The rubrospinal tract collateralizes to the interposed nucleus. A, Schematic of experiment. The cre-expressing virus AAV8.hSyn1.mCherry.Cre was injected into the RN and a Cre-dependent retrogradely transported virus, AAV1.CAG.Flex.eGFP, was injected into the IN to label IN-projecting neurons in the RN. B, Example coronal section showing GFP expression in the RNm (−3.80 mm from bregma; n = 7). Scale bar, 200 μm. C, GFP-expressing axons in the rubrospinal tract at C5 level of the spinal cord. Scale bar, 10 μm. D, GFP-expressing axon terminals in the ventral horn gray matter at the C5–C7 level (n = 4). Scale bar, 10 μm. RST, Rubrospinal tract; CE, cervical enlargement; 3V, third ventricle.