(A) Left, Schematic of a parasagittal section of the cerebellum showing injection site for anterograde tracing from mCbN. Middle, Confocal image of the mCbN after injection of AAV-hSyn-ChR2-eYFP virus. Dotted line, mCbN boundaries. Right, Confocal image of virally labeled mCbN axons in the vlPAG. (B) Left, Schematic of a coronal section of the PAG showing injection site for retrograde tracing from the vlPAG. Middle, Example injection site of CTb-GFP in the vlPAG. Dotted line, approximate boundaries of vlPAG. Right Retrogradely labeled neurons in the mCbN. Dotted line, boundaries of mCbN. (C) Top, Low magnification confocal image of the PAG in a Chx10-tdT mouse, showing the distribution of Chx10 neurons in the ventrolateral PAG. Bottom, High magnification image of the white box in the upper panel. (D) Left, Schematic of the PAG showing injection site for anterograde tracing in Chx10-cre mice. Right, Confocal image of the vlPAG after viral labeling of Chx10 neurons. (E) Left, Schematic of a coronal section of the brainstem showing the approximate rostro-caudal position of Chx10-positive axons in the magnocellular reticular nucleus (Mc), approximately −6.6 mm from bregma. Right, Axonal labeling of Chx10-positive axons in the Mc. Arrows indicate labeled axons. (F) Left, EPSCs evoked in Mc neurons by optogenetic stimulation of Chx10-ChR2 axons. Right, Population data for first evoked EPSC in Mc neurons. Open symbols, individual cells, solid symbols, mean ± SEM.