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Synaptic pruning, a phase in the development of the nervous system, is the process of synapse elimination that occurs between early childhood and the onset of puberty in many mammals, including humans. [1]
Synaptogenesis is particularly important during an individual's critical period, during which there is a certain degree of synaptic pruning due to competition for neural growth factors by neurons and synapses. Processes that are not used, or inhibited during their critical period will fail to develop normally later on in life.
In addition to synaptic pruning, the brain undergoes myelination, which influences the speed of information flow across brain regions. Myelination involves neuronal axons connecting certain brain areas to become insulated with a white, fatty substance called myelin that increases the speed and efficiency of transmission along axons.
This was observed primarily in the frontal and parietal cortices. Theories as to why this occurs vary. One thought is that the intracortical myelination paired with increased axonal calibre increases the volume of white matter tissue. Another is that synaptic reorganization occurs from proliferation and then pruning. [8]
Myelin's best known function is to increase the rate at which information, encoded as electrical charges, passes along the axon's length. Myelin achieves this by eliciting saltatory conduction. [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.
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.
The myelin sheath that surrounds and protects nerve cells is made by cells called oligodendrocytes. In a person with MS, these cells are lost, so damaged myelin sheaths cannot be repaired.
Analysis confirmed the massive rewiring, 10-fold decrease in the number of synapses, that takes place as axons prune their motor units but add more synaptic areas at the NMJs with which they remain in contact. [37]