The left ventricular papillary muscles of rats were studied under the electron microscope. The tissue was fixed in glutaraldehyde 6.25%, postfixed in osmium tetroxide 2% and embedded in Epon 812. Sections were stained with lead according to Karnovsky.
The terminal distribution of nerve fibers and their relationship with target cells represented by cardiac and vascular smooth muscles were investigated. Nerve bundles followed the course of vessels to the perivascular spaces around the smallest arterioles. At this level the bundles became progressively thinner and individual axons were observed leaving the bundles and penetrating between myocardial cells. In the micrographs, the unmedullated axons contained in the bundles were of three different types. Axons of type 1, chiefly containing neurofilaments, were considered the most proximal portion of the nerve fibers involved in the conduction of impulses. Axons of type 2 were characterized by the appearance of agranular and granular vesicles. Axons of type 3, or nerve endings, represented the anatomical endpoint of individual axons. True myoneural junctions (nerve-muscle close contact) have been detected. More frequently, axons of type 3 end in the intercellular space (loose contact). Only a loose contact relationship was observed between the axons and vessels walls.