Can nonadrenergic inhibitory varicosities be identified structurally.

The distribution of vesicles in axon varicosities was examined in rabbit jejunum and opossum esophagus. Circular muscle cells of both rabbit jejunum and opossum esophagus responded to stimulation of intrinsic nerves with inhibitory junction potential in nearly all cases. Inhibitory junction potentials were rare and small in longitudinal muscle cells of rabbit jejunum. There were few axon varicosities with a predominance of large opaque vesicles or large dense-cored vesicle in any muscle layer of rabbit or opossum or in the plexuses of rabbit jejunum. There were almost as high a proportion of such varicosities in longitudinal as in circular muscle of rabbit jejunum, and there were almost none in circular muscle of opossum esophagus. The distribution of vesicles in varicosities was found often to be heterogeneous and could account for the observed occurrence of a small proportion of varicosities with a predominance of large vesicles. It is suggested that there is no structural distinction in vertebrates between axons which are cholinergic and axons which are nonadrenergic inhibitory.