On the ultrastructure of the enteric nerve ganglia

This brief article reviews some aspects of the ultrastructure of the enteric ganglia which have been brought to light by electron microscopy. The ganglion neurons are surrounded by a vast neuropil in which axons, dendrites and glial cells with their processes are tightly packed together. Blood vessels and connective tissue do not penetrate into the ganglia but lie outside them. The exclusion of connective tissue from the ganglia takes place during embryonic development and is complete soon after birth. By ultrastructural criteria it has proven difficult to classify neuronal cell types, in spite of the differences in their functional specializations and their projections. The glial cells are rich in gliofilaments and have processes which are firmly anchored to the surface of the ganglion. Glial cells outnumber ganglion neurons and probably confer structural stability to the ganglia and at the same time allow substantial changes in the shape of the ganglia and in the arrangements of its elements to occur when the adjacent muscle layers contract. Numerous specialized contacts are found between vesicle-containing nerve endings and glial cell bodies or glial processes. The question of how freely substances diffuse inside a ganglion, along the narrow spaces between processes and cell bodies, remains to be answered. Since parts of the surface of many neurons are directly covered by the basal lamina of the ganglion and by connective tissue, certain substances of the interstitial space may have direct access to the nerve cells. Investigations on the intraganglionic nerve endings have provided several classifications of axonal types, mainly based on the appearance of their vesicles. However, these classifications are tentative and are in many respects still uncertain. The great majority of endings originate from intrinsic neurons and synaptic specializations are commonly encountered.