Closely Spaced Nodes of Ranvier in the Teleost Brain

Abstract

ALTHOUGH there have been several studies of internode length of myelinated fibres in peripheral nerve^1,2 and in main fibre tracts of the central nervous system^3,4, there has been only one report of internode lengths within neuropil and nuclear regions of the vertebrate central nervous systems (CNS)⁵. Internode lengths in uninjured peripheral nerves of fishes are always greater than 200 µm (ref. 1), and studies with rabbit spinal cord suggest that 200 µm is also a minimum periodicity in main fibre tracts of the CNS⁴. Bodian's study⁵ of internode lengths in CNS included obssrvations of myelinated fibres in the preoptic area, hypothalamus, hypoglossal root and pyramidal tract of the adult opossum. Of fifty-four fibres represented in his chart 1, six had internode lengths of approximately 100 µm or less; none had internode distances of less than 50 µm. During a study of the oculomotor nucleus of the spiny boxfish Chilomycterus with the electron microscope, I have made numerous observations of fine myelinated fibres with internodes less than 20 µm long and occasionally approximately 5 µm long. Tissue was fixed by perfusion with buffered solutions of osmium tetroxide or glutaraldehyde followed by osmium tetroxide and prepared for microscopy as reported before⁶.