THE PERIPHERAL AND CENTRAL PROJECTIONS OF THE EDINGER-WESTPHAL NUCLEUS IN THE RAT - A LIGHT AND ELECTRON-MICROSCOPIC TRACING STUDY

The peripheral and central efferent projections of the rostral part of the Edinger-Westphal nucleus in the rat were investigated at the ligh t and electron microscopic level by means of iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin and retro grade tracer injections of Fast blue and Nuclear yellow into the facia l nucleus and into the principal olive. Two pathways leaving the rostr al part of the Edinger-Westphal nucleus were studied, a peripheral and a central descending pathway. Fluorescent experiments demonstrated th at the central pathway fibers originated from distinct individual Edin ger-Westphal neurons. These neurons were mainly distributed throughout the rostral part of the Edinger-Westphal nucleus and had fusiform cel l bodies. The neurons rarely form collateral projections. The central descending pathway left the Edinger-Westphal nucleus medially and term inated bilaterally in the principal olive, in the subnuclei A, B and C of the inferior olive and ipsilaterally in the medial accessory olive . The central pathway also terminated contralaterally in the lateral p arabrachial nucleus, the facial nucleus, the trigeminal brainstem nucl ear complex, the lateral reticular nucleus and the rostroventral retic ular nucleus. The projection to the facial nucleus provides evidence f or the existence of a polysynaptic loop forming the central part of th e corneal blink reflex. Projections from the Edinger-Westphal nucleus to the cerebellar cortex or the deep nuclei, as described in cat and p rimate, could not be confirmed. The peripheral pathway left the Edinge r-Westphal nucleus ventrally and terminated on dendrites of ciliary ga nglion cells, along smooth muscle cells of ciliary ganglion associated arterioles and in the proximity of ciliary ganglion associated venule s. The central and peripheral terminals that originate in the Edinger- Westphal nucleus all had similar ultrastructural features: clear, roun d vesicles and electron dense mitochondria. The terminals originating from the central descending pathway were often found to be arranged in glomerular-like structures. The central and peripheral terminals made asymmetric synaptic membrane specializations (Gray type one), except terminals innervating the ciliary ganglion associated vessels, which s howed no synaptic contacts.