Clearly, communication in the nervous system is fundamental to behavior. So far we have looked at how individual cells communicate with one another. In this section, we examine the organization of the nervous system as a whole. Figure 1.
Figure 1 Organization of the human nervous system.
presents an organizational chart that shows the relationships a all parts of the nervous system.
The Peripheral Nervous System
The first and most important cut separates the central nervous system (the brain and spinal cord) from the peripheral nervous system. The peripheral nervous system is made up all those nerves that lie outside the brain and spinal cord. Nerves are bundles of neuron fibers (axons) that are routed together in the peripheral nervous system. The peripheral nervous system can be subdivided into the sounatic nervous system and the autonomic nervous system.
The Somatic Nervous System
The somatic nervous system is made up of nerves that connect to voluntary skeletal muscles and to sensory receptors. These nerves are the cables that carry information from receptors in the skin, muscles, and joints to the central nervous system and that carry commands from the central nervous system to the muscles. These functions require two kinds of nerve fibers. Afferent nerve fibers are axons that carry information inward to the central nervous system from the periphery of the body. Efferent nerve fibers are axons that carry information outward from the central nervous system to the periphery of the body.
The Autonomic Nervous System (ANS)
The autonomic nervous system (ANS) is made up of nerves that connect to the heart, blood vessels, smooth muscles, and glands. As its name hints, the autonomic system is a separate (autonomous) system, although it is ultimately controlled by the central nervous system. The autonomic nervous system controls automatic, involuntary, visceral functions that people don’t normally think about, such as heart rate, digestion, and perspiration.
The autonomic nervous system can be subdivided into two branches: the sympathetic division and the parasympathetic division (see Figure 2). The sympathetic division is the branch of the autonomic nervous system that mobilizes the body’s resources for emergencies. It creates the fight-or-flight response. Activation of the sympathetic division slows digestive processes and drains blood from the periphery, lessening bleeding in the case of an injury. Key sympathetic nerves send signals to the adrenal glands, triggering the release of hormones that ready the body for exertion. In contrast, the parasympathetic division is the branch of the autonomic nervous system that generally conserves bodily resources. It activates processes that allow the body to save and store energy. For example, actions by parasympathetic nerves slow heart rate, reduce blood pressure, and promote digestion.
Figure 2 The Autonomic Nervous System.
The Central Nervous System
The central nervous system is the portion of the nervous system that lies whinin the skull and spinal column. Thus, the central nervous system (CNS) consists of the brain and the spinal cord. The spinal cord connects the brain to the rest of the body through the peripheral nervous system. Although the spinal cord.
looks like a cable from which the somatic nerves branch, it is part of the central nervous system. The spinal cord runs from the base of the brain to just below the level of the waist. It houses bundles of axons that carry the brain’s commands to peripheral nerves and that relay sensations from the periphery of body to the brain. Many forms of paralysis result from spinal cord damage, a fact that underscores the critical role it plays in transmitting signals from the brain to the neurons that move the body’s muscles.
Anatomically, the brain is the part of the central nervous system that fills the upper portion of the skull. Although it
weighs only about three pounds and could be held in one hand, the brain contains billions of interacting cells that integrate information from inside and outside the body, coordinate the body’s action, and enable us to talk, think, remember, plan, create, and dream.