The need for speed. II. Myelin in calanoid copepods

Speed of nerve impulse conduction is greatly increased by myelin, a multi-l ayered membranous sheath surrounding axons. Myelinated axons are ubiquitous among the vertebrates, but relatively rare among invertebrates. Electron m icroscopy of calanoid copepods using rapid cryofixation techniques revealed the widespread presence of myelinated axons. Myelin sheaths of up to 60 la yers were found around both sensory and motor axons of the first antenna an d interneurons of the ventral nerve cord. Except at nodes, individual lamel lae appeared to be continuous and circular, without seams, as opposed to th e spiral structure of vertebrate and annelid myelin. The highly organized m yelin was characterized by the complete exclusion of cytoplasm from the int racellular spaces of the cell generating it. In regions of compaction, extr acytoplasmic 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 d esigned to minimize both resistive and capacitive current flow through the internodal membrane, greatly speeding nerve impulse conduction. Copepod mye lin differs from that of any other group described, while sharing features of every group.