key concept 44.1 Neurons and Glia Are Unique Cells of Nervous Systems

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Nervous systems are informational systems. They encode, process, and store a wide variety of information from the external and internal environments, and they use that information to control and regulate the physiology and behavior of the organism. The nervous systems of all animals can carry out these functions because of the properties of a unique class of cells: nerve cells, or neurons. Vertebrate nervous systems also include another unique class of cells called glial cells or glia.

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  • The structure of a neuron defines its functions in receiving, integrating, and communicating information.

  • Specific types of glial cells have unique roles within the nervous system to support, insulate, and nurture neurons and to affect neuronal signaling.

There are many types of neurons, which vary in size, shape, and function, but they all generate and conduct electric signals. Glia do not generate and conduct electric signals, and there are two major types—macroglia and microglia. Macroglia support and modulate the functions of neurons. Microglia are phagocytic cells derived from blood-forming stem cells. Microglia are motile, are activated by foreign matter or damage to neural tissue, and are the first and major immune defense mechanism in the nervous system.

Nervous systems in all but the simplest invertebrates are divided into central and peripheral divisions. The central division includes dense assemblages of neurons that process and store information and generate commands. In lower invertebrates these assemblages of neurons are called ganglia. With increased complexity, some large ganglia of invertebrates are designated as brains. In vertebrates the central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) provides communication between the central division and all of the rest of the body.