The Origin of Inputs to the Monkey Central Mesencephalic Reticular Formation (cMRF)

Abstract

The central mesencephalic reticular formation (cMRF) is a midbrain region reciprocally connected to the superior colliculus (SC) where stimulation elicits horizontal saccades. Here we sought to identify its sources of input, by injecting retrograde tracers into the cMRF of M. fascicularis monkeys. In the diencephalon, labeled neurons were observed in the zona incerta, hypothalamus and the ventral lateral geniculate, ipsilaterally. The main source of midbrain input to the cMRF outside the SC was the ipsilateral substantia nigra pars reticulata, and contralateral cMRF. Within the pons, the paramedian reticular formation displayed a few cells, mostly ipsilaterally. Labeled neurons were found bilaterally in the nucleus prepositus hypoglossi and in the interposed and fastigial nuclei. A few neurons were present at medullary levels. It has been suggested that the cMRF might provide a conduit to the SC for ascending information on saccade duration and direction. While some feedback was observed for most of the cMRF's downstream targets, extensive connections to support this proposed function were not found. Instead, the cMRF seems to receive its greatest input from motor, as opposed to sensory, structures that also supply the SC. If the cMRF reticulotectal projection is inhibitory, then the GABAergic cells in the nigra and zona incerta are in position to release this inhibition via the cMRF.Support: NIH Grant EY014263