Nervous tissue

Nervous tissue forms the parenchyma of the nervous system, and it is composed of neurons and glial cells. The major glial cell types of the central nervous system (CNS) are the astrocytes (astroglia), oligodendrocytes (oligodendroglia) microglia and ependymal cells, while peripheral nervous system (PNS) contains Schwann cells and satellite cells. Detailed description of the cells populating the nervous system is given in Chapter I (basic tissues). It is not the primary goal of histology to elaborate on neuroanatomical details, including the complex circuitry of the central nervous system; therefore, only the basic microanatomy of the cerebrum, cerebellum, and the spinal cord (CNS), along with the histology of ganglia and peripheral nerves (PNS) are described below.

Peripheral nerve

Peripheral somatic nerves are composed of axons of neurons whose cell bodies are located in the central or peripheral nervous systems. These axons are surrounded by myelin sheath formed by numerous layers of the cell membrane duplications of Schwann cells that also envelope the myelin sheath itself. This Schwann sheath or neurilemma contains the cytoplasm and the elongated nuclei of Schwann cells. Indeed, the vast majority of nuclei that populate the parenchyma of the peripheral nerve are nuclei of Schwann cells.

The myelin sheath is formed by multiple sections that represent the myelinization process carried out by a single Schwann cell; the gaps between the sections are covered with Schwann sheath and contain an axonal swelling, the node of Ranvier. Circularly arranged cytoplasmatic remains between the myelin sheath layers form the Schmidt-Lanterman clefts as well as the perinodal cytoplasmatic remnants at the Ranvier nodes.

Peripheral nerves are surrounded by several succeeding layers of connective tissue envelopes. Endoneurium is formed around individual Schwann cells surrounding the myelin and Schwann sheaths, and it is composed of thin, type III collagen fibers that are probably secreted by the Schwann cells. Occasional fibroblasts can also be observed among the myelinated neurites. Fascicles of nerve fibers are enveloped by the perineurium formed by fibroblasts and collagen fibers along with flattened contractile cells. The outermost envelope, the epineurium encloses the entire nerve into a dense connectivetissue sheath that often extends into the nerve between the fascicles and is continuous with the dura mater and the underlying arachnoid. Epineurium contains vessels (vasa nervorum) and nerves (nervi nervorum); the latter providing the intrinsic innervation of the nerve sheaths.

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