Fenestration in the myelin sheath of nerve fibers of the shrimp: a novel node of excitation for saltatory conduction.
Giant nerve fibers of the shrimp family Penaeidae conduct impulses at the velocity highest among all animal species (approximately 210 m/s; highest in mammals = 120 m/s). We examined these giant and other small nerve fibers morphologically using a differential interference contrast microscope as well as an electron microscope, and found a very specialized form of excitable membrane that functions as a node for saltatory conduction of the impulse. This node appeared under the light microscope as a characteristic pattern of concentrically aligned rings in a very small spot of the myelin sheath. The diameter of the innermost ring of the node was about 5 microns, and the distance between these nodes was as long as 12 mm. Via an electron microscope, these nodes were characterized by a complete lack of the myelin sheath, forming a fenestration that has a tight junction with an axonal membrane. Voltage clamp measurements by a sucrose gap technique demonstrated that the axonal membrane at these fenestration nodes is exclusively excitable and that the large submyelinic space is a unique conductive pathway for loop currents for saltatory conduction through such fenestration nodes.
Hsu
K
K
Department of Neuropharmacology, Shanghai Institute of Physiology, China.
Terakawa
S
S
D000818:Animals; D033364:Decapoda / Q000502:physiology*; D066298:In Vitro Techniques; D008854:Microscopy, Electron; D008870:Microtubules / Q000502:physiology / Q000648:ultrastructure; D009186:Myelin Sheath / Q000502:physiology* / Q000648:ultrastructure; D009412:Nerve Fibers / Q000502:physiology* / Q000648:ultrastructure; D009431:Neural Conduction / Q000502:physiology*; D018408:Patch-Clamp Techniques; D019108:Tight Junctions / Q000502:physiology / Q000648:ultrastructure