Excitation in unstriated muscle

Summary

Excitatory phenomena in unstriated muscle can be explained if it is assumed that the muscle consists of two zones, outer and inner, and excitation be due to difference in concentration of ions in these two zones. Moderate increase in permeability would diminish the excitability to alternating current and increase that to potassium; great increase would dimmish the excitability to both. An increase in the permeability of the outer membrane by physiological action, injury, asphyxia would cause excitation or inhibition. Spontaneous contractions are caused by increase in permeability, not great enough to cause continuous tension. Substances to which the muscle is moderately permeable, such as sodium and barium, produce continuous tension as they are unable to enter the inner zone. Substances to which the muscle is more permeable such as ammonium or potassium produce only a temporary contraction.