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A neurite or neuronal process refers to any projection from the cell body of a neuron. This projection can be either an axon or a dendrite. The term is frequently used when speaking of immature or developing neurons, especially of cells in Cell culture, because it can be difficult to tell axons from dendrites before differentiation is complete.
Young neurites are often packed with microtubule bundles, the growth of which is stimulated by neurotrophic factors, such as nerve growth factor (NGF).Bear, Mark F; Connors, Barry W.; Paradiso, Michael A., Neuroscience, Exploring the Brain, Philadelphia : Lippincott Williams & Wilkins; Third Edition (February 1, 2006). Tau proteins can aid in the stabilization of microtubules by binding to the microtubules, protecting them from microtubule severing proteins. Even after the microtubules have stabilized, the cytoskeleton of the neuron remains dynamic. Actin filaments retain their dynamic properties in the neurite that will become the axon in order to push the microtubules bundles outward to extend the axon. In all other neurites however, the actin filaments are stabilized by myosin. This prevents the development of multiple axons.
The neural cell adhesion molecule N-CAM simultaneously combines with another N-CAM and a fibroblast growth factor receptor to stimulate the tyrosine kinase activity of that receptor to induce the growth of neurites.
There are several software kits available to facilitate neurite tracing in images such as NeuronJ (an ImageJ plugin), Neuromantic, and the Neurolucida system.
Weak endogenous electric fields may be used to both facilitate and direct the growth of projections from cell soma neurites, EFs of moderate strength have been used to direct and enhance neurite outgrowth in both Mouse, or mouse, and xenopus models. Co-culture of neurons with electrically aligned glial tissue also directs neurite outgrowth, as it is rich in that promote nerve growth .
Whatever extracellular signals may be involved in inducing axon formation are transduced through at least 4 different pathways: the Rac-1 pathway, the Ras-mediated pathway, the Cyclic AMP-liver kinase B1 pathway, and the calcium/calmodulin-dependent protein kinase pathway. A deficiency in any of these pathways would lead to the inability to develop a neuron.
After forming one axon, the neuron must prevent all other neurites from becoming axons as well. This is known as global inhibition. It has been suggested that global inhibition is achieved by a long-range negative feedback signal released from the developed axon and taken up by the other neurite. However, no long range signaling molecule has been discovered. Alternatively, it has been suggested that the buildup of axonal growth factors in the neurite destined to become the axon means there is a depletion of axonal growth factors by default, as they must compete for the same proteins. This causes the other neurites to develop into dendrites as they lack sufficient concentrations of axonal growth factors to become axons. This would allow for a mechanism of global inhibition without the need for a long range signaling molecule.
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