Although the shape and arrangement of cells in specific muscle types (e.g., smooth muscle, skeletal muscle, and cardiac muscle) are quite different, all muscle types share a common characteristic: The bulk of the cytoplasm consists of the contractile proteins actin and myosin. Although these proteins are ubiquitous in all cells, only in muscle cells are they present in such large amounts and organized in such highly ordered arrays that their contractile activity can produce movement in an entire organ or organism.
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 at which electrical impulses are transferred from one cell to the next by secretion of neuromediators. These chemical substances are released at synapses to generate electrical impulses in the adjacent communicating neuron.
In the central nervous system (CNS), which comprises the brain and spinal cord, the supporting cells are called neuroglial cells. In the peripheral nervous system (PNS), which comprises 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. 4.4a). 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. 4.4b).
Neurons and supporting cells are derived from neuroectoderm, which forms the neural tube in the embryo. Neuroectoderm originates by invagination of an epithelial layer, the dorsal ectoderm of the embryo. Some nervous system cells, such as ependymal cells and cells of the choroid plexus in the CNS, retain the absorptive and secretory functions characteristic of epithelial cells.
FIGURE 4.4  Nerve tissue. a. A Mallory-stained section of a peripheral nerve. Nerve tissue consists of a vast number of threadlike myelinated axons held together by connective tissue. The axons have been cross-sectioned and appear as small, red, dotlike structures. The clear space surrounding the axons previously contained myelin that was dissolved and lost during preparation of the specimen. The connective tissue is stained blue. It forms a delicate network around the myelinated axons and ensheathes the bundle, thus forming a structural unit, the nerve. 270. b. An Azan-stained section of a nerve ganglion, showing the large, spherical nerve cell bodies and the nuclei of the small satellite cells that surround the nerve cell bodies. The axons associated with the nerve cell bodies are unmyelinated. They are seen as nerve fiber bundles (NFB) between clusters of the cell bodies. 270.