Swallowing, salivating, breathing, eye movements, blood pressure regulation, and gut activity are only a few of the many autonomic functions that are localized in the medulla, the pons, and the midbrain. To carry out these functions, the brainstem has its own components of the PNS, the 12 paired cranial nerves. You encountered the olfactory nerve, the optic nerve, and the auditory nerve (cranial nerves I, II, and VIII) in Chapter 45. Another one, cranial nerve X, is called the vagus (“wandering”) nerve because it travels into the body cavity and communicates with many of the organs, including the heart and the gut. You will encounter the vagus nerve in subsequent chapters.

Anatomically distinct groups of neurons in the CNS that work together as a unit are called a nucleus (not to be confused with the nucleus of a single cell). Axons from the neurons of many brainstem nuclei extend to various regions of the brain and modulate brain activity; for example, brainstem nuclei are involved in keeping the higher brain areas awake or allowing them to sleep. All of the sensory information coming up the neural axis from the spinal cord passes through the brainstem on its way to the forebrain, and many of these ascending neuronal tracts give off branches to the awake-promoting nuclei in the brainstem. Because the neuronal circuitry in this part of the brain is so complicated and because activity in these ascending sensory pathways can promote wakefulness, the core of the brainstem has been termed the reticular-activating system (“reticular” means netlike). Damage to the brain or spinal cord below the reticular-activating system can result in paralysis but leave sleep–wake cycle behavior normal. Damage above the level of the reticular-activating system can interrupt ascending wake-promoting activity resulting in a coma.