The nervous system is an essential component of the human body and is divided into two main systems: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is composed of the brain and the spinal cord and acts as the command center of the organism. The PNS is a network of nerves that extends from the spinal cord throughout the body and regulates both voluntary and involuntary movements, such as digestion, heart rate, breathing, and body temperature. Within the PNS, there are two subdivisions: the somatic nervous system and the autonomic nervous system. The somatic system controls voluntary movements, while the autonomic system operates automatically and regulates essential involuntary functions like heart rate and digestion. It is further divided into two branches, the sympathetic and parasympathetic systems, which work together to maintain homeostasis. Alterations in the autonomic nervous system can lead to various diseases, as these two branches play a crucial role in regulating the organs and systems of the body. For example, an imbalance in the sympathetic system can result in excessive heart rate and blood pressure, while a dysfunction in the parasympathetic system can lead to digestive problems. Therefore, understanding its divisions and functions is essential for the diagnosis and treatment of diseases caused by autonomic nervous system dysfunction. This book provides a comprehensive overview of this system and its functions.