Chapter 111. Divisions of the Nervous System

1. The nervous system is the human body’s fast communication system. It consists of the central nervous system (brain and spinal cord) and the peripheral nervous system (all the other nerves in the body). In all, the nervous system contains around 85 billion individual nerve cells called neurons.

2. The central nervous system (often referred to as the CNS) includes most of the neurons in the body. It handles the information-processing and decision-making tasks. Sensory information from the outside world and from inside the body is channeled through the peripheral nervous system (PNS) and gets analyzed in the CNS. If some action is required, the CNS organizes a response and communicates the message through the peripheral nervous system to the appropriate muscles or internal organs.

3. The peripheral nervous system is subdivided into the somatic nervous system (which carries sensory messages from skin and muscles, and controls the body’s movements) and the autonomic nervous system (which communicates with and regulates the internal organs, largely outside conscious awareness).

4. The autonomic nervous system (sometimes referred to as the ANS) controls the glands and muscles of our internal organs. It regulates the automatic behaviors necessary for survival. (Autonomic means "self-regulating.") The autonomic nervous system has two subdivisions, one responsible for mobilizing us during stressful situations, and the other for calming us when the emergency has passed.

5. The sympathetic division of the autonomic nervous system arouses the body during emergencies by accelerating heartbeat, slowing digestion, raising blood sugar, dilating arteries, and creating perspiration. This prepares the body for an energetic physical response to a threat or challenge, including the challenge of sexual intercourse.

6. The parasympathetic division of the autonomic nervous system relaxes the body after the emergency has passed, by producing the opposite effects—slowing the heartbeat, stimulating digestion, lowering blood sugar, contracting the arteries, and shutting down perspiration. The parasympathetic system is also responsible for the localized sexual arousal of the genitals in both females and males. Although this connection to sexual arousal may seem an exception to the usual tendency of the parasympathetic division to calm the body, it makes sense in terms of survival—having engorged genitals would not be a useful response in the presence of danger, when the sympathetic division is activated.

7. Here’s a memory aid—a sentence that may help you remember the general functions (except for sexual responses) of the two parts of the autonomic nervous system: The sympathetic division makes you sizzle (arouses you for action); the parasympathetic division pacifies you (calms you down).