Myelin
However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon.At the nodes of Ranvier, which are approximately one thousandth of a mm (one micrometre (μm) in length, the axon's membrane (axolemma) is bare of myelin.[1] Saltatory conduction refers to the fact that electrical impulses 'jump' along the axon, over long myelin sheaths, from one node of Ranvier to the next.At the molecular level, the myelin sheath increases the distance between extracellular and intracellular ions, reducing the accumulation of electrical charges.[10] Due to its high prevalence, multiple sclerosis, which specifically affects the central nervous system (brain, spinal cord and optic nerve), is the best known disorder of myelin.Myelin was first described in 1854 by Rudolf Virchow,[17] although it was over a century later, following the development of electron microscopy, that its glial cell origin and its ultrastructure became apparent.Blood vessels provide the route for oxygen and energy substrates such as glucose to reach these fibre tracts, which also contain other cell types including astrocytes and microglia in the CNS and macrophages in the PNS.[25] Positively charged sodium ions can enter the axon through these voltage-gated channels, leading to depolarisation of the membrane potential at the node of Ranvier.[29][30] In other words, the myelinating cell seems to act as a local "fueling station" for the axon, which uses a great deal of energy to restore the normal balance of ions between it and its environment,[31][32] following the generation of action potentials.Typical symptoms include blurriness in the central visual field that affects only one eye, may be accompanied by pain upon eye movement, double vision, loss of vision/hearing, odd sensation in legs, arms, chest, or face, such as tingling or numbness (neuropathy), weakness of arms or legs, cognitive disruption, including speech impairment and memory loss, heat sensitivity (symptoms worsen or reappear upon exposure to heat, such as a hot shower), loss of dexterity, difficulty coordinating movement or balance disorder, difficulty controlling bowel movements or urination, fatigue, and tinnitus.Techniques include surgically implanting oligodendrocyte precursor cells in the central nervous system and inducing myelin repair with certain antibodies.While results in mice have been encouraging (via stem cell transplantation), whether this technique can be effective in replacing myelin loss in humans is still unknown.[38] Glycogen synthase kinase 3β inhibitors such as lithium chloride have been found to promote myelination in mice with damaged facial nerves.
- Axon
- Nucleus of Schwann cell
- Schwann cell
- Myelin sheath
- Neurilemma
SystemAnatomical terminologynerve cellinsulateaction potentialsnodes of Ranviermicrometreaxolemmasaltatory conductioninternodal segmentaxon terminalneurotransmitterssynapsereceptorsmyocytesecretory cellglial cellsnutritionalhomeostaticcentral nervous systemspinal cordoptic nervesoligodendrocytesperipheral nervous systemSchwann cellsmotor functionsensory functionhearingcognitionleukodystrophiesdemyelinating disordermultiple sclerosisperipheral neuropathiesMyelinogenesisoligodendrocyte progenitor cellsdifferentiatespeech acquisitiongrey mattercerebral cortexvertebratesgnathostomesRudolf Virchowglial cellastrocyteoligochaetepenaeidspalaemonidscalanoidsKuruma shrimpTransmission electron micrographTrinity CollegeHartford, ConnecticutSchwann cellMyelin sheathNeurilemmawhite matteroptic nervecorticospinal tractcorpus callosumsciatic nerveauditory nerveastrocytesmicrogliamacrophagesproteinmyelin basic proteinmyelin oligodendrocyte glycoproteinproteolipid proteinmyelin protein zeroglycolipidgalactocerebrosidesphingomyelinCholesterolcapacitanceelectrical resistancevoltage-gated sodium channelsdepolarisationresting membrane potentialpotassium channelscytoplasmdistaldiffusionmembrane potentialphosphorylationneurofilamentscytoskeletalorganellesmitochondriaDemyelinating diseaseneurodegenerativeautoimmuneacute disseminated encephalomyelitisneuromyelitis opticatransverse myelitischronic inflammatory demyelinating polyneuropathyGuillain–Barré syndromecentral pontine myelinosisleukodystrophyCharcot–Marie–Tooth diseasepernicious anaemiaSubacute combined degeneration of spinal cordcognitiveCanavan diseaseimmune systemcytokinestumor necrosis factorinterferonDemyelinationneuropathyRemyelinationoligodendrocyte precursor cellsmyelin repairantibodiesstem cellCholinergic treatmentsacetylcholinesterase inhibitorsoligodendrocyteAChEIsnicotineGlycogen synthase kinase 3βlithium chloridevitamin B12lesionsshiverer mousePelizaeus–Merzbacher diseasephenylketonuria