The periaxonal space of crayfish giant axons.
The influence of the glial cell layer on effective external ion concentrations has been studied in crayfish giant axons. Excess K ions accumulate in the periaxonal space during outward K+ current flow, but at a rate far below that expected from the total ionic flux and the measured thickness of the space. At the conclusion of outward current flow, the external K+ concentration returns to normal in an exponential fashion, with a time constant of approximately 2 ms. This process is about 25 times faster than is the case in squid axons. K+ repolarization (tail) currents are generally biphasic at potentials below about -40 mV and pass through a maximum before approaching a final asymptotic level. The initial rapid phase may in part reflect depletion of excess K+. After block of inactivation and reversal of the Na+ concentration gradient, we could demonstrate accumulation and washout of excess Na ions in the periaxonal space. Characteristics of these processes appeared similar to those of K+. Crayfish glial cell ultrastructure has been examined both in thin sections and after freeze fracture. Layers of connective tissue and extracellular fluid alternate with thin layers of glial cytoplasm. A membranous tubular lattice, spanning the innermost glial layers, may provide a pathway allowing rapid diffusion of excess ions from the axon surface.
D000200:Action Potentials; D000818:Animals; D003400:Astacoidea; D001369:Axons / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D004594:Electrophysiology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism*; D008564:Membrane Potentials; D009457:Neuroglia / Q000378:metabolism* / Q000648:ultrastructure; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D007473:Ion Channels; D012964:Sodium; D011188:Potassium