An electron microscopic study of synaptic morphology in the oculomotor nuclei of three inframammalian species.

Abstract

AbstractThe fine structure and distribution of synapses in the oculomotor nuclei of three inframammalian species (spiny boxfish Chilomycterus schoepfi, grass frog Rana pipiens, American chameleon Anolis carolilzensis) were studied by electron microscopy. Oculomotor neurons in Chilomycterus are approximately 40 μ in diameter, and have one or a few large dendrites which extend in a ventro‐ lateral direction and ramify in the tegmental neuropil. Axons establish numerous en passant synapses and sometimes extend into indentations in the surfaces of the motoneurons. Approximately one‐sixth of synaptic profiles include regions of close membrane apposition (gap junctions). Presynaptic processes at these synapses contain electron‐lucent and dense core vesicles. Similar synapses occur on fine dendrites in the neuropil but are rare. Somatosomatic and dendroden‐ dritic close appositions are not present, so that electrotonic coupling between oculomotor neurons occurs via a presynaptic pathway. Synapses commonly arise at nodes of Ranvier. Rana oculomotor nucleus consists of motoneurons about 30 p in diameter with one or a few dendrites which ramify in the ventrolateral neuro‐ pil, and smaller and less common multipolar interneurons. The two neuronal types differ in their relationships with glial elements. The majority of synapses on large neurons exhibit distinct synaptic clefts. Close membrane appositions are present at approximately 8% of synaptic contacts on large neurons, but are rare on small diameter dendrites. Subsynaptic cisterns are present at some synapses, but were never observed subjacent to close appositions. Axosomatic synapses are uncommon on the smaller interneurons. A cryptlike extension of the cerebral aqueduct separates contralateral oculomotor nuclei in Anolis. Oculomotor neu‐ rons in dorsolateral and ventromedial divisjons of the nucleus are multipolar and approximately 25 p in diameter. Large areas of the cell bodies are covered by glial lamellae, and axodendritic synapses predominate. At 5% of synapses on somata and proximal dendrites, there are close appositions. Subjunctional dense bodies are present at other synapses. Previous electrophysiological studies have demonstrated electrotonic coupling between homonymous oculomotor neurons, and between neurons and presynaptic fibers in teleosts. The present results pro‐ vide morphological evidence for electrotonic transmission in the oculomotor nuclei of three diverse species of vertebrates.