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NERVE TISSUE

June 22, 2024

Nerve Tissue

Nervous system is the most complex system in the human body and is formed by a network of more than 100 millioerve cells (neurons), assisted by much numerous glial cells. Each neuron has a thousand interconnections with other neurons, forming a very complex system for communication. Neurons provide rapid communication between groups of serially disposed cells, permitting rapid transmission of information over long distances.

Nerve tissue is distributed throughout the body as an integrated communications network. Nerve tissues develop from embryonic ectoderm that is induced to differentiate by the underlying notochord. Structurally, nerve tissue consists of two cell types: nerve cells, or neurons, which usually show numerous long processes; and several types of glial cells, which have short processes, support and protect neurons, and participate in neural activity, neural nutrition, and the defense processes of the central nervous system.

Nerve cells, or neurons, are independent anatomic and functional units with complex morphologic characteristics. They are responsible for the reception, transmission, and processing of stimuli; the triggering of certain cell activities; and the release of neurotransmitters and other informational molecules.

Most neurons consist of three parts: the dendrites, which are multiple elongated processes specialized in receiving stimuli from the environment; the cell body, or perikaryon, which represents the trophic center for the whole nerve cell and is also receptive to stimuli; and the axon, which is a single process specialized in generating or conducting nerve impulses to other cells (nerve, muscle, and gland cells). Axons may also receive information from other neurons; this information mainly modifies the transmission of action potentials to other neurons. The distal portion of the axon is usually branched and constitutes the terminal arborization. Each branch of this arborization terminates on the next cell in dilatations called end bulbs (boutons), which interact with other neurons or nonnerve cells, forming structures called synapses. Synapses transmit information to the next cell in the circuit.

Neurons and their processes are extremely variable in size and shape. Cell bodies can be spherical, ovoid, or angular; some are very large, measuring up to 150 mm in diameter—large enough to be visible to the naked eye. Other nerve cells are among the smallest cells in the body; for example, the cell bodies of granule cells of the cerebellum are only 4-5 mm in diameter.

According to the size and shape of their processes, most neurons can be placed in one of the following categories: multipolar neurons, which have more than two cell processes, one process being the axon and the others dendrites; bipolar neurons, with one dendrite and one axon; and pseudounipolar neurons, which have a single process that is close to the perikaryon and divides into two branches. The process then forms a T shape, with one branch extending to a peripheral ending and the other toward the central nervous system. In pseudounipolar neurons, stimuli that are picked up by the dendrites travel directly to the axon terminal without passing through the perikaryon.

During the maturation process of pseudounipolar neurons, the central (axon) and the peripheral (dendrite) fibers fuse, becoming one single fiber. In these neurons, the cell body does not seem to be involved in the conduction of impulses, although it does synthesize many molecules, including neurotransmitters, that migrate to the peripheral fibers.

Simplified view of the 3 main types of neurons, according to their morphologic characteristics.

Most neurons of the body are multipolar. Bipolar neurons are found in the cochlear and vestibular ganglia as well as in the retina and the olfactory mucosa. Pseudounipolar neurons are found in the spinal ganglia (the sensory ganglia located in the dorsal roots of the spinal nerves). They are also found in most cranial ganglia.

Neurons can also be classified according to their functional roles. Motor (efferent) neurons control effector organs such as muscle fibers and exocrine and endocrine glands. Sensory (afferent) neurons are involved in the reception of sensory stimuli from the environment and from within the body. Interneurons establish relationships among other neurons, forming complex functional networks or circuits (as in the retina).

CELL BODY, OR PERIKARYON

The cell body is the part of the neuron that contains the nucleus and surrounding cytoplasm, exclusive of the cell processes. It is primarily a trophic center, although it also has receptive capabilities. The perikaryon of most neurons receives a great number of nerve endings that convey excitatory or inhibitory stimuli generated in other nerve cells.

Motor neuron. The myelin sheath is produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. The neuronal cell body has an unusually large, euchromatic nucleus with a well-developed nucleolus. The perikaryon contains Nissl bodies, which are also found in large dendrites. An axon from another neuron is shown at upper right. It has 3 end bulbs, one of which forms a synapse with the neuron. Note also the 3 motor end-plates, which transmit the nerve impulse to striated skeletal muscle fibers. Arrows show the direction of the nerve impulse.

