Simple reflexes may be organized by neurons within the segment of entry and exit of related sensory and motor nerves. More complex reflexes require intersegmental organization and, quite frequently, inputs from the brain. The brainstem , consists of the medulla, pons and midbrain . The major ascending and descending fiber tracts that interconnect the cerebrum and cerebellum with the spinal cord travel near the surface of the brainstem. Its central region contains mainly nuclei.
Most cranial nerves enter and exit the brain from the medulla and rostrally adjacent pons.
The medulla contains the first brain relay stations for the somatosensory pathways that ascend in the spinal cord. Most motor fibers from the cerebrum also cross to the opposite side in the medulla. The brainstem is the source and target of the cerebellar peduncles, which interconnect the cerebellum with the rest of the central nervous system. The cerebellum consists of a central mass of four pairs of nuclei, surrounded by a deeply convoluted cellular cortex. The cerebellum has two distinct hemispheres, attached to a central region called vermis.