Periaxonal ion regulation in the squid
In the majority of animal groups, axons release K+ and take up Na+ during action potential production, with a tendency for an accumulation of K+ and depletion of Na+ in the narrow extracellular spaces adjacent to the axon. Homeostatic mechanisms are present to reduce the severity of these changes, and so limit their undesirable effects on neuronal function. This chapter reviews studies in squid giant axons to show that, under normal physiological conditions, the Schwann cell sheath around the axon has powerful mechanisms for regulating the [K+] in the periaxonal space, using a combination of passive diffusion (particularly via the transcellular glial tubular system and across its membranes) and carrier-mediated transport. The significance of this regulation for the normal function of the giant axon system in swimming and escape responses is discussed. Axons in vertebrates and invertebrates use a sequence of voltage-dependent ionic currents to generate propagating action potentials, generally an early inward Na+ current followed by a late outward K+ current. If the axon is surrounded by a narrow interstitial space, there is a tendency for K+ accumulation and Na+ depletion in the space during activity. If the axon is surrounded by a narrow interstitial space, there is a tendency for K+ accumulation and Na+ depletion in the space during activity. The axonal resting potential is relatively insensitive to small changes of [K+] in the physiological range, because the significant Na+ permeability of the membrane flattens the Nernst plot of membrane potential versus log [K+] in this range.
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