ABSTRACT
An electrophysiological method was used to estimate the half-times for sodium and potassium entry to, and efflux from, the extra-axonal space in peripheral nerve and central nervous connectives of two species of crustacean. Results from crab (marine) and crayfish (fresh water) were qualitatively similar.
Peripheral nerve showed no evidence for diffusion barriers, potassium entry and efflux being rapid, and proceeding at comparable rates.
In connective, potassium entry was extremely slow, with a half-time greater than 100 min, while potassium efflux was relatively rapid
. Sodium movements were less restricted, but sodium entry was more rapid than sodium efflux.The potassium experiments were compared with the behaviour of a theoretical model system. Evidence is presented for diffusional restriction to potassium at the connective perineurial layer. The mechanism of restriction may involve changes in permeability or activation of an ion pump in the perineurial layer.
The physiological significance of these findings is discussed.
In this study, the effects of replacing normal saline by isotonic KCL are interpreted as the effects of high potassium (K+ depolarization), assuming that the contribution of Na+ ions to resting potentials is negligible.
The words ‘entry’ or ‘access’ are used here and in analogous places instead of ‘uptake’ to show that it is purely the change in concentration in the extracellular space adjacent to the axons which is being considered; no attempt is made to relate this to the behaviour of any ‘compartment’ identified by tracer measurements.
Note that this statement is not incompatible with the barrier having a specific potassium permeability; potassium ions can only enter the system as fast as sodium ions can escape, so that the determining factor is really PNa-hence the factor A in equation (2) of Methods.
