The human nervous system, by far the most complex system in the body, is formed by a network of many billion nerve cells (neurons), all assisted by many more supporting cells called glial cells. Each neuron has hundreds of interconnections with other neurons, forming a very complex system for processing information and generating responses.
Nerve tissue is distributed 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 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 that are 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:
Sensory division (afferent)
Somatic – sensory input perceived consciously (eg, from eyes ears, skin, musculoskeletal structures)
Visceral – sensory input not perceived consciously (eg, from internal organs and cardiovascular structures)
Motor division (efferent)
Somatic – motor output controlled consciously or voluntarily (eg, by skeletal muscle effectors)
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.
Cells in both central and peripheral nerve tissue are of two kinds: neurons, which typically have numerous long processes, and various glial cells (Gr. glia, glue), which have short processes, support and protect neurons, and participate in many neural activities, neural nutrition, and defense of cells in the CNS.
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, 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 ...