Nerve-Purkinje fiber relationship in the moderator band of bovine and caprine heart.

The moderator band in the heart of the ox and goat contains bundles of Purkinje fibers and nerve fibers separated by connective tissue. The axons are mostly unmyelinated and embedded in the cytoplasm of Schwann cells. Small bundles of axons run close to the Purkinje fibers. The axons dilate into varicosities 0.5 to 1.6mu in diameter (mean 0.95mu), containing three types of vesicles: 1)agranular vesicles with a diameter of 400-500 A, 2) large dense-cored vesicles with a diameter of 800-1200 A, 3) small dense-cored vesicles with a diameter of 500 A. Most varicosities contain agranular vesicles together with a few large dense-cored vesicles. The gap between the varicosities and the nearest Purkinje fiber is unusually wide and normally varies between 0.3mu and 0.8mu. No intimate nerve-Purkinje fiber contacts, with a cleft of 200 A, were observed.