The nervous system, responsible for sensing, integrating, and responding to various stimuli, is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS has two functional divisions: the sensory or afferent division and the motor or efferent division.

The sensory division transmits information from sensory receptors in the body to the CNS. It provides the CNS with knowledge about somatic senses (such as tactile, thermal, pain, and proprioceptive sensations) and special senses (like smell, taste, vision, hearing, and equilibrium).

On the other hand, the motor division carries output from the CNS to effectors like muscles and glands. This division further splits into the somatic nervous system (SNS) and the autonomic nervous system (ANS).

The SNS directs output from the CNS to skeletal muscles. The ANS controls output from the CNS to smooth muscles, cardiac muscles, and glands. The ANS comprises two main branches: the sympathetic nervous system and the parasympathetic nervous system. These usually have opposing actions. The parasympathetic nervous system manages "rest-and-digest" activities, while the sympathetic nervous system supports exercise or emergency actions, also known as "fight-or-flight" responses.

Lastly, there is the enteric nervous system (ENS), a network of over 100 million neurons confined to the wall of the gastrointestinal tract. The ENS helps regulate the activity of the smooth muscle and glands of the GI tract and communicates with and is regulated by the other branches of the ANS.