Abstract
The fine innervation of the heart in dog and rabbit was investigated by the electron microscope. It was concluded that:
1. Not only precapillary but also true capillary in the area of sino-atrial node of dog and rabbit are innervated by cholinergic and adrenergic nerve fibers.
2. Two types of synaptic vesicles were identified, the one (cholinergic) has the average diameter of 500Å, while the other (adrenergic) 1, 000Å. The latter synaptic vesicles contain granules in the core. Dense granules in the core of the vesicles may be associated with catecholamine.
3. Possible role of the synaptic vesicles to the true capillary without muscular elements was discussed briefly.
