Intro | Cytoplasm | Endoplasmic Reticulum | Golgi Complex | Microtubules | Mitochondria | Nucleus | Plasma Membrane | Ribosomes
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Microtubules are small tubes formed by thirteen filamentous strands. Each filament is composed of a chain of protein called tubulin. Microtubules in a neuron are used to transport substances to different parts of the cell. For example, neurotransmitters are made in the cell body close to the nucleus, but need to travel long distances to the end of axons where they will be used for synaptic transmission. Microtubules are the route via which these essential molecules are transported from the site of synthesis to where they are needed.
Microtubules, neurofilaments, and microfilaments compose the cytoskeletal elements of a neuron. The earliest accounts of these cellular structures included the writing of not only Ramon y Cajal, the neuroanatomist, but also Freud and Alzheimer in their studies of cellular motility and tubules. Very recent molecular research has expanded our understanding of these elements of cell architecture, motility, and shape (Nixon, 1998). Neurons appear to differ in the properties of their cytoskeleton that may reflect functional characteristics, including susceptibility to injury and ability to repair following injury.
Cytoskeletal elements are synthesized in the cell body of a neuron, but delivered throughout the length of the neuron's axon (which composes approximately 99% of the neuron's structure) where they form large molecular assemblies or matrices. Until recently it was believed that the cytoskeletal structure was fairly homogeneous throughout the length of the axon. More recent evidence, however, indicates otherwise. For example, in mammalian peripheral neurons neurofilament protein content decreases almost 2-fold distally (away from the cell body), while microtubule content increases moving away from the cell body. Neurofilaments collect locally in regions of myelination in response to signals emanating from the oligodendrocyte. The neurofilament also increases the caliber of the axon underlying myelinated segments of normal neurons.
The dynamic behavior of neuronal filament proteins, including regional accumulation and the formation of cross-links to stabilize matrices, is under the control of protein kinases and phosphatases, enzymes that regulate the molecular activity at the ends of these structural proteins. Our understanding of these enzymes will yield useful information for the treatment of a number of degenerative neurological diseases (Alzheimer's Disease, Lewy body dementia, amyotropic lateral sclerosis) that are characterized by the abnormal accumulation of cytoskeletal elements along the axon (Julien, 1999; McShea, Wahl & Smith, 1999; Mukaetova-Ladinska, et al., 2000).
Julien, J.P. (1999). Neurofilament functions in health and disease. Current Opinions in Neurobiology, 9(5), 554-560.
McShea, A., Wahl, A.F. & Smith, M.A. (1999). Re-entry into the cell cycle: a mechanism for neurogeneration in Alzheimer disease. Medical Hypotheses, 52(6), 525-527.
Mukaetova-Ladinska, E.B., Hurt, J., Jakes, R., Xuereb, J., Honer, W.G. & Wischik, C.M. (2000). Alpha-synuclein inclusions in Alzheimer and Lewy body diseases. Journal of Neuropathology and Experimental Neurology, 59(5), 408-417.
Nixon, R.A. (1998). Dynamic behavior and organization of cytoskeletal proteins in neurons: reconciling old and new findings. BioEssays, 20(10), 798-807.