Nervous system development and reaction to injury. A Nervous system development and reaction to injury. A. Development of the nervous system from the neural tube and neural crest. B. Selection and refinement of connections. A group of nerve cells born at the same time will be programmed to send axonal projections toward target cells. The axons that arrive early and form successful connections receive trophic support from the targets. These neurons survive a period of cell death that occurs naturally for all groups of neurons sometime after their birth. Neurons that fail to make enough connections die. This selection establishes the initial wiring of neural circuits. The early connections tend to be widely dispersed. Subsequent electrical and synaptic activity in the circuit eliminates some synapses and strengthens others, reducing the spread of the initial connections and producing functional circuits. C. Reactions of neurons to injury. When a central or peripheral axon is severed, its distal portion degenerates and the myelin wrappings are lost. The cell body undergoes chromatolysis; the Nissl substance (rough endoplasmic reticulum) is greatly reduced, the nucleus moves eccentrically, and the cell body swells. In the peripheral nervous system, axons will eventually begin growing from the proximal stump. If newly proliferated Schwann cells provide proper guidance channels, the regenerating axon can find its target and reinnervate it. In the central nervous system, many neurons die after injury, and barriers produced by glial scars and inhibitory molecules on oligodendrocytes often interfere with axon regrowth, even when the neuron survives. Source: NERVOUS TISSUE, The Big Picture: Histology Citation: Ash R, Morton DA, Scott SA. The Big Picture: Histology; 2017 Available at: https://accessphysiotherapy.mhmedical.com/DownloadImage.aspx?image=/data/books/2058/ashhist_ch6_f006.png&sec=155784206&BookID=2058&ChapterSecID=155784107&imagename= Accessed: November 16, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved