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Mammalian neurons usually do not divide, and their degeneration represents a permanent loss. Peripheral nerve fibers can regenerate if their perikaryons are not destroyed.
In contrast to nerve cells, neuroglia of the central nervous system—and Schwann cells and ganglionic satellite cells of the peripheral nervous system—are able to divide by mitosis. Spaces in the central nervous system left by nerve cells lost by disease or injury are invaded by neuroglia.
Because nerves are widely distributed throughout the body, they are often injured. When a nerve axon is transected, degenerative changes take place, followed by a reparative phase.
In a wounded nerve fiber, it is important to distinguish the changes occurring in the proximal segment from those in the distal segment. The proximal segment maintains its continuity with the trophic center (perikaryon) and frequently regenerates. The distal segment, separated from the nerve cell body, degenerates.
Axonal injury causes several changes in the perikaryon: chromatolysis, ie, dissolution of Nissl substances with a consequent decrease in cytoplasmic basophilia; an increase in the volume of the perikaryon; and migration of the nucleus to a peripheral position in the perikaryon.
In the nerve stub distal to the injury, both the axon and the myelin sheath degenerate completely, and their remnants, excluding their connective tissue and perineurial sheaths, are removed by macrophages. While these regressive changes take place, Schwann cells proliferate within the remaining connective tissue sleeve, giving rise to solid cellular columns. These rows of Schwann cells serve as guides to the sprouting axons formed during the reparative phase.
After the regressive changes, the proximal segment of the axon grows and branches, forming several filaments that progress in the direction of the columns of Schwann cells. Only fibers that penetrate these Schwann cell columns will continue to grow and reach an effector organ.
The choroid plexus consists of invaginated folds of pia mater, rich in dilated fenestrated capillaries, that penetrate the interior of the brain ventricles. It is found in the roofs of the third and fourth ventricles and in part in the walls of the lateral ventricles.
The choroid plexus is composed of loose connective tissue of the pia mater, covered by a simple cuboidal or low columnar epithelium made of ion-transporting cells .
The main function of the choroid plexus is to elaborate cerebrospinal fluid, which contains only a small amount of solids and completely fills the ventricles, central canal of the spinal cord, subarachnoid space, and perivascular space. Cerebrospinal fluid is important for the metabolism of the central nervous system and acts as a protective device against mechanical shocks.
Cerebrospinal fluid is clear, has a low density (1.004–1.008 g/ mL), and is very low in protein content. A few desquamated cells and two to five lymphocytes per milliliter are also present. Cerebrospinal fluid is continuously produced and circulates through the ventricles, from which it passes into the subarachnoid space. There, arachnoid villi provide the main pathway for absorption of cerebrospinal fluid into the venous circulation.
The blood–brain barrier is a functional barrier that prevents the passage of some substances, such as antibiotics and chemical and bacterial toxic matter, from the blood to nerve tissue.
The blood–brain barrier results from the reduced permeability that is characteristic of blood capillaries of nerve tissue. Occluding junctions, which provide continuity between the endothelial cells of these capillaries, represent the main structural component of the barrier. The cytoplasm of these endothelial cells does not have the fenestrations found in many other locations, and very few pinocytotic vesicles are observed. The expansions of neuroglial cell processes that envelop the capillaries (astrocytes) are partly responsible for their low permeability.
The skull and the vertebral column protect the central nervous system. It is also encased in membranes of connective tissue called the meninges . Starting with the outermost layer, the meninges are the dura mater, arachnoid, and pia mater. The arachnoid and the pia mater are linked together and are often considered a single membrane called the pia-arachnoid.
The dura mater is the external layer and is composed of dense connective tissue continuous with the periosteum of the skull. The dura mater that envelops the spinal cord is separated from the periosteum of the vertebrae by the epidural space, which contains thin-walled veins, loose connective tissue, and adipose tissue. The dura mater is always separated from the arachnoid by the thin subdural space.
The arachnoid has two components: a layer in contact with the dura mater and a system of trabeculae connecting the layer with the pia mater. The cavities between the trabeculae form the subarachnoid space, which is filled with cerebrospinal fluid and is completely separated from the subdural space. This space forms a hydraulic cushion that protects the central nervous system from trauma. The subarachnoid space communicates with the ventricles of the brain. The arachnoid is composed of connective tissue devoid of blood vessels. In some areas, the arachnoid perforates the dura mater, forming protrusions that terminate in venous sinuses in the dura mater. These protrusions, which are covered by endothelial cells of the veins, are called arachnoid villi. Their function is to reabsorb cerebrospinal fluid into the blood of the venous sinuses.
