Spinal cord: Long tube-shaped bundle of nerves running through the spine or backbone that transmits impulses to and from the brain and controls some reflex actions.
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The central nervous system is located centrally in the body, as its name suggests. It consists of two major parts: the brain and the spinal cord. The CNS acts as the command center of the body. It interprets incoming sensory information and sends out instructions on how the body should respond to the information.
The peripheral nervous system is the part of the nervous system outside the CNS. It consists mainly of nerves that extend from the brain and spinal cord to the rest of the body. Cranial nerves carry impulses to and from the brain while spinal nerves carry impulses to and from the spinal cord. The PNS can be divided into two systems: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls the voluntary movements of the skeletal muscles. The autonomic nervous system controls activities in the body that are involuntary or automatic. These include the actions of the heart, the glands, and the digestive organs and associated parts.

The brain stem is the stalk-shaped lower portion of the brain and is a continuation of the spinal cord. It consists of the medulla oblongata, pons, and midbrain. The medulla oblongata is actually a portion of the spinal cord that extends into the brain. All messages that are transmitted between the brain and spinal cord pass through the medulla. Nerves on the right side of the medulla cross to the left side of the brain, and those on the left cross to the right. The result of this arrangement is that each side of the brain controls the opposite side of the body.
Three vital centers in the medulla control heartbeat, rate of breathing, and diameter of the blood vessels. Centers that help coordinate swallowing, vomiting, hiccuping, coughing, sneezing, and other basic functions of life are also located in the medulla. A region within the medulla helps to maintain the conscious state. The pons (the Latin word for bridge) conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. The midbrain conveys impulses from the hypothalamus to the pons and spinal cord. It also contains visual and auditory (hearing) reflex centers involving the movement of the eyeballs and head.
Brain and spinal cord
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The brain and the spinal cord are the two major parts of the central nervous system, or CNS. The CNS acts as the command center of the body.
Illustration by Kopp Illustration, Inc.

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