Most nerve cells have a spherical, unusually large, dichromatic (pale-staining) nucleus with a prominent nucleolus. Binuclear nerve cells are seen in sympathetic and sensory ganglia. The chromatin is finely dispersed, reflecting the intense synthetic activity of these cells.

There are such structural types of nerve cells:

1) due to their size: large (Purcinje cells, pyramidal cells), middle sized and small (corn cells);

2) due to the shape of perikaryon: stellate, pyramidal cells, horizontal.

The cell body contains a highly developed rough endoplasmic reticulum organized into aggregates of parallel cisternae. In the cytoplasm between the cisternae are numerous polyribosomes, suggesting that these cells synthesize both structural proteins and proteins for transport. When appropriate stains are used, rough endoplasmic reticulum and free ribosomes appear under the light microscope as basophilic granular areas called Nissl bodies. The number of Nissl bodies varies according to neuronal type and functional state. They are particularly abundant in large nerve cells such as motor neurons. The Golgi complex is located only in the cell body and consists of multiple parallel arrays of smooth cisternae arranged around the periphery of the nucleus. Mitochondria are especially abundant in the axon terminals. They are scattered throughout the cytoplasm of the cell body.

Neurofilaments are abundant in perikaryons and cell processes. When impregnated with silver, they form neurofibrils that are visible with the light microscope. The neurons also contain microtubules that are identical to those found in many other cells. Nerve cells occasionally contain inclusions of pigments, such as lipofuscin, which is a residue of undigested material by lysosomes.

Photomicrograph of a motor neuron, a very large cell, from the spinal cord. The cytoplasm contains a great number of Nissl bodies. The large cell process is a dendrite. Note the large, round, stained nucleus, with a central dark-stained nucleolus. Pararosaniline–toluidine blue (PT) stain. Medium magnification.

Functionally nerve cells are dividing in sensory (afferent), associative and motor (efferent) neurons, according to the place in reflex arc, and excitatory and inhibiting due to their activity.

Ultrastructure of a neuron. The neuronal surface is completely covered either by synaptic endings of other neurons or by processes of glial cells. At synapses, the neuronal membrane is thicker and is called the postsynaptic membrane. The neuronal process devoid of ribosomes (lower part of figure) is the axon hillock. The other processes of this cell are dendrites.

DENDRITES & AXONS

Dendrites are usually short and divide like the branches of a tree. Most nerve cells have numerous dendrites, which considerably increase the receptive area of the cell. The arborization of dendrites makes it possible for one neuron to receive and integrate a great number of axon terminals from other nerve cells.

Unlike axons, which maintain a constant diameter from one end to the other, dendrites become thinner as they subdivide into branches. The composition of dendritic cytoplasm is similar to that of the perikaryon; however, dendrites are devoid of Golgi complexes.

Most neurons have only one axon; a very few have no axon at all. An axon is a cylindrical process that varies in length and diameter according to the type of neuron. Although some neurons have short axons, axons are usually very long processes. For example’, axons of the motor cells of the spinal cord that innervate the foot muscles may have a length of up to 100 cm. All axons originate from a short pyramid-shaped region, the axon hillock, which usually arises from the perikaryon. The plasma membrane of the axon is called the axolemma; its contents are known as axoplasm. Ieurons that give rise to a myelinated axon, the portion of the axon between the axon hillock and the point at which myelination begins is called the initial segment. This is the site where various excitatory and inhibitory stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate or not to propagate – an action potential, or nerve impulse. In contrast to dendrites, axons have a constant diameter and do not branch profusely. Occasionally, the axon, shortly after its departure from the cell body, gives rise to a branch that returns to the area of the nerve cell body. All axon branches are known as collateral branches.

GLIAL CELLS

Although neurons are the principal cells of nerve tissue, glial cells play an important supporting role. These cells are 10 times more abundant in the mammalian brain thaeurons; they surround both cell bodies and their axonal and dendrite processes that occupy the interneuronal spaces.

Nerve tissue has only a very small amount of extracellular matrix, and glial cells furnish a microenvironment suitable for neuronal activity.

