1.2 Organization of the Nervous System

The nervous system is subdivided into two systems: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is made of the brain and the spinal cord. The CNS is protected by the bony structures of the skull and spine and the cerebral spinal fluid that surrounds the CNS.  The CNS is the part of the nervous system that allows us to integrate, process, and respond to information from our internal and external environment. Specific functions of the brain are described in the section “Organization of the Brain.”

The peripheral system comprises the nerves and neurons throughout the body that are not part of the brain or spinal cord. They send information between the body and the central nervous system. The peripheral nervous system is organized into subsystems. The somatic nervous system and the autonomic nervous system.

The somatic nervous system functions to process our actions and thoughts that are voluntary or under conscious control. The somatic nervous system has sensory neurons that receive sensory information from our environment. These signals are carried toward the central nervous system. The central nervous system signals the motor neurons that carry information to the muscles allowing the body to respond. In between the motor and sensory neurons are interneurons, these make up most of neurons in our bodies.

The autonomic nervous system controls the functions, such as digestion and heart rate, which generally are not under our voluntary control. This system has two subsystems: the sympathetic and the parasympathetic systems. The sympathetic nervous system is the system of activation and arousal. It prepares the body for when energy or a quick response is needed. This system is activated in exciting and in stressful situations. The sympathetic system is associated with the fight or flight response; the body uses energy, pupils dilate, heart rate and blood pressure increase. This allows the person to respond quickly to a stressor or danger, such as realizing you forgot you had a paper due tomorrow or encountering a bear at your campsite. The parasympathetic nervous system is the balance to the sympathetic nervous system, it is activated during periods of rest and routine functions. When there is no immediate danger or excitement, the body digests food and stores energy for later use, the blood pressure and heart rate function at normal rates.