Kainalu Rista BIO 206 Writing assignment #3
Detailed focus question: How does Wallerian degeneration and axonal regeneration repair severed axon/nerve fibers in the peripheral nervous system?
I. Introduction. In order to explain the cellular and molecular features of my focus questions I will need to introduce and provide background for three key concepts: Schwann cells, Macrophages, and general information on Wallerian degeneration and regeneration. A. General Information
1. The term Wallerian degeneration refers to the innate-immune response of a traumatic nerve injury in both central nervous system (CNS) and the peripheral nervous system (PNS); however, the mechanisms that undergo Wallerian degeneration between the 2 nervous…show more content… A. Anterograde/Degenerating Axon 1. Initial reaction to injury (~24 hours) Demyelination and
a. Upon nerve injury, degeneration is initiated by macrophages which move into injured area to remove myelination and axonal debris from damaged nerves (Gaudet, A. D., Popovich, P. G., & Ramer, M. S., 2011).
b. Detached axon segments remain attached for a few days after nerve injury (Rotshenker, S., 2011).
2. Axonal skeleton disintegrates (~1 week)
a. Macrophages Eventually, axons undergo catastrophic granular disintegration of the cytoskeleton, axon of the neuron, which causes the damaged axon distal to the neuron head to degrade into fine debris. (Rotshenker, S. 2011).
b. All but the axon’s neurolemma which is that the outermost layer of the neuron made up of Schwann cells don’t degenerate and remains as a hollow scaffold/tube providing a path for regenerating axons (Frostick, S. P., Yin, Q., & Kemp, G. J., 1998).
c. Ca2+ dependent protease, calpains, is a key player and responsible in the degeneration of the axon and synapse during Wallerian degeneration (Ma, M., Ferguson, T. A., Schoch, K. M., Li, J., Qian, Y., Shofer, F. S., . . . Neumar, R. W., 2013).
3. Debris Removal
a. Injured axons activate macrophages which in turn clear myelin and axon debris efficiently away from damaged nerve. Simultaneously, macrophages produce growth factors that facilitate Schwann cell migration and axon regeneration (Rotshenker, S. 2011).
III. Axonal Regeneration.