Schematic of the corticospinal tract (CST) trajectory. The course of the CST is shown in (a) a sagittal view of the mouse brain (black line) and in (b) a three-dimensional schematic of the medulla (red line). (a) The CST begins in the motor cortex (mcx), where layer V neurons project axons through the internal capsule (ic) and cerebral peduncles (cp) to the level of the mid-hindbrain boundary (MHB), just rostral to the pontine nuclei (pn), where they turn medially and ventrally to project along the ventral surface of the medulla oblongata (mo) as the pyramidal tracts. Within the caudal medulla the CST is surrounded laterally and dorsally by the inferior olives (not shown but visible in Figures 2d and 6). (b) At the boundary between the medulla and the spinal cord CST axons turn dorsally and cross the midline, forming the pyramidal decussation and subsequently project caudally into the dorsal funiculus (df).