The human nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is composed of the brain and spinal cord, while the PNS contains nerve cells, clusters of nerve cells, and the sensory receptors that are outside the CNS. The PNS has two types of nerve cells: sensory (afferent) and motor (efferent). Sensory cells send signals to the CNS from receptors, and motor cells carry signals from the CNS to organs, muscles, and glands. The PNS is further divided into three sections: the autonomic nervous system (ANS), the somatic nervous system (SNS), and the enteric nervous system (ENS). The ANS regulates involuntary functions for homeostasis, such as heart rate, gland secretion, and digestion. The SNS mainly controls skeletal muscles, managing both voluntary and involuntary contractions. The ENS contains many nerve cells in the gastrointestinal (GI) tract and controls involuntary actions such as peristaltic movement of the GI tract and hormone secretion from endocrine cells.

The ANS consists of two divisions: sympathetic and parasympathetic. Sympathetic neurons originate from the spinal cord's thoracic and lumbar segments, while parasympathetic neurons originate from cranial and sacral segments. These divisions have preganglionic neurons connecting to ganglia near the spinal cord or within target organs. The ANS uses neurotransmitters acetylcholine and noradrenaline. Sympathetic activation triggers a "fight-or-flight" response, while parasympathetic activation induces a "rest-and-digest" response. Most organs receive signals from both divisions. The ANS is vital for maintaining bodily functions and coordinating responses.