The hydration of sodium ions crossing the nerve membrane.

The sodium channel of the excitability mechanism in nerve membranes is about as permeable to hydroxylamine and hydrazine cations as it is to sodium ions. It is impermeable to methylamine cations. This selectivity is explained by supposing that an oxygen group in the channel must receive a hydrogen bond from the permeating cation at the same time as the cation lies against another negatively charged oxygen acid. If these conditions are not satisfied the cation cannot permeate. Sodium ions can satisfy this hydrogen-bonding requirement if they have a water of hydration. The H(2)O.Na complex also has almost the same dimensions as the hydroxylamine and hydrazine cations. This hydrated ion is probably part of the critical complex between sodium ions and the selectivity mechanism of the sodium channel.