Observations on the synaptology of intracellularly injected spinothalamic tract neurons in the cat.

Single spinothalamic tract cells in the deep lumbar group have been injected with horseradish peroxidase and their dendritic trees examined on the electron microscope. Large dendrites are surrounded by rosettes of from 5 to 12 synaptic terminals. Most terminals contain round vesicles but some terminals contain flattened clear vesicles and others contain membraneous tubes or cisterns. Large (mean diameter 100 nm) round, dense-core vesicles are found in both terminals containing clear round vesicles and terminals containing flattened vesicles. As yet none of those terminals on STT cell dendrites have been found participating in axo-axonic synapses, suggesting that presynaptic inhibition or facilitation plays no role in modulating the responses of these cells to presynaptic activity.