|MadSci Network: Neuroscience|
When a nerve is damaged a clearly defined sequence of events take place. Severing the axon (the cable along which neural signals are transmitted) causes a number of degenerative changes to take place. Firstly communication between the damaged neuron and all other neurons to which it is connected ceases. The distal portion (or the section of the severed axon that is furthest from the cell body) slowly starts degenerating. The cells that had synaptic connections (points of cell-cell communication) with the damaged cell also start to degenerate. The degenerating material attracts scavenging cells from the nervous system (glia) and the immune system (macrophages). Glia and macrophages begin the process of clearing up the debris caused by the injury. What happens next depends on whether the damaged neurons were in the central nervous system (brain and spinal cord) or peripheral nervous system (sensory and motor neurons). In the central nervous system neurons that are damaged have very little ability to regenerate axons and make new connections. Thus, people who have brain damage or damage their spinal cords often remain permanently disabled. It is thought that there are signals in the central nervous system that prevent neuronal recovery from injury. Some of the signals that 'stop' regrowth in its path are found in the scars that form from glia that remain hanging around after they have finished scavenging dead matter. Much scientific research is underway to understand these 'stop' signals and find ways of overcoming them so the brain and spinal cord can heal itself after injury. To understand the signals that prevent regrowth scientists are examining recovery from injury in the peripheral nervous system where neurons CAN regenerate. Here there are special chemicals that prevent the degeneration of axons after they are damaged and promote the regrowth of new connections. Scientists have shown that transplanting pieces of peripheral nerve into a damaged central nerve can promote the regrowth of the central axons.
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