Myenteric plexus of frog large intestine: light and electron microscopy of fiber system and neurons.

The myenteric plexus of the frog large intestine was studied by means of silver impregnation, NADH-diaphorase histochemistry and electron microscopy. The main fiber bundles of the myenteric plexus are situated between the longitudinal and circular smooth muscle layers. Most of the branches of this main plexus could be followed into the circular muscle layer. The diameter of the large nerve bundles was demonstrated by impregnation to be 7-8 microns. NADH-diaphorase histochemistry revealed a random distribution of the nerve cells; the nerve cells did not form ganglia. The maximal profile of these mostly multipolar cells was 75-100 microns 2. The packing density of nerve cells was 710 +/- 23 per cm2. At the electron microscopic level, the plexus consisted of a number of unmyelinated nerve profiles, a few myelinated axons, Schwann cells and neurons. The proportion of myelinated fibers did not exceed 1%. The nerve profiles contained vesicles, small agranular vesicles or mixed vesicle populations. Synapses were extremely rare in the neuropil. The possibility and importance of non-synaptic transmitter release and presumed transmitter substances are discussed.