Abstract
1. Results from the electron microscopic observations of the autonomic nerve terminals in general were summarized and discussed with relation to the concept of “the autonomic ground plexus” proposed by HILLARP (1959, 1960).
2. The fine structure of adrenergic and cholinergic nerve terminals and their distribution within the vertebrate heart have been described. From the amphibian, avian and mammalian myocardium, evidence has been obtained which indicates the two types of autonomic axon terminals to exist in pairs sharing a common Schwann cell sheath.
3. Special attention was focused upon the particularly dense innervation of the cardiac pacemaker regions which have been observed in the heart of lower as well as higher vertebrates.
4. Very little information has been available concerning the fine structure of sensory nerve endings within autonomic effectors. Axon profiles, packed with an unusual abundance of mitochondria and a few vesicles and tentatively interpreted as representing a peripheral portion of the cardiac afferents, have been found in the guinea pig pulmonary valve and in the human ventricular myocardium.