Silver-stained section of cerebral cortex showing many pyramid-shaped neurons with their processes and a few glial cells. Medium magnification.

There are macroglial and microglial cells ierve tissue. First group includes astrocytes (fibrous and protoplasmic), oligodendrocytes (mantial and lemmocytes or Schwann cells) and ependymal cells. Almost all of them originate from nerve tube. Small microglial ce4lls belong to macrophagic system being glial macrophages.

Drawings of neuroglial cells as seen in slides stained by metallic impregnation. Note that only astrocytes exhibit vascular end-feet, which cover the walls of blood capillaries.

Photomicrographs (prepared with Golgi stain) of glial cells from the cerebral cortex. A: Fibrous astrocytes, showing blood vessels (BV). x1000. B: Protoplasmic astrocyte showing brain surface (arrow). x1900. C: Microglial cell. x1700. D: Oligodendrocytes. x1900.

Oligodendrocytes

Oligodendrocytes produce the myelin sheath that provides the electrical insulation of neurons in the central nervous system. These cells have a few small processes that wrap around axons, producing a myelin sheath.

Schwann cells have the same function as oligodendrocytes of central nerve system but are located around axons in the peripheral nervous system. One Schwann cell (lemmocyte) forms myelin around one axon, in contrast to the ability of oligodendrocytes to branch and serve more than one neuron and its processes.

Subtype of oligodendrocytes in dorsalroot ganglia are named mantial cells or satellite cells of ganglia. They have flattened bodies, which lie ower the pseudounipolar cell.

Astrocytes

Astrocytes are star-shaped cells, because of their multiple radiating processes. These cells have bundles of intermediate filaments made of glial fibrillary acid protein that reinforce their structure. Astrocytes bind neurons to capillaries and to the pia mater (a thin connective tissue that covers the central nervous system). Astrocytes with few long processes are called fibrous astrocytes and are located in the white matter; protoplasmic astrocytes, with many short-branched processes, are found in the grey matter. Astrocytes, compared to other glial cells, are by far the most numerous and exhibit an exceptional morphological and functional diversity.

In addition to their structural functions, astrocytes participate in controlling the ionic and chemical environment of neurons. One type of astrocyte develops processes with expanded end-feet that are linked to endothelial cells by junctional complexes. It is believed that through the end-feet, astrocytes transfer molecules and ions from the blood to the neurons.

Expanded processes are also present at the external surface of the central nervous system, where they make a continuous layer. Furthermore, when the central nervous system is damaged, astrocytes proliferate to form cellular scar tissue.

Astrocytes also play a role in regulating the numerous functions of the central nervous system. Astrocytes in vitro exhibit adrenergic receptors, amino acid receptors (e.g., y-aminobutyric acid [GABA]), and peptide receptors (including natriuretic peptide, angiotensin II, endothelins, vasoactive intestinal peptide, and thyrotropin-releasing hormone). The presence of these and other receptors on astrocytes provides them with the ability to respond to several stimuli.

Brain section prepared with Rio Hortega silver stain showing fibrous astrocytes with their processes ending on the external surface of blood vessels. Medium magnification.

Astrocytes can influence neuronal survival and activity through their ability to regulate constituents of the extracellular environment, absorb local excess of neurotransmitters, and release metabolic and neuroactive molecules. The latter molecules include peptides of the angiotensinogen family, vasoactive endothelins, opioid precursors called enkephalins, and the potentially neurotrophic somatostatin. On the other hand, there is some evidence that astrocytes transport energy-rich compounds from the blood to the neurons and also metabolize glucose to lactate, which is then supplied to the neurons.

Finally, astrocytes are in direct communication with one another via gap junctions, forming a network through which information can flow from one point to another, reaching distant sites. For example, by means of gap junctions and the release of various cytokines, astrocytes can interact with oligodendrocytes to influence myelin turnover in both normal and abnormal conditions.

Ependymal Cells

Ependymal cells are low columnar epithelial cells lining the ventricles of the brain and central canal of the spinal cord. In some locations, ependymal cells are ciliated, which facilitates the movement of cerebrospinal fluid.

