The nodes of Ranvier

Abstract

At a node of Ranvier the axon is reduced in diameter; on the largest mammalian fibres the reduction is to less than half of the internodal diameter. The axoplasm at the node has optical properties different from the internodal axoplasm, but there is no visible transverse boundary across it. The length of the portion of axon left uncovered at the node is less than 0·5μ in the largest mammalian fibres; it is somewhat greater on the smaller fibres. The area of the cylinder of axon membrane exposed at the node increases only slightly with increasing fibre diameter. This area is 4μ², and the area of the surface of the narrow portion of the axon is about 90μ², in the largest mammalian fibres. The exposed portion of the axon is probably not covered by Schwann cell protoplasm, but is surrounded by a 'cementing disk’ composed of scleroprotein material, continuous with the neurilemma (inner endoneurium). This disk has a radial thickness of 5μ in the largest mammalian fibres. Outside this disk is a perinodal space with no stainable contents, bounded externally by the collagenous outer endoneurium (sheath of Key & Retzius). Methylene blue and silver nitrate entering the fibres at the node stain the whole narrow region, suggesting that this, rather than the myelin-free area, may constitute the effective nodal membrane. Periodic interruptions of the myelin occur along fibres of the spinal cord of rabbits, probably on all fibres. On central fibres of 3 to 15μ in diameter the internodal lengths vary from 300 to 1700μ. As in peripheral nerves internodal length increases with growth; in smaller rabbits this length is less for any given fibre diameter than in larger animals. In new -born rabbits the smallest fibres have internodes as short as 200μ.