Electrical coupling in smooth muscles. Is it universal?

There is strong experimental evidence for electrical coupling in all types of smooth muscle. In some publications, and particularly in physiological textbooks, smooth muscles are still divided into those that are electrically coupled and those that are not. In this article we review the evidence for the universal presence of coupling in smooth muscles and the underlying mechanism, which, in most cases, appears to be gap junctions. We propose a classification of smooth muscles based on the mechanisms that initiate their activity. The two main types of smooth muscle according to this classification are neurogenic (e.g., iris, arterioles, vas deferens) and myogenic (e.g., urinary bladder, intestine, most blood vessels).