The document discusses the main parts and functions of the nervous system. It describes the central nervous system as consisting of the brain and spinal cord. The peripheral nervous system connects the central nervous system to the rest of the body and consists of cranial nerves, spinal nerves, and the autonomic nervous system. The autonomic nervous system regulates involuntary body functions and is divided into the sympathetic and parasympathetic nervous systems, which work in opposition to activate the fight-or-flight response or rest and digest responses, respectively.