Efferent projections of the cat oculomotor reticular omnipause neuron region: an autoradiographic study

Omnipause neurons (OPNs) are midline pontine neurons that are thought to be instrumental in the generation of saccadic eye movements. Inhibition of the tonically active OPNs is postulated to disinhibit the burst neurons that cause the saccadic discharge in motoneurons, leading to a saccade. To test whether the anatomical connections of OPNs are consistent with this scheme, we studied the efferent projections of the OPN region by using the technique of anterograde transport of tritiated amino acids. Injections into the OPN region yield a distinct pattern of labeled tracts and terminal fields that is different from the patterns following control injections in the surrounding reticular formation. Caudally, there are terminal fields over the paramedian reticular formation, the caudal part of the medial accessory nucleus of the inferior olivary complex, the nucleus prepositus hypoglossi, and the nucleus reticularis paragigantocellularis dorsalis caudal and ventromedial to the abducens nuclei. Rostrally, terminal label is distributed over parts of the nuclei reticularis pontis caudalis and oralis, the nucleus raphe pontis, the nucleus reticularis tegmenti pontis, the mesencephalic reticular nucleus, the central gray, and the nucleus of the H-field. Thus, there are direct projections from the OPN region to all areas known to contain burst neurons. In addition, OPNs also apparently have indirect access to the spinal cord and cerebellum. Many of the latter connections parallel the efferent projections of the superior colliculus.