Electrical Excitability of Developing Cardiac Muscle in Chick Embryos

Abstract

The resting and action potentials and the strength-duration relation to square current pulse stimulation were investigated on the chick embryo's heart from 2-day stage (7-somite stage) to near the hatching. From the embryos older than 3-day the resting and action potentials were recorded, and the magnitude of both potentials increased with the age. Before commencement of the spontaneous contraction (9-somite stage) electric stimulation failed to elicit contraction. After the 10-somite stage all-or-none contraction was produced by electric stimulation, and the strength-duration cruves were constructed at all stages by passing cathodal current to the cardiac muscle from 50μ micropipette. The rheobasic current was several tens of μA at the earlier stages, decreasing rapidly for the first several days, and then fell gradually to less than 1μA at the later stages. The ehronaxie was also found to shorten gradually from about 5 to 1 msec during the development. Since chronaxie is proportional to the membrane time constant, this characteristic change in chronaxie (membrane time constant) and rheobase (inverse of excitability) was suggested to correlate to structural change of the cell membrane.