Porth's Essentials of Pathophysiology, 4e

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Nervous System

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Neuroglial Cells of the Central Nervous System The neuroglial cells of the CNS consists of four types of cells: oligodendrocytes, astrocytes, microglia, and epen- dymal cells (Fig. 34-3). The oligodendrocytes form the myelin in the CNS. Instead of forming a myelin cover- ing for a single axon, these cells reach out with several processes, each wrapping around and forming a multi- layered myelin segment around several different axons. As within the PNS, the covering of axons in the CNS increases the velocity of nerve conduction. Astrocytes are the largest and most numerous of neu- roglia and are particularly prominent in the gray mat- ter of the CNS. They form a network within the CNS and communicate with neurons to support and modulate their activities. Astrocytes have many processes, some stretching their processes from blood vessels to neurons and others filling most of the intercellular space within the CNS. It is now thought that astrocytes play an impor- tant role in the movement of metabolites and wastes to and from neurons and regulate ionic concentrations in the intercellular compartment. In addition, astrocytes take up neurotransmitters from synaptic zones after their release and thereby help regulate synaptic activity. They also have a role in maintaining the tight junctions of cap- illaries that form the blood-brain barrier. Astrocytes are also the principal cells responsible for repair and scar formation in the brain. They can fill their cytoplasm with microfibrils (i.e., fibrous astrocytes), and masses of these cells form the special type of scar tissue that develops in the CNS when tissue is destroyed, a process called gliosis . A third type of neuroglia, the microglia, the microglial cells are the resident macrophages of the central nervous system. They are available for cleaning up debris after cellular damage, infection, or cell death. The fourth type

Each of the Schwann cells along a peripheral nerve fiber is encased in a continuous tube of basement mem- brane, which in turn is surrounded by a multilayered sheath of loose connective tissue known as the endoneu- rium (see Fig. 34-2). The endoneurial sheath, which is essential to the regeneration of peripheral nerves, pro- vides a collagenous tube through which a regenerating axon can again reach its former target. The endoneu- rial sheath does not penetrate the CNS. The absence of the endoneurial sheaths is thought to be a major factor in the limited axonal regeneration of CNS nerves com- pared with those of the PNS. The endoneurial sheaths are bundled with blood vessels into small clusters of nerves called fascicles . In the nerve, fascicles consisting of bundles of nerve fibers are surrounded by another protective covering called the perineurium . Usually, several fascicles are further surrounded by the heavy, protective epineu- rial sheath of the peripheral nerve. The protective lay- ers that surround the peripheral nerve processes are continuous with the connective tissue capsule of the sensory nerve endings and the connective tissue that surrounds the effector structures, such as the skeletal muscle cell. Centrally, the connective tissue layers con- tinue along the dorsal and ventral roots of the nerve and fuse with the meninges that surround the spinal cord and brain. Satellite cells are a type of neuroglial cell that sur- round the cell body of neurons in the PNS, including those with ganglia. They have been found to serve a variety of roles, including control over the microen- vironment of sympathetic ganglia. Satellite cells are thought to have a role similar to that of astrocytes in the CNS. They supply nutrients to the surrounding neurons, and they also act as protective, cushioning cells.

Microglial cell

Oligodendrocyte

Astrocyte

FIGURE 34-3. The supporting neuroglial cells of the central nervous system (CNS). Diagrammatic view of relationships between the glial elements (astrocyte, oligodendrocyte, microglial cell, and ependymal cells), capillaries, cerebrospinal fluid, and cell bodies of CNS neurons.

Capillary

Ependymal cells

Cerebrospinal fluid

Neuron