Tonic and reflex synaptic activity recorded in ciliary ganglion cells of anaesthetized rabbits

We have studied patterns of synaptic activity in rabbit ciliary ganglion cells by intracellular recording in vivo, and have examined the morphology of functionally characterized neurones by intracellular injection of horseradish peroxidase. Nearly all of the neurones studied (293 of 300) received tonic synaptic input from preganglionic neurones. This tonic activity was not decreased by darkness or by acute optic nerve section. The rate of tonic synaptic activity recorded in the vast majority of neurones (94%) changed in response to retinal illumination. Most ganglion cells showed an increased rate; some cells, however, showed decreased activity during illumination. The rate of synaptic activity recorded in ciliary neurones tended to be progressively higher in neurones with more complex geometries. Neurones with similar reflex properties included cells that lacked dendrites and cells with complex dendritic arborizations; conversely, neurones with similar geometries often had different reflex characteristics. The synaptic activity arising from different preganglionic axons innervating the same ganglion cell was not temporally linked in any obvious way. The relevance of these results to the regulation of the number of axons that innervate target neurones is discussed.