Microglia

Microglia (Gr. micros, small, + glia) are small elongated cells with short irregular processes. They can be recognized in routine haematoxylin-and-eosin (H&E) preparations by their dense elongated nuclei, which contrast with the spherical nuclei of other glial cells. Microglia, phagocytic cells that represent the mononuclear phagocytic system ierve tissue, are derived from precursor cells in the bone marrow. They are involved with inflammation and repair in the adult central nervous system, and they produce and release neutral proteases and oxidative radicals. When activated, microglia retracts their processes and assume the morphologic characteristics of macrophages, becoming phagocytic and acting as antigen-presenting cells. Microglia secretes a number of immunoregulatory cytokines and dispose of unwanted cellular debris caused by central nervous system lesions.

In multiple sclerosis, the myelin sheath is destroyed by an unknown mechanism with severe neurologic consequences. In this disease, microglia phagocize and degrade myelin debris by receptor-mediated phagocytosis and lysosomal activity- In addition, AIDS dementia complex is caused by HIV-I infection of the central nervous system. Overwhelming experimental evidence indicates that perivascular and multinucleated microglia are infected by HIV-1. A number of cytokines, such as interleukin-1 and tumor necrosis factor-a, activate and enhance HIV replication in microglia.

NERVE FIBERS

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. Nerve fibers exhibit differences in their enveloping sheaths, related to whether the fibers are part of the central or the peripheral nervous system. Most axons in adult nerve tissue are covered by single or multiple folds of a sheath cell. In peripheral nerve fibers, the sheath cell is the Schwann cell, and in central nerve fibers it is the oligodendrocyte. Axons of small diameter are usually unmyelinated nerve fibers.

MYELINATED FIBERS

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, a lipoprotein complex whose lipid component can be partly removed by standard histologic procedures.

Myelin consists of many layers of modified cell membranes. These membranes have a higher proportion of lipids than do other cell membranes. Central nervous system myelin contains two major proteins: myelin basic protein and proteolipid protein. Several human demyelinating diseases are due to an insufficiency or lack of one or both of these proteins.

Each axon is surrounded by myelin formed by a sequential series of Schwann cells. 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 myelin sheath is formed by the processes of the oligodendrocytes. Oligodendrocytes differ from Schwann cells in that different branches of one cell can envelop segments of several axons.

Myelin sheath of the central nervous system. The same oligodendrocyte forms myelin sheaths for several (3–50) nerve fibers. In the central nervous system, processes of other cells sometimes cover the nodes of Ranvier, or there is considerable extracellular space (ES) at that point. The axolemma shows a thickening where the cell membrane of the oligodendrocyte comes into contact with it. This limits the diffusion of materials into the periaxonal space between the axon and the myelin sheath. At upper left is a surface view of the cell body of an oligodendrocyte. Cyt, cytoplasm of the oligodendrocyte.

The center drawing shows a myelinated peripheral nerve fiber as seen under the light microscope. The process is the axon enveloped by the myelin sheath and by the cytoplasm of Schwann cells. A Schwann cell nucleus, the Schmidt-Lanterman clefts, and a node of Ranvier are shown. The upper drawing shows the ultrastructure of the Schmidt-Lanterman cleft. The cleft is formed by Schwann cell cytoplasm that is not displaced to the periphery during myelin formation. The lower drawing shows the ultrastructure of a node of Ranvier. Note the appearance of loose interdigitating processes of the outer leaf of the Schwann cells’ cytoplasm (SC) and the close contact of the axolemma. This contact acts as a sort of barrier to the movement of materials in and out of the periaxonal space between the axolemma and the membrane of the Schwann cell. The basal lamina around the Schwann cell is continuous. Covering the nerve fiber is a connective tissue layer—mainly reticular fibers—that belong to the endoneurial sheath of the peripheral nerve fibers.

UNMYELINATED FIBERS

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.

Upper: The most frequent type of unmyelinated nerve fiber, in which isolated axons are surrounded by a Schwann cell and each axon has its own mesaxon. Lower: Many very thin axons are sometimes found together, surrounded by the Schwann cell. In such cases, there is one mesaxon for several axons.

The central nervous system is rich in unmyelinated axons; unlike those in the peripheral system, these axons are not sheathed. In the brain and spinal cord, unmyelinated axonal processes run free among the other neuronal and glial processes.

