The velocity of development of the demarcation current in the frog's sartorius

Abstract

Urano and Fahr have experimentally established the fact that the potassium ion is almost exclusively the only cation within the muscle cell of the frog's sartorius. Overton has shown that the demarcation current of the frog's sartorius may be inhibited or have its sign reversed by replacing the lymph fluid surrounding the muscle cell by a fluid containing K ions in place of the normally Na ions. This action is reversible. The author has repeated Overton's experiments and in addition to confirming them found that the relation of K in the muscle cell bathing fluid to the potential developed by injury of the muscle is a quantitative one and also that it is possible to get not only a change of direction for the demarcation current by replacing Na in the cell bathing fluid by sufficient K but also to get a pseudo action current under these circumstances. This pseudo action current only travels a short distance along the muscle, is of slow rate of progression and is accompanied by a small, slow contraction.

From the above experiments it was concluded that a displacement of K ions across a semi-permeable cell boundary was responsible for the demarcation current of muscle. At the moment of cutting or injuring a muscle cell there is opportunity for ion equilibrium at the cut surface, whereas there is ion and thus electric strain at the uncut surface. If this hypothesis is true the velocity with which the demarcation current develops to its maximum intensity is of the order of 1 /100,000 of seconds because of the speed of ions and the distance to be travelled by them. Garten has cut the surface of the frog's sartorius and determined the speed with which the demarcation current rises to its maxirrium value by means of the capillary electrom'eter.