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[3] [4] [5] Once there, the electrical signal provokes the release of chemical neurotransmitters across the synapse, which bind to receptors on the post-synaptic cell (e.g. another neuron, myocyte or secretory cell). Myelin is made by glial cells, which are non-neuronal cells that provide nutritional and homeostatic support to the axons
Myelin is formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.Therefore, the first stage of myelinogenesis is often defined as the differentiation of oligodendrocyte progenitor cells (OPCs) or Schwann cell progenitors into their mature counterparts, [4] followed by myelin formation around axons.
A single oligodendrocyte can extend its processes to cover up to 40 axons, that can include multiple adjacent axons. [2] The myelin sheath is not continuous but is segmented along the axon's length at gaps known as the nodes of Ranvier. In the peripheral nervous system the myelination of axons is carried out by Schwann cells. [1]
Schwann cells are a variety of glial cells that keep peripheral nerve fibres (both myelinated and unmyelinated) alive. In myelinated axons, Schwann cells form the myelin sheath. The sheath is not continuous. Individual myelinating Schwann cells cover about 1 mm of an axon [3] – equating to about 1000 Schwann cells along a 1-m length of the axon.
Oligodendrocyte progenitor cells (OPCs), also known as oligodendrocyte precursor cells, NG2-glia, O2A cells, or polydendrocytes, are a subtype of glia in the central nervous system named for their essential role as precursors to oligodendrocytes and myelin. [1] They are typically identified in the human by co-expression of PDGFRA and CSPG4.
The first is oligodendrocyte, which can be found in the mammalian Central Nervous System (CNS). The second is Schwann cells, which are found in the Peripheral Nervous System (PNS). Myelination of axons by these Schwann cells are essential for normal nerve function. Peripheral nerves rely on communication between axons and Schwaan cells. [2]
Myelin basic protein (MBP) is a protein believed [weasel words] to be important in the process of myelination of nerves in the nervous system.The myelin sheath is a multi-layered membrane, unique to the nervous system, that functions as an insulator to greatly increase the velocity of axonal impulse conduction. [5]
In the PNS, specialized microvilli project from the outer collar of Schwann cells and come very close to nodal axolemma of large fibers. The projections of the Schwann cells are perpendicular to the node and are radiating from the central axons. However, in the CNS, one or more of the astrocytic processes come in close vicinity of the nodes.