Myelin Sheath
The myelin sheath is composed of multi-layered lipid & protein covering produced by neuroglia.
In the PNS, myelin formation begins with Schwann cells draped around a solitary axon. It continues to wrap around the axon several times, extruding cytoplasm from between the membranes so that only concentric lamellae of plasma membrane remain. The outermost layer is the cytoplasm & nucleus of the Schwann cell, called the neurolemma. The function of the neurolemma is to aid in repair and regeneration of the axon. The Nodes of Ranvier are physical gaps in the sheath where the axon cell membrane is exposed to the extracellular fluid. In the CNS, the oligodendrocytes produce the myelin sheath in a similar way to that of the PNS. However, there is no neurolemma and fewer Nodes of Ranvier. This is why little to no repair and regeneration occur in the CNS after injury. Functionally, the myelin sheath provides electrical insulation to the axon and increases the speed of impulse conduction. The amount of myelination increases from birth to maturity and this is why baby’s responses to stimuli are not as quick or coordinated as an adult.
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