Segments of the PICA. A, Left posterolateral view. The cerebellum was removed by dividing the cerebellar peduncles. The p1 arises anterior to the level of the inferior olive. The p2 courses between the most prominent part of the olive and the origin of CNs IX–XI from the medulla. The p3 courses along the posterolateral medulla and inferior cerebellar peduncle. The p4 courses within the telovelotonsillar cleft and forms the cranial loop, and the p5 is composed of the cortical branches. B, Posterior view. The p2 courses between the most prominent part of the olive and the origin of CNs IX–XI. The p3 courses along the posterolateral medulla and inferior cerebellar peduncle (Inf. Ped.). C, Superior view of an axial section through the medulla. The p1 arises along the anterolateral margin of the medulla and is related to the rootlets of the hypoglossal nerve. The p2 is situated between the most prominent part of the olive and the origin of CNs IX–XI from the medulla. D, Posterior view of the relationship between the p1 to p3 and to the lower CNs. The p1 arises anterior to the midportion of the olive and passes by the rootlets of CN XII. The p2 courses along the lateral medulla and the p3 along the posterolateral medulla and inferior cerebellar peduncle. E, The right half of the cerebellum has been removed to show p3 to p5. F, Posterior view of the suboccipital surface of the cerebellum. The p3 courses along the caudal margin of the tonsil, and the p5 is composed of the cortical branches. (Images courtesy of AL Rhoton, Jr.)