The pia mater is a loose connective tissue containing many blood vessels. Although it is located quite close to the nerve tissue, it is not in contact with nerve cells or fibers. Between the pia mater and the neural elements is a thin layer of neuroglial processes, adhering firmly to the pia mater and forming a physical barrier at the periphery of the central nervous system. This barrier separates the central nervous system from the cerebrospinal fluid. The pia mater follows all the irregularities of the surface of the central nervous system and penetrates it to some extent along with the blood vessels. Squamous cells of mesenchymal origin cover pia mater. Blood vessels penetrate the central nervous system through tunnels covered by pia mater—the perivascular spaces. The pia mater disappears before the blood vessels are transformed into capillaries. In the central nervous system, the blood capillaries are completely covered by expansions of the neuroglial cell processes.
Ganglia are ovoid structures containing neuronal cell bodies and glial cells supported by connective tissue. Because they serve as relay stations to transmit nerve impulses, one nerve enters and another exits from each ganglion. The direction of the nerve impulse determines whether the ganglion will be a sensory or an autonomic ganglion.
Sensory ganglia receive afferent impulses that go to the central nervous system. Two types of sensory ganglia exist. Some are associated with cranial nerves (cranial ganglia); others are associated with the dorsal root of the spinal nerves and are called spinal ganglia. A connective tissue framework and capsule support the ganglion cells. The neurons of these ganglia are pseudounipolar and relay information from the ganglion's nerve endings to the gray matter of the spinal cord via synapses with local neurons.
Autonomic ganglia appear as bulbous dilatations in autonomic nerves. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the intramural ganglia. These ganglia are devoid of connective tissue capsules, and their cells are supported by the stroma of the organ in which they are found. Autonomic ganglia usually have multipolar neurons. As with craniospinal ganglia, autonomic ganglia have neuronal perikaryons with fine Nissl bodies.A layer of satellite cells frequently envelops the neurons of autonomic ganglia. In intramural ganglia, only a few satellite cells are seen around each neuron.
In the peripheral nervous system, the nerve fibers are grouped in bundles to form the nerves. Except for a few very thin nerves made up of unmyelinated fibers, nerves have a whitish, homogeneous, glistening appearance because of their myelin and collagen content.
Nerves have an external fibrous coat of dense connective tissue called epineurium, which also fills the space between the bundles of nerve fibers. Each bundle is surrounded by the perineurium, a sleeve formed by layers of flattened epitheliumlike cells. The cells of each layer of the perineurial sleeve are joined at their edges by tight junctions, an arrangement that makes the perineurium a barrier to the passage of most macromolecules and has the important function of protecting the nerve fibers from aggression. Within the perineurial sheath run the Schwann cell-sheathed axons and their enveloping connective tissue, the endoneurium. The endoneurium consists of a thin layer of reticular fibers, produced by Schwann cells.
The nerves establish communication between brain and spinal cord centers and the sense organs and effectors (muscles, glands, etc). They possess afferent and efferent fibers to and from the central nervous system. Afferent fibers carry the information obtained from the interior of the body and the environment to the central nervous system. Efferent fibers carry impulses from the central nervous system to the effector organs commanded by these centers. Nerves possessing only sensory fibers are called sensory nerves; those composed only of fibers carrying impulses to the effectors are called motor nerves. Most nerves have both sensory and motor fibers and are called mixed nerves; these nerves have both myelinated and unmyelinated axons .
The main components of the peripheral nervous system are the nerves, ganglia, and nerve endings. Nerves are bundles of nerve fibers surrounded by connective tissue sheaths.
Nerve fibers consist of axons enveloped by a special sheath derived from cells of ectodermal origin. Groups of nerve fibers constitute the tracts of the brain, spinal cord, and peripheral nerves.
In myelinated fibers of the peripheral nervous system, the plasmalemma of the covering Schwann cell winds and wraps around the axon . The layers of membranes of the sheath cell unite and form myelin. Myelin consists of many layers of modified cell membranes. These membranes have a higher proportion of lipids than do other cell membranes. The myelin sheath shows gaps along its path called the nodes of Ranvier; these represent the spaces between adjacent Schwann cells along the length of the axon. Interdigitating processes of Schwann cells partially cover the node. The distance between two nodes is called an internode and consists of one Schwann cell. The length of the internode varies between 1 and 2 mm. There are no Schwann cells in the central nervous system; there, the processes of the oligodendrocytes form the myelin sheath. Oligodendrocytes differ from Schwann cells in that different branches of one cell can envelop segments of several axons .
In both the central and peripheral nervous systems, not all axons are sheathed in myelin. In the peripheral system, all unmyelinated axons are enveloped within simple clefts of the Schwann cells. Unlike their association with individual myelinated axons, each Schwann cell can sheathe many unmyelinated axons. Unmyelinated nerve fibers do not have nodes of Ranvier, because abutting Schwann cells are united to form a continuous sheath.
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