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The rabbit SA-node was outlined electrophysiologically and its adrenergic and cholinergic innervation patterns were studied with the electron microscope. Differentiation between adrenergic and cholinergic terminals was achieved by fixation of the specimens in KMnO4 which produces dense-cored synaptic vesicles in adrenergic terminals, whereas synaptic vesicles in cholinergic terminals are empty. It was found that adrenergic and cholinergic nerve terminals often come in close apposition to each other, the distance between adjoining membranes being in the order of 250 Å. At times, faint membrane thickenings could be seen in these places. The available pharmacological, physiological and morphological evidence leaves little room for doubt that cholinergic terminal fibers can influence the adrenergic ones. From mainly morphological evidence it is also postulated that adrenergic terminals influence cholinergic terminals.

The relations between adrenergic and cholinergic terminals were studied in rat iris and rat heart with the electron microscope. Adrenergic terminals were identified by treating the animals with 5-hydroxydopamine, which produces dense-cored synaptic vesicles in adrenergic terminals in tissues fixed in glutaraldehyde and osmium. The specificity of this observation was verified. It was found that adrenergic and cholinergic nerve terminals often come in close contact with one another, the distance between the adjoining membranes being about 250 Å. At times, faint membrane thickenings could be observed in these places. The available pharmacological, physiological, and morphological evidence leaves little room for doubt that cholinergic terminal fibres can influence the adrenergic fibres. From mainly morphological evidence, it is also postulated that adrenergic terminals influence cholinergic ones.

Plexuses of cholinergic nerve terminals were demonstrated (acetylcholinesterase staining) in pial arteries (down to a diameter of about 15µ) at the base of the brain and on the brain convexities of mice, rats, rabbits, hamsters, guinea-pigs, and cats. The pial veins were less well supplied than the arteries. Consecutive formaldehyde gas treatment (to visualize adrenergic nerves) and acetylcholinesterase staining revealed that the adrenergic and cholinergic plexuses followed each other closely, the axon terminals running together in the same Schwann cell strands. This was confirmed by electron microscopy after KMnO4 fixation or 5-hydroxydopamine treatment. The varicosities of cholinergic and adrenergic axons were sometimes seen as close as 250 Å. In the neuro-effector area, the terminals of both nerve types (naked or surrounded by an incomplete Schwann cell covering) approached the smooth muscle cells as close as 800–1100 Å, and they were separated from the latter only by the fused neuronal and muscular basement membranes. In this area axo-axonal contacts were observed. The adrenergic, but not the cholinergic, nerves disappeared after bilateral removal of the superior cervical sympathetic ganglia. Isolated cat middle cerebral artery contracted strongly with acetylcholine, and the effect was inhibited by atropine.

In the oxyntic gland area of the rat stomach the histamine-containing epithelial cells (also referred to as enterochromaffin-like cells because of their morphologic similarity with the 5-hydroxytryptamine-storing enterochromaffin cells) constitute the system of argyrophil cells in this area as previously shown by the combined use of fluorescence and light microscopic techniques. By performing the argyrophil staining reaction directly on ultra-thin sections it could be demonstrated in the electron microscope that the argyrophil cells have features suggesting that they are endocrine. Based on the ultrastructure of their secretory granules at least two such endocrine cell systems—both argyrophil—could be recognized in the oxyntic glands. The silver deposits were accumulated over the secretory granules of both these cell systems.