Transition of the spinal cord to the brainstem shows a continuum of neuronal groups and pathways. This is due to the continuous space provided by the neurotube architecture of the brain and spinal cord. Axons can project long distance to interconnect the most caudal levels of the spinal cord to the most rostral levels of the brain.
While the spinal cord interfaces with body structures through spinal nerves, the brain has coverage of the head through 12 cranial nerves. Ten cranial nerves course to and from the brainstem through the subarachnoid space forming a loose shearing zone that prevents breaking of nerve fibers.
Neuronal groups define functional nuclei (groups) along the course of the brainstem. Cranial nerves are more complicated than spinal nerves due to added senses and motor, special senses, components for parasympathetic system, gill arch sensory nerves and additional gill arch motor expressions.