Axons

Abstract

Axons, or nerve fibres, are neuronal extensions that serve as conduits for the transfer of information from one nerve cell to others.

Keywords: nerve fibre; myelin; node of ranvier; action potential

Figure 1.

Simplified model of ion channel organization of the mammalian myelinated fibre. Sodium channels (gNa) are aggregated in high density in the axon membrane at the node of Ranvier, but are present in much lower densities in the paranodal and internodal axon membrane, under the myelin. Fast potassium channels (gK) have a complementary distribution and are present in the axon membrane under the myelin sheath. This complex pattern of ion channel organization has important implications for impulse conduction, and for the abnormalities of conduction that occur in axons following demyelination.

Figure 2.

Relationship between conduction velocity and fibre diameter for myelinated and nonmyelinated fibres at 37°C. Above a critical diameter of approximately 0.2 μm, myelinated fibres conduct impulses more rapidly than nonmyelinated fibres of the same size; interestingly, myelinated fibres as small as 0.2 μm are observed in the CNS, but myelinated fibres smaller than this are not present.

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Further Reading

Hodgkin AL and Huxley AF (1952) A quantitative description of membrane current and its application to conduction and excitation in nerve. Journal of Physiology 117: 500–536.

Imaizumi T, Lankford KL, Waxman SG, Greer CA and Kocsis JD (1998) Transplanted olfactory ensheathing cells remyelinate and enhance axonal conduction in the demyelinated dorsal columns of the rat spinal cord. Journal of Neuroscience 18: 6176–6185.

Morell P (ed.) (1985) Myelin, 2nd edn. New York: Plenum Press.

Ritchie JM and Rogart RB (1977) The density of sodium channels in mammalian myelinated nerve fibers and the nature of the axonal membrane under the myelin sheath. Proceedings of the National Academy of Sciences of the USA 74: 211–215.

Rosenbluth J (1976) Intramembranous particle distribution at the node of Ranvier and adjacent axolemma in myelinated axons. Journal of Neurocytology 5: 731–745.

Waxman SG (1978) Physiology and Pathobiology of Axons. New York: Raven Press.

Waxman SG (1998) Demyelinating diseases: new pathological insights, new therapeutic targets. New England Journal of Medicine 338: 323–325.

Waxman SG, Kocsis JD and Stys PK (eds) (1995) The Axon. New York: Oxford University Press.

Waxman SG, Dib‐Hajj S, Cummins TR and Black JA (1999) Sodium channels and pain. Proceedings of the National Academy of Sciences of the USA 96: 7635–7639.

Zagoren J and Fedoroff S (eds) (1984) The Node of Ranvier. New York: Academic Press.

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How to Cite close
Waxman, Stephen G(Apr 2001) Axons. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0000019]