Nerve tissue consists of nerve cells (neurons) and associated supporting cells of several types
Although all cells exhibit electrical properties, nerve cells or neurons are highly specialized to transmit electrical impulses from one site in the body to another; they are also specialized to integrate those impulses. Nerve cells receive and process information from the external and internal environment and may have specific sensory receptors and sensory organs to accomplish this function. Neurons are characterized by two different types of processes through which they interact with other nerve cells and with cells of epithelia and muscle. A single, long axon (sometimes longer than a meter) carries impulses away from the cell body, which contains the neuron's nucleus. Multiple dendrites receive impulses and carry them toward the cell body. (In histologic sections, it is usually impossible to differentiate axons and dendrites because they have the same structural appearance.) The axon terminates at a neuronal junction called a synapse, where electrical impulses are transferred from one cell to the next by secretion of neuromediators. These chemical substances are released at synapses to generate electric impulses in the adjacent communicating neuron.
In the central nervous system (CNS), i.e., the brain and spinal cord, the supporting cells are called neuroglial cells. In the peripheral nervous system (PNS), i.e., the nerves in all other parts of the body, the supporting cells are called Schwann (neurilemmal) cells and satellite cells. Supporting cells are responsible for several important functions. They separate neurons from one another, produce the myelin sheath that insulates and speeds conduction in certain types of neurons, provide active phagocytosis to remove cellular debris, and contribute to the blood-brain barrier in the CNS.
In an ordinary hematoxylin and eosin (H&E)-stained section, nerve tissue may be observed in the form of a nerve, which consists of varying numbers of neuronal processes along with their supporting cells (Fig. 3.4a). Nerves are most commonly seen in longitudinal or cross sections in loose connective tissue. Nerve cell bodies in the PNS, including the autonomic nervous system (ANS), are seen in aggregations called ganglia, where they are surrounded by satellite cells (Fig. 3.4b).
Neurons and supporting cells are derived from neuroectoderm, which forms the neural tube in the embryo. Neuroectoderm originates by invagination of an epithelial
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