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Journal of Comparative Physiology A

, Volume 186, Issue 4, pp 347–357 | Cite as

The need for speed. II. Myelin in calanoid copepods

  • T. M. Weatherby
  • A. D. Davis
  • D. K. Hartline
  • P. H. Lenz
ORIGINAL PAPER

Abstract

Speed of nerve impulse conduction is greatly increased by myelin, a multi-layered membranous sheath surrounding axons. Myelinated axons are ubiquitous among the vertebrates, but relatively rare among invertebrates. Electron microscopy of calanoid copepods using rapid cryofixation techniques revealed the widespread presence of myelinated axons. Myelin sheaths of up to 60 layers were found around both sensory and motor axons of the first antenna and interneurons of the ventral nerve cord. Except at nodes, individual lamellae appeared to be continuous and circular, without seams, as opposed to the spiral structure of vertebrate and annelid myelin. The highly organized myelin was characterized by the complete exclusion of cytoplasm from the intracellular spaces of the cell generating it. In regions of compaction, extracytoplasmic space was also eliminated. Focal or fenestration nodes, rather than circumferential ones, were locally common. Myelin lamellae terminated in stepwise fashion at these nodes, appearing to fuse with the axolemma or adjacent myelin lamellae. As with vertebrate myelin, copepod sheaths are designed to minimize both resistive and capacitive current flow through the internodal membrane, greatly speeding nerve impulse conduction. Copepod myelin differs from that of any other group described, while sharing features of every group.

Key words Crustacean Sensorimotor Ultrastructure Multilamellar sheath Myelinated axons 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2000

Authors and Affiliations

  • T. M. Weatherby
    • 1
  • A. D. Davis
    • 2
  • D. K. Hartline
    • 2
  • P. H. Lenz
    • 2
  1. 1.Biological Electron Microscope Facility, Pacific Biomedical Research Center, University of Hawaii at Manoa, 1993 East-West Rd., Honolulu, HI 96822, USAUS
  2. 2.Békésy Laboratory of Neurobiology, Pacific Biomedical Research Center, University of Hawaii at Manoa, 1993 East-West Rd., Honolulu, HI 96822, USA e-mail: petra@pbrc.hawaii.edu Fax: +1-808-956-6984US

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