Neuroanatomy is the branch of anatomy that studies the anatomical organization of the nervous system. In vertebrate animals, the routes that the myriad nerves take from the brain to the rest of the body (or "periphery"), and the internal structure of the brain in particular, are both extremely elaborate. As a result, the study of neuroanatomy has developed into a discipline in itself, although it also represents a specialization within neuroscience. The delineation of distinct structures and regions of the brain has figured centrally in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions.
The peripheral nervous system is made up of all the neurons in the body outside of the central nervous system, and is further subdivided into the somatic and autonomic nervous systems. The somatic nervous system is made up of afferent neurons that convey sensory information from the sense organs to the brain and spinal cord, and efferent neurons that carry motor instructions to the muscles.
The autonomic nervous system also has two subdivisions. The sympathetic nervous system is a set of nerves that activate what has been called the "fight-or-flight" response that prepares the body for action. The parasympathetic nervous system instead prepares the body to rest and conserve energy.