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The nervous system is comprised of cellular networks that collect and integrate information, direct behavior, and create concepts and memories. You are reading these words using input from the sensory receptors in your eyes to acquire visual signals for interpretation by your brain in concert with the output of motor systems to control eye movements required to track the words. At the same time, other components of your nervous system are managing "housekeeping" functions, including maintaining posture, breathing, and moving food through your digestive system. This is a huge task; when a body is at rest, the brain by itself is responsible for one-fifth of the body's oxygen consumption.

The nervous system consists of two kinds of cells, neurons and glial cells. Neurons, or nerve cells, are the functional units of information and signal processing, and glial cells provide essential support to neurons. There are approximately 1011 neurons and 1012 glial cells in the body. Neurons are electrically excitable cells linked together into computational circuits, with each cell having hundreds or thousands of specific connections. Simple circuits, for example, can involve only sets of the few cells that are required to make the leg extend when the patellar tendon is stretched. The structures involved in more complex activities such as reading likely involve at least hundreds of circuits and millions of cells.

The basic organization of the nervous system is described from both anatomic and functional perspectives. The nervous system has two anatomic realms.

  • The peripheral nervous system (PNS) receives sensory information from the outside world and from conditions within the body. It also transmits commands to effectors such as muscles and glands.

  • The central nervous system (CNS), which consists of the brain and the spinal cord, integrates sensory inputs and produces coordinated outputs. The sensory systems of the eye and ear are also part of the CNS.

The nervous system also has two functional divisions; one includes a subjective component.

  • The somatic or voluntary nervous system is comprised of central elements that produce output that can be controlled by thinking.

  • The autonomic nervous system (ANS) consists of both central and peripheral elements that control involuntary activities such as gut motility. The ANS has parasympathetic and sympathetic divisions that often have opposed actions.

The nervous system is often compared to a digital computer because both are involved with information processing. Digital computers are built to precise specifications, with the location of each component and its connections predetermined. However, there is far too little genetic information to direct complete specification of the nervous system, and a selective strategy is used for its construction. After the developmental period, which extends for years after birth, subtle cellular changes must be involved in memory formation and learning; any larger plasticity required for regeneration is severely limited, however, and damage to nervous tissue is ...

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