Axon
Published: Jul 16, 2023
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Updated: Jul 21, 2023
Written by Oseh Mathias
Founder, SpeechFit
Axons, the long, thread-like extensions of neurons, serve as the main transmission lines of the nervous system and are vital for interneuronal communication, facilitating the transfer of electrochemical signals to other neurons, muscle cells, or gland cells [1].
Each neuron usually has a single axon which can vary in length from a mere millimeter to up to a meter in humans [2]. This axon originates from a region of the neuron called the axon hillock and can branch out to create multiple terminal branches, each ending in a synaptic bouton or terminal. These boutons or terminals interface with the dendrites or soma of other neurons at a specialized junction known as a synapse [3].
Axons are crucial for the propagation of action potentials, which are rapid, transient changes in membrane potential that travel down the length of the axon [5]. Myelin, a lipid-rich substance that insulates the axon and significantly enhances the speed of signal transmission, aids in this propagation [6]. Myelin is produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system, and it is laid down in segments, forming distinct nodes of Ranvier, which regenerate the action potential for rapid, saltatory conduction [7].
The axon's interior, or axoplasm, contains many cytoskeletal proteins like microtubules and neurofilaments, as well as mitochondria and other organelles [8]. The axonal cytoskeleton and related motor proteins, such as kinesin and dynein, are involved in axonal transport – a key process that ensures the delivery of nutrients, organelles, and other cellular components to and from the neuron's cell body [9].
Axon damage or degeneration, referred to as axonopathy, can lead to several neurological disorders, including multiple sclerosis (a demyelinating disease), peripheral neuropathies, and various neurodegenerative diseases [10].
Oseh is a software engineer, entrepreneur and founder of SpeechFit. Oseh is passionate about improving health and wellbeing outcomes for neurodiverse people and healthcare providers alike.
References
Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Structure and Function of Neurons. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26935/
Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 21.3, Overview of Neuron Structure and Function. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21535/
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Synapses. Available from: https://www.ncbi.nlm.nih.gov/books/NBK10869/
Queensland Brain Institute. (n.d.). [Illustration of axons and their properties]. Axons: The Cable Transmission of Neurons. Retrieved July 19, 2023, from https://qbi.uq.edu.au/brain/brain-anatomy/axons-cable-transmission-neurons
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Action Potentials. Available from: https://www.ncbi.nlm.nih.gov/books/NBK11051/
Hartline DK, Colman DR. Rapid Conduction and the Evolution of Giant Axons and Myelinated Fibers. Curr Biol. 2007 Jan 23;17(2):R29-35. doi: 10.1016/j.cub.2006.11.042.
Nave KA. Myelination and the trophic support of long axons. Nat Rev Neurosci. 2010 Apr;11(4):275-83. doi: 10.1038/nrn2797.
Hirokawa N, Noda Y, Tanaka Y, Niwa S. Kinesin superfamily motor proteins and intracellular transport. Nat Rev Mol Cell Biol. 2009 Oct;10(10):682-96. doi: 10.1038/nrm2774.
Encalada SE, Goldstein LS. Biophysical challenges to axonal transport: motor-cargo deficiencies and neurodegeneration. Annu Rev Biophys. 2014;43:141-69. doi: 10.1146/annurev-biophys-051013-022746.
Conforti L, Gilley J, Coleman MP. Wallerian degeneration: an emerging axon death pathway linking injury and disease. Nat Rev Neurosci. 2014 Jun;15(6):394-409. doi: 10.1038/nrn3680.