A horseradish peroxidase study of the innervation of the internal structures of the eye. Evidence for a direct pathway.

The sources of innervation of the internal structures of the eye in monkey and rabbit were investigated by injecting horseradish peroxidase (HRP) intraocularly and monitoring its retrograde transport. HRP-labeled cells were found in the ciliary ganglia and in the midbrain throughout the dorsoventral extent of the midline of the oculomotor complex. The results suggest that a direct pathway exists from the midbrain, and it is argued that this pathway is for the control of accomodation. Either this pathway is direct, with no synapse interposed, or the postsynaptic neurons are near or in the intrinsic musculature. A third but not so likely possibility is that retrograde transsynaptic transport of HRP occurred across electrotonic synapses in the ciliary ganglion. These findings may challenge the commoly held view that the entire pathway for the control of the intrinsic musculature synapses in the ciliary ganglion. On the other hand, it may be that this direct path is a sensory system much like the mesencephalic nucleus of the trigeminal nerve.