Internode (neurobiology)

In neurobiology , the term internode refers to the myelinated section of the axon of a nerve cell between two successive Ranvier cord rings for myelinated nerve fibers from vertebrates .

The electrically insulating myelin sheath around the axon is formed by special glial cells of the nervous system, in the peripheral nervous system the Schwann cells , in the central nervous system the oligodendrocytes . This happens when a glial cell or parts of it wrap around a certain axon section. Thereby, narrow gaps remain between the individual medullary sheath sections encasing the axon as axial cylinders, which occur at regular intervals along the nerve fiber as constricted areas of the glial sheath, where the insulating medullary lamellae are missing.

Only in these so-called Ranvier rings is the axon free enough to build up an action potential in the saltatory conduction of excitation . The corresponding axon section shows a fluctuation in caliber in thicker medullary nerve fibers, the Ranvier node ( node of Ranvier ). In terms of structure and function, the nodal sections assigned to a Ranvier cord ring (only) in a myelinated axon differ from the internodal sections between the nodes. Thus belongs an isolated Axonabschnitt to each of a glial cell insulating portion of the myelin sheath formed between two Ranvier constriction rings, on which in myelinated nerve fibers for the conduction must be established an internode no action potential more.

The length of internodes varies in human nerve fibers and the like. a. depending on the fiber diameter and is 0.5–1 mm in the middle course of the peripheral nerves.

literature

• Christopher D. Moyes, Patricia M. Schulte (Ed.): Animal Physiology. Pearson Studium, Munich 2008, ISBN 978-3-8273-7270-3 , p. 182.
• Wolfgang Zenker (Ed.): Benninghoff: Macroscopic and microscopic anatomy of humans , volume 3 nervous system, skin and sensory organs . Urban and Schwarzenberg, Munich 1985, ISBN 3-541-00264-6 .