The neuronal pathway subserving the pupillary light reflex.

The pupillary light reflex was investigated using electrical stimulation along the pathway and recording in the short ciliary nerves. The discharge of single units in the ciliary ganglion was compared during diffuse light stimuli and electrical stimuli. It was concluded that the early reflex discharge in the short ciliary nerves following electrical stimulation on the optic tract is due to excitation of fibres active during the pupillary light reflex. The light reflex is conveyed by slow (less than 10 m/sec) optic tract fibres which synapase in the medial part of the pretectal area. In turn, pretectal neurones with conduction velocities of about 6 m/sec pass to the Edinger-Westphal complex from which the preganglionic 'pupilloconstrictor' neurones originate. Latency measurements show that there are synapses in the pretectal region and the Edinger-Westphal nucleus. The amplitude of the 'pupilloconstrictor' responses in the short ciliary nerves can be used as a measure of the excitability in the pathway of the pupillary light reflex under various conditions, e.g. following conditioning stimuli of other interacting pathways. In addition to the 'pupilloconstrictor' response there is also another short-latency discharge in the short ciliary nerves following stimulation of the posterior commissure and the Edinger-Westphal nucleus. That discharge is presumably due to activation of fibres which cause accommodation of the lens.