Ultrastructure of the autonomic innervation apparatus in the main pial arteries of rats and cats

The autonomic nerve supply, particularly its terminal portion, was studied by electron microscopy of the anterior and middle cerebral arteries of rats and cats. In agreement with previous histochemical information, the arteries received a considerable number of non-myelinated axons that were enclosed in a Schwann cell plasmodium. Close to the vascular wall, varicosed terminals were found together with smooth preterminal fibres. In KMnO₄-fixed material from untreated animals, about half of the terminals contained an abundance of dense-cored, 50 nm synaptic vesicles, whereas the synaptic vesicles in the remainder of the terminals were empty. In terminals of both types, a smaller number of larger electron-dense vesicles of 100 nm was present. After treatment of the animals with 5-hydroxydopamine followed by glutaraldehyde and osmium fixation, dense-cored vesicles with a diameter of 50 nm up to 80 nm appeared in about the same number of terminals as were previously found to contain the electron-dense synaptic vesicles. The rest of the axon terminals remained unchanged. It is concluded that the terminals with dense-cored vesicles belonged to adrenergic nerves, whereas those with the empty 50 nm vesicles were cholinergic. Near the vessel, the Schwann cell sheath of the terminals was usually incomplete, and varicosities of either the same or different (i.e., adrenergic/cholinergic) type came in close apposition to each other with a distance between adjacent membranes of approximately 25 nm. This arrangement offers a structural possibility for an interaction mechanism between the two types of axon terminals. In the adventitia the terminals had either an incomplete Schwann cell sheath, or were completely naked. Varicosities of the terminals of both types of axons approached the outer muscular layer of the media with a distance of about 100 nm. In this neuro-effector area, axo-axonal appositions were also found. The pial arteries possessed a neuro-effector apparatus that fulfilled the criteria of a true autonomic innervation in the same way as with arterial vessels in various peripheral organs.