Abstract
1. The transmembrane action potentials driven by extracellular electrical stimulations were recorded with flexibly mounted microelectrode in the isolated uterine muscle of the mouse at latter half period of pregnancy.
2. Single action potential was usually produced by an electrical shock, but a series of discharges on some occasions. Magnitude of action potential was 38.2±0.38mV on an average, while that of resting potential was 51.3±0.35mV at this stage of pregnancy.
3. Duration of the action potential at 10% level of spike height was 69±0.40 msec. on an average but varied markedly in individual cases, mostly because of variable magnitude of the after-potential and of the peculiar slow potential superposed on the rising or falling phase of the spike.
4. With gradual increase in the intensity of extracellular stimulation, a peculiar slow potential first made its appearance and then a spike when the slow potential exceeded a critical depolarization. It was also at the threshold stimulus that two or three slow potentials were frequently produced.
5. Further increase in the intensity, however, usually caused a shortening of latency of both slow and spike potentials and finally set up a simple typical action potential.
6. When too strong stimulus was applied to a portion near the recordingelectrode, the action potential appeared mostly flattened and markedly deformed showing double peaks in some instances.
7. The refractory period was about 0.1sec. for producing an effective second response and 0.2sec. for producing the second full spike.
8. The conduction velocity of excitation was 10.2±0.41cm per second in the same muscle bundle, but much slower and variable between different muscle bundles.
9. Discussions were made on the nature of the peculiar slow potential.