Ultrastructural features of myelinated (A) and unmyelinated (B) nerve fibers. (1) Nucleus and cytoplasm of a Schwann cell; (2) axon; (3) microtubule; (4) neurofilament; (5) myelin sheath; (6) mesaxon; (7) node of Ranvier; (8) interdigitating processes of Schwann cells at the node of Ranvier; (9) side view of an unmyelinated axon; (10) basal lamina.

Electron micrograph of a peripheral nerve containing both myelinated (M) and unmyelinated (U) nerve fibers. The reticular fibers (RF) seen in cross section belong to the endoneurium. Near the center of the figure is a Schwann cell nucleus (S). The perineurial cells (P [over a nucleus], arrows) form a barrier that controls access of materials to nerve tissue. x30,000. Inset: Part of an axon, where numerous neurofilaments and microtubules are seen in cross section. x60,000.

Electron micrographs of a myelinated nerve fiber. Top: x20,000.

Bottom: x80,000.

Nerve endings

Morphologic (structural) classification of nerve endings includes 3 main types: sensory (receptors), associative (synapses) and effectory (motor or secretory.

1. Sensory endings (receptors) are classifying by their

A. Disposition: 1. Interoceptors

2. Proprioceptors

3. Exteroceptors

B. Feelings: 1. Pain

2. Pressure

3. Temperature

C. Structure: 1. Simple (free) consist of free dendrites.

2. Compound (nonfree) have dendrites: noncapsulated (with glial sheath) and encapsulated whose inner bulb is cowered with connective tissue capsule.

Drawing of a Golgi tendon organ (sensory ending). This structure collects information about differences in tension among tendons and relays data to the central nervous system, where they are processed and help to coordinate fine muscular contractions.

Several types of sensory skierve endings. Free endings– nocioreceptor, Pacinian Meissner bodies and bublb of Krause – encapsulated nonfree endings.

2. SYNAPSES (chemical and electric) are classifying by their:

– Structure

– Functions: excitatory, inhibiting

– Mediators: acetylcholine, adrenalin, bombesin …

The synapse is responsible for the unidirectional transmission of nerve impulses. Synapses are the sites where contact occurs betweeeurons or betweeeurons and other effector cells (e.g., muscle and gland cells). The function of the synapse is to convert an electrical signal (impulse) from the presynaptic cell into a chemical signal that can be transferred to the postsynaptic cell. Most synapses transmit information by releasing chemical messengers during the signalling process.

The synapse itself is formed by an axon terminal (presynaptic terminal) that delivers the signal; a region on the surface of another cell where a new signal is generated (postsynaptic terminal); and a thin intercellular space called the synaptic cleft. If an axon forms a synapse with a cell body, it is called an axosomatic synapse; with a dendrite, axodendritic; or with an axon, axoaxonic. Although most synapses are chemical synapses and use chemical messengers, a few synapses transmit ionic signals through gap junctions that cross the pre- and postsynaptic membranes, thereby conducting neuronal signals directly. These synapses are called electrical synapses.

The presynaptic terminal always contains synaptic vesicles and numerous mitochondria. The vesicles contaieurotransmitters (acetylcholine, epinephrine and norepinephrine, so on); the mitochondria furnish energy for synaptic activity.

Types of synapses. The axon terminals usually transmit the nerve impulse to a dendrite or to a nerve cell body; less frequently, they make a synapse with another axon.

Examples of excitatory and inhibitory synapses in a motor neuron.

Electron micrograph of a rotary-replicated freeze-etched synapse. Synaptic vesicles surround a mitochondrion (M) in the axon terminal. x25,000.

Adrenergic nerve ending. There are many 50-nm-diameter vesicles (arrow) with dark, electron-dense cores containing norepinephrine. x40,000.

