Direct visualization by scanning electron microscopy of the preganglionic innervation and synapses on the true surfaces of neurons in the frog heart.

A simple and flexible technique is reported for scanning electron microscopic observation of the true surfaces of neurons, and the distribution of glial cells, preganglionic nerve fibers and synapses on them. The interatrial septum of the frog was chosen as a convenient preparation, and connective tissue and glial cells were successively removed by incubation in collagenase and protease solutions. The three-dimensional relationships between presynaptic fibers and target neurons can be observed with high resolution over long distances. Some neurons have multiple innervation. Varicosities can be seen clearly, and where their interior has been exposed, synaptic vesicles can be discerned.