Excitability and of its Influencing Physico-chemical Factors

Nerve fibres have two properties, excitability and conductivity. These properties are characterised, accompanied by, and perhaps actually due to, electrical changes. As to the excitation of nerve fibres by electrical stimulation, Nernst put forward the hypothesis that electrical excitation depended on an electrolytic concentration of ions at membranes impermeable to them. On the other hand, recently Lapicque came to the conclusion that there are intimate relations between excitability of living tissues and the duration. and strength of the stimulating current, formulated the law of chronaxie, which as may be stated as follows; the chronaxie may be defined as the shortest duration of twice the rheobasic strength (galvanic threshold) which will produce stimulation. According to him the chronaxie of a tissue is a definite measure of its excitability, since in that time a current of twice the rheobasic strength is to able to produce those physico-chemical alterations which determine excitation. The author performed investigations for the purpose of examining these two opinions upon the excitation of nerve, using a nerve model of Hermann's type, modified for this purpose and nerves of different animals (sciatics of frog and of toad, vagus of tortoise). Especially precise determinations were made on the relations between some physico-chemical conditions and these influences upon the excitation of nerve fibres. From the results obtained the following may be concluded. 1) If a galvanic current of known strength and of know duration be passed through a nerve model, filled with electrolytic solution, the polarisation potential developed through the current, dimiuishes as the viscosity of the solution increases. 2) It is ascertained that there is a definite relation between chronaxie and polarisation of nerve fibres, i.e. the polarisation potential developed in nerve fibres, through the passage of a galvanic current of known strength and of known duration, is higher in the more excitable nerve with shorter chronaxie than the less excitable one with longer chronaxie. 3) The nerve model of larger calibre has a shorter chronaxie and a lower rheobase than that of smaller calibre. This fact may be considered due to their having different capacities and resistances for electricity, according as their calibres are larger or smaller. 4) The chronaxie of the nerve model is longer when it is measured in electrolytic solution than when it is measured in the air. It increases to a certain extent as the ionic concentration of the solution increases, until the ionic dissociation reaches its maximum. 5) The influence of electrolytic solution upon the increase of both the chronaxie and rheobase of nerve model varies inversely with calibre of the nerve model