subcortical white matter

24 Months:

24 Months

Terminal zones:

Terminal zones These are areas of known slow myelination within the brain and should not be mistaken for areas of ischemia. They are seen from about age 16 months until age 10 years.

Terminal zones:

Terminal zones Persistent signal intensity in lateral, superior, and posterior to the lateral ventricles, particularly in the region of trigones .

Perivascular spaces:

Perivascular spaces Curvilinear periventricular areas that are iso intense to CSF on all imaging sequences.

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Anatomic Region T1WI T2WI Superior cerebellar peduncle 28 gest wks 27 gest wks Median longitudinal fasciculus 25 gest wks 29 gest wks Medial lemnisci 27 gest wks 30 gest wks Lateral lemnisci 26 gest wks 27 gets wks Myelination Patterns on MRI

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Anatomic Region: Brain T1 T2 Cerebellar peduncle, middle 0 0-2 months Cerebral White matter 0-4 months 3-5 months Posterior limb internal capsule Anterior Portion Posterior Portion 1 month 0 4-7 months 0-2 months Anterior limb internal capsule 2-3 months 7-11 months Corpus callosum , genu 4-6 months 5-8 months Corpus callosum, splenium 3-4 months 4-6 months Occipital white matter Central Peripheral 3-5 months 4-7 months 9-14 months 11-15 months Frontal white matter Central Peripheral 3-6 months 7-11 months 11-16 months 14-18 months Centrum semiovale 2-4 months 7-11 months Myelination Patterns on MRI

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