3. EFFECTORY endings (effectors) consist of axon with or without glial sheath.

Due to the location and evidence of their activity effectors may be of two types:

– Motor (in muscles), which promote contractions of muscular tissue

– Secretory (in glands), stimuli secretion of glandular cells

Ultrastructure of the motor end-plate and the mechanism of muscle contraction. The drawing at the upper right shows branching of a small nerve with a motor end-plate for each muscle fiber. The structure of one of the bulbs of an end-plate is highly enlarged in the center drawing. Note that the axon terminal bud contains synaptic vesicles. The region of the muscle cell membrane covered by the terminal bud has clefts and ridges called junctional folds. The axon loses its myelin sheath and dilates, establishing close, irregular contact with the muscle fiber. Muscle contraction begins with the release of acetylcholine from the synaptic vesicles of the end-plate. This neurotransmitter causes a local increase in the permeability of the sarcolemma. The process is propagated to the rest of the sarcolemma, including its invaginations (all of which constitute the T system), and is transferred to the sarcoplasmic reticulum (SR). The increase of permeability in this organelle liberates calcium ions (drawing at upper left) that trigger the sliding filament mechanism of muscle contraction. Thin filaments slide between the thick filaments and reduce the distance between the Z lines, thereby reducing the size of all bands except the A band. H, H band; S, sarcomere.

Muscle spindle showing afferent and efferent nerve fibers that make synapses with the intrafusal fibers (modified muscle fibers). Note the complex nerve terminal on the intrafusal fibers. The two types of intrafusal fibers, one with a small diameter and the other with a dilation filled with nuclei, are shown. Muscle spindles participate in the nervous control of body posture and the coordinate action of opposing muscles.

DEGENERATION & REGENERATION OF NERVE TISSUE

Although it has been shown that neurons can divide in the brain of adult birds, mammaliaeurons usually do not divide, and their degeneration represents a permanent loss. Neuronal processes in the central nervous system are, within very narrow limits, replaceable by growth through the synthetic activity of their perikaryons. Peripheral nerve fibers can also regenerate if their perikaryons are not destroyed.

Death of a nerve cell is limited to its perikaryon and processes. The neurons functionally connected to the dead neuron do not die, except for those with only one link. In this latter instance, the isolated neuron undergoes transneuronal degeneration.

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.

Since nerves are widely distributed throughout the body, they are often injured. When a nerve axon is transsected, 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, i.e., 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. The proximal segment of the axon degenerates close to the wound for a short distance, but growth starts as soon as debris is removed by macrophages. Macrophages produce interleukin-1, which stimulates Schwann cells to secrete substances that promote nerve growth.

In the nerve stub distal to the injury, both the axon (now separated from its trophic center) 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.

When there is an extensive gap between the distal and proximal segments, or when the distal segment disappears altogether (as in the case of amputation of a limb), the newly growerve fibers may form a swelling, or neuroma, that can be the source of spontaneous pain.

Main changes that take place in an injured nerve fiber. A: Normal nerve fiber, with its perikaryon and effector cell (striated skeletal muscle). Note the position of the neuron nucleus and the quantity and distribution of Nissl bodies. B: When the fiber is injured, the neuronal nucleus moves to the cell periphery, and Nissl bodies become greatly reduced in number. The nerve fiber distal to the injury degenerates along with its myelin sheath. Debris is phagocytosed by macrophages. C: The muscle fiber shows a pronounced denervation atrophy. Schwann cells proliferate, forming a compact cord penetrated by the growing axon. The axon grows at the rate of 0.5–3 mm/day. D: Here, the nerve fiber regeneration was successful. Note that the muscle fiber was also regenerated after receiving nerve stimuli. E: When the axon does not penetrate the cord of Schwann cells, its growth is not organized.

Regeneration is functionally efficient only when the fibers and the columns of Schwann cells are directed to the correct place. The possibility is good, however, since each regenerating fiber gives origin to several processes, and each column of Schwann cells receives processes from several regenerating fibers. In an injured mixed nerve, however, if regenerating sensory fibers grow into columns connected to motor end-plates that were occupied by motor fibers, the function of the muscle will not be re-established.

References

1. Douglas F. Paulsen. Basic Histology. – Prentice – Hall International Inc. – 1990.

2. Johanes, A.Y., and Rhodin M.D.: An Atlas of Ultrastructure. Philadelphia, London, Saunders Co., 1963.

3. Wheater P.R., Burkitt, H.Y., Daniels V.Y.: Functional Histology: Text and Colour Atlas. 2nd ed. Edinburg, London, Melburn, New York, 1987.