Axosomatic synapses of the rat ciliary ganglion

We have identified four different types of axosomatic synapses within the rat ciliary ganglion, and present the three-dimensional relationships of both pre- and postsynaptic elements. The majority of axosomatic synapses are situated on small postsynaptic spines that simply appose the axon (termed somatic spine), or are situated within an axonal invagination (termed invaginating somatic spine). The somatic spine synapse predominates, composing 70% of the population, which may be due to simplicity of construction as it usually forms only one active zone. In contrast, the invaginating somatic spine forms multiple active zones and accounts for only 22% of the population. Synapses involving a regular nonspinous portion of the cell membrane were rarely encountered (6%; termed somatic), as were those of axon branches situated within tubular invaginations of the cell body (2%; termed tunnelling). Synapses were differentially distributed, occurring four times more frequently on that portion of neuronal cell body membrane adjacent to the glial cell perinuclear area. However, there was no preferred location by synapse type, suggesting that this unequal distribution was the result of a general mechanism. The neuronal cells of the rat ciliary ganglion apparently constitute a single population, at least on the basis of cell size, shape, and organelle content.