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The human nervous system, by far the body’s most complex system, comprises a network of many billion nerve cells (neurons), all supported by many more cells called glial cells. Each neuron has hundreds of interconnections with other neurons, forming an extremely intricate system for processing information and generating responses.

Neurons and their processes become localized throughout the body as an integrated communications network. Anatomically, the general organization of the nervous system (Figure 9–1) has two major divisions:

  • Central nervous system (CNS), consisting of the brain and the spinal cord

  • Peripheral nervous system (PNS), composed of the cranial, spinal and peripheral nerves conducting impulses to and from the CNS (sensory and motor nerves, respectively) and ganglia consisting of small aggregates of nerve cells outside the CNS


The general organization of the nervous system.

Anatomically the nervous system is divided into the CNS and PNS, which have the major components shown in the diagram.

Functionally the nervous system consists of the following:

  1. Sensory division (afferent)

    1. Somatic—sensory input perceived consciously (eg, from eyes ears, skin, and musculoskeletal structures)

    2. Visceral—sensory input not perceived consciously (eg, from internal organs and cardiovascular structures)

  2. Motor division (efferent)

    1. Somatic—motor output controlled consciously or voluntarily (eg, by skeletal muscle effectors)

    2. Autonomic—motor output not controlled consciously (eg, by heart or gland effectors)

The autonomic motor nerves, comprising what is often called the autonomic nervous system (ANS), all have pathways involving two neurons: a preganglionic neuron with the cell body in the CNS and a postganglionic neuron with the cell body in a ganglion. The ANS has two divisions: (1) The parasympathetic division, with its ganglia within or near the effector organs, maintains normal body homeostasis. (2) The sympathetic division has its ganglia close to the CNS and controls the body’s responses during emergencies and excitement. ANS components located in the wall of the digestive tract are sometimes referred to as the enteric nervous system.

In both central and peripheral nerve tissue, neurons typically have numerous long processes for intercellular communication and the various glial cells (Gr. glia, glue) have shorter, more variable processes involved in many neural activities, neural maintenance, and defense.

Neurons respond to environmental changes (stimuli) by altering the ionic gradient that exists across their plasma membranes. All cells maintain such a gradient, also called an electrical potential, but cells that can rapidly change this potential in response to stimuli (eg, neurons, muscle cells, and some gland cells) are said to be excitable or irritable. Neurons react promptly to stimuli with a reversal of the ionic gradient (membrane depolarization) that generally spreads from the place that received the stimulus and propagates across the neuron’s entire plasma membrane. This propagation, called the action ...

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