Conduction in smooth muscle: comparative structural properties

Abstract

The individual fibers of smooth muscle of high propagation velocity (e.g. esophagus) are longer and more closely packed than in those of low velocity (e.g. cat intestine). The space between fibers and within bundles calculated from electron micrographs is 4.4% for pig esophagus muscularis mucosae, 9.0–12.5% for taenia coli and cat intestine, 18.2% for the slow dog retractor penis and 39% for the nonconducting carotid artery. Predominant intercellular separation for esophagus and taenia is 700 A°, for cat longitudinal intestinal 1600 A°, dog retractor 3000 A° and carotid artery 10,000 A°. Two types of interfiber ‘bridges’ both crossed by membranes were observed. These may represent relatively low resistance pathways between fibers during conduction or they may serve a mechanical function. Vesicles are abundant beneath cell membranes and are arranged in longitudinal ridges, some 20 rows/cell. Among the myofilaments are found dark bodies which appear to be composed of noncontractile protein.