Differentiation of Synaptic Bulbs in Clarke's Column

INVESTIGATIONS of the fine structure of synaptic junctions have revealed a morphological differentiation of apposed synaptic membranes and associated presynaptic and postsynaptic elements1–4. Presynaptic terminals have been shown to contain agranular synaptic vesicles, although larger granular vesicles have also been reported5,6. Two kinds of axonal terminals were recently identified, one of which contains spherical, and the other flattened, agranular vesicles. The observation was made in boutons synapsing with dendrites in the molecular layer of the cerebellum7 and in boutons synapsing with anterior horn cells of the spinal cord8. This communication reports observations on two forms of synaptic vesicles which can be used to differentiate two types of boutons found within Clarke's column of the monkey (Saimiri sciureus). This nucleus of the dorsal column is found in the spinal cord at thoracic and upper lumbar levels. It receives proprioceptive impulses chiefly from neuromuscular spindles and Golgi tendon organs. These impulses are relayed to the cerebellum by spinocerebellar tracts.