Enlarged View of the Right PICA. Left, Enlarged view of the right PICA. The PICA penetrates the dura and passes in front of the spinal portion of the accessory nerve and behind the upper attachment of the dentate ligament. The VA penetrates the dura in front of both the accessory nerve and the dentate ligament. The C-1 nerve root passes below the extradural origin of the PICA. Right, The left PICA penetrates the dura medial to the atlantooccipital joint and has no extradural branches. The PICA shares the site of dural penetration with the VA and the C-1 nerve root. After penetrating the dura, the PICA loops caudally to the level of the C-2 dorsal root. (Images courtesy of AL Rhoton, Jr.)