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| − | ===<u>The Schwann Cell</u>===
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| − | * Schwann cells provide myelination in the PNS.
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| − | ** They are derived from neural crest cells.
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| − | * During development of the nervous system, Schwann cells interact with many small axons.
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| − | ** Schwann cells eventually relate to only one axon, as axonal diameter increases with maturation of the system.
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| − | ***[[Neurons - Anatomy & Physiology#Oligodendrocytes|Oligodendrocytes]], that myelinate the CNS, differ from Schwann cells in that they interact with many axons.
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| − | ====The Process of Myelination====
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| − | # Initially, the single axon to be myelinated by the Schwann cell sits in a trough formed by the Schwann cell processes.
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| − | # The processes come together to enclose the axon, forming an inner mesaxon.
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| − | # The leading-edge process continues to move over the axon, creating a spiral of Schwann cell around it.
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| − | # Cytoplasm within the proces is extruded, meaning the axon is wrapped in Schwann cell membrane (myelin) alone.
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| − | #* The internal surfaces of the membrane, now vacated of cytoplasm, come together as the major dense line.
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| − | #* The outer membrane forms the intraperiod line.
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| − | # *The alternating pattern of these major dense line and the intraperiod line form the lamellae of compacted myelin.
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| − | # The myelin sheath remains attached to, and is an integral part of, the Schwann cell.
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| − | #* The myelin is therefore dependent on the Schwann cell for its maintenance.
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| − | ====Relationship With Axons====
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| − | * A single Schwann cell forms a single myelin sheath or internode.
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| − | ** One cell does not myelinate the whole length of an axon!
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| − | * Myelin thickness is related to internodal length, which in turn is associated with axon calibre:
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| − | ** Large axons have long, thick myelin sheaths, and therefore also conduct more rapidly.
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| − | * The internodes do not abut one another but are separated by an exposed area of axon called the '''Node of Ranvier'''.
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| − | ** Action potentials are able to leap between Nodes of Ranvier in saltatory conduction. This increases conduction speed.
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| − | * If the axon diameter remains small, then a Schwann cell will continue to associate with many axons, although none of them are fully myelinated.
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| − | ** Thus, even unmyelinated axons retain a Schwann cell ensheathment.
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| − | ** These non-myelinating Schwann cells are sometimes referred to as '''Remak cells'''.
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| | ===<u>Fibre Types</u>=== | | ===<u>Fibre Types</u>=== |