Myelin Sheath

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Many neurons in vertebrates are electrically insulated by a fatty layer called the myelin sheath produced by supportive cells called glial cells. There are different types of glial cells, oligodentrocytes surround neurons in the Central  nervous system and schwann cells surround periphral neurons[1], they produce the myelin sheath by the plasma membrane migrating around the neurons axon in a tight spiral. A single oligodentrocyte can myelinate up to 10-15 neurons whereas schwann cells only myelinate one[2]. Myelinated neurons are refered to as white matter as the myeline sheath is white[3].            

Between the glial cells are gaps called Nodes of Ranvier where voltage gated sodium ion channels are.

An action potential travels down a myelinated axon by saltatory conduction which is where an action potential depolarises the axon enough to initiate the opening of the voltage gated sodium ion channels at the next node. This jumping of the action potential results in faster propagation and conservation of energy.

Multiple Sclerosis is an autoimmune disease[4] where the myeline sheath is degenerated, causing random areas of inflammation called lesions[5]. The glial cells can also be destroyed so regeneration cannot occur so the myelin sheath is replaced by scar tissue[6]. The degenerated neurons can't transmit the action potential as quickly causing symptoms such as loss of control of muscles and numbness. Preventing brain atrophy may have important clinical implications affecting treatment decisions in the future. In recent years, research efforts have directed towards finding agents to modify the disease and reduce brain volume loss. Intravenous immunoglobulin (IVIg) may have some potential roles in this regard.[7]

Invertebrates do not have myelinated neurons.

The myelin sheath ensures an increased rate of action potential propagation. In peripheral nerves the myelin sheath is formed from Schwann cell (specialised glial cells) and acts as an insulating layer to the axonal membrane in preventing current from leaking across it [8].


  1. Molecular Biology of the Cell, Alberts, 5th edition, Garland publishers, ISBN 9780815341062
  7. Zare-Shahabadi, Rashidian, Sahraian, Rezaei (2015)(Potential role of immunoglobulin replacement therapy on MRI measures in multiple sclerosis: a systematic review.)
  8. B Alberts, A Johnson, J Lewis, M Raff, K Roberts, P Walter, 2008, Molecular Biology of the Cell 5th Edition, New York : Garland Science (P678)


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