Topographical organization of cat mesodiencephalic areas for monosynaptic activation of vertical oculomotoneurons.

This study investigated the topographical organization of mesodiencephalic areas for monosynaptic activation of vertical eye movement-related oculomotoneurons (OMNs) in anaesthetised cats. Systematic mapping of effective sites for inducing monosynaptic, excitatory field potentials in vertical OMN pools was performed bilaterally using one concentric bipolar stimulating electrode. These OMN pools included superior rectus, inferior oblique, inferior rectus and superior oblique pools of the oculomotor and trochlear nuclei. All the field potentials induced in each OMN pool were negative and had a latency of 0.8-1.2 ms, indicating monosynaptic activation of the OMNs. Effective stimulating sites for inducing the field potentials were distributed at a rostrocaudal level of A 7.0-9.0 and 1.0-4.0 mm lateral from the midline, but especially at A 7.0-8.0 and 2.0-3.0 mm lateral from the midline. Histologically, they were located within and near the medial part of the Forel's field H. Stimulation at a more caudal level of A 5.0-6.0 and 1.0-2.0 mm from the midline was also particularly effective. This area correspond to the interstitial nucleus of Cajal and the nearby reticular formation. For each OMN pool, the above field potentials were induced bilaterally, but predominantly ipsilaterally. These results suggest the existence of a bilateral, but ipsilaterally predominant, monosynaptic excitatory pathway from Forel's field H to each vertical OMN pool.