NERVE TISSUE. NERVE CELLS. NEUROGLIAL CELLS. NERVE FIBERS. NERVE ENDINGS.
Using lectures (on the web-page of the department posted the presentation text and lectures), books, additional literature and other sources, students must to prepare the following theoretical questions:
1. General morphofunctional characteristic of the nervous tissue. Histogenesis of nervous tissue.
2. Neurons morphologic and functional classification.
3. Neuron micro- and ultrastructure. General and special organelles, their functions.
4. Neuroglia classification, structure and functions.
5. Nervous cells regeneration peculiarities.
6. General features and classification of nerve fibres.
7. Myelinated nerve fibers micro- and ultrastructure.
8. Mechanism of the myelinated nerve fibers formation.
9. Non-myelinated nerve fibers histophysiology.
10. Regeneration and degeneration of nerve fibers.
11. Nervous endings general morphofunctional characteristic.
12. Receptors classification and structure.
13. Effectors classification and structure.
14. Synapse. Interneuronal synapses classification and structure.
15. Symple reflectory ark compounds. Synapses functions.
16. Neuronal theory.
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.
Although the nervous system is very complex, there are only two main types of cells ierve tissue. The actual nerve cell is the neuron. It is the “conducting” cell that transmits impulses and the structural unit of the nervous system. The other type of cell is neuroglia, or glial, cell. The word “neuroglia” means “nerve glue.” These cells are nonconductive and provide a support system for the neurons. They are a special type of “connective tissue” for the nervous system.
Pyramidal cell from the cerebral cortex (Golgi stain). Dendrites (D) arise from the apex and the side, a single axon (A) extends from the base
The Purkinje cells of the cerebellum prepared with a Golgi stain.
These cells have an extraordinary, finely branching dendritic tree (D) at one end, and a single axon (A) at the other. This stain reveals the shape of the cell (the axon is not identifiable in standard preparations) but obscures cytological detail (Purkinje cells have a nucleus with nucleoli just like other nerve cells).
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 ieural activity, neural nutrition, and the defense processes of the central nervous system.
Nerve tissues develop from embryonic ectoderm that is induced to differentiate by the underlying notochord. First, a neural plate forms; then the edges of the plate thicken, forming the neural groove. The edges of the groove grow toward each other and ultimately fuse, forming the neural tube. This structure gives rise to the entire central nervous system, including neurons, glial cells, ependymal cells, and the epithelial cells of the choroids plexus.
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).
Dendrites and axons are cytoplasmic extensions, or processes, that project from the cell body. They are sometimes referred to as fibers. Dendrites are usually, but not always, short and branching, which increases their surface area to receive signals from other neurons. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the neuron cell body. There is only one axon that projects from each cell body. It is usually elongated and because it carries impulses away from the cell body, it is called an efferent process.
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 arborisation. 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.
An axon may have infrequent branches called axon collaterals. Axons and axon collaterals terminate in many short branches or telodendria. The distal ends of the telodendria are slightly enlarged to form synaptic bulbs. Many axons are surrounded by a segmented, white, fatty substance called myelin or the myelin sheath. Myelinated fibers make up the white matter in the CNS, while cell bodies and unmyelinated fibers make the gray matter. The unmyelinated regions between the myelin segments are called the nodes of Ranvier.
In the peripheral nervous system, the myelin is produced by Schwann cells. The cytoplasm, nucleus, and outer cell membrane of the Schwann cell form a tight covering around the myelin and around the axon itself at the nodes of Ranvier. This covering is the neurilemma, which plays an important role in the regeneration of nerve fibers. In the CNS, oligodendrocytes produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate.
Functionally, neurons are classified as afferent, efferent, or interneurons (associatioeurons) according to the direction in which they transmit impulses relative to the central nervous system. Afferent, or sensory, neurons carry impulses from peripheral sense receptors to the CNS. They usually have long dendrites and relatively short axons. Efferent, or motor, neurons transmit impulses from the CNS to effector organs such as muscles and glands. Efferent neurons usually have short dendrites and long axons. Interneurons, or associatioeurons, are located entirely within the CNS in which they form the connecting link between the afferent and efferent neurons. They have short dendrites and may have either a short or long axon.
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
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.
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 centres, 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.
Most nerve cells have a spherical, unusually large, dichromatic (pale-staining because of euchromatine) 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.
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.
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 and have numerous well prominent cristae and electron dense matrix.
Neurofilaments are abundant in perikaryons and cell processes. When impregnated with silver, they form neurofibrils that are visible with the light microscope. Neurofibrils lie paralelly in processes of nerve cell and irregularly in cell body. 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.
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 amacrine cells of retina). An axon is a cylindrical nonbranched 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
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, which lye in organs.
SYNAPTIC COMMUNICATION
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).
Examples of excitatory and inhibitory synapses in a motor neuron.
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 contain neurotransmitters; the mitochondria furnish energy for synaptic activity. The postsynaptic membrane has receprtors adequate to to transmitter synthesyzing in presynaptic pert.
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 interneuron spaces.
1. Astrocytes – two types
a. protoplasmic astrocytes
*granular cytoplasm, many branches on short processes
*some of processes are closely applied to neurons, while others form intimate contacts with blood vessels.
*thought to form a conduit for nutrients from blood vessels to neurons.
*found in gray matter.
b. fibrous astrocytes
*long slender processes
*functioot well understood
*found chiefly in white matter.
3. oligodendroglia (also called oligodendrocytes)
a. Smaller than astrocytes, fewer processes
b. found in both gray and white matter
c. In white matter, these cells form the myelin sheaths that are around many axons, in gray mater they may lightly myelinate some dendrites.
d. Anaologous to Schwann cells of peripheral nervous system
e. These cells must be cultured with neurons in order to get neurons to grow in tissue culture. Suggests intimate interactive association.
4. microglia
a. small cell body that is elongated
b. Elongate nucleus with mostly heterochromatin
c. Can be differentiated from other glia by elongate nucleus. Other glia have a spherical nucleus
d. Many of what were thought to be microglia under the light microscope, have turned out to be oligodendroglia when cells were examined with EM.
5. ependymal cells
a. ciliated cells forming single layer of cuboidal epithelium that lines the entire neurocoel
b. ciliary action acts to circulate cerebral spinal fluid.
|
Glial Cell Type |
Origin |
Location |
Main Function |
|
Oligodendrocyte |
Neural Tube |
Central Nervous System |
Myelin Production, electric insulation |
|
Schwann Cell |
Neural Tube |
Peripheral nerves |
Myelin Production, electric insulation |
|
Astrocyte |
Neural Tube |
Central Nervous System |
Structural support, repair processes Blood-brain barrier, metabolic exchanges |
|
Ependymal cell |
Neural Tube |
Central Nervous System |
Lining cavities of central nervous system |
|
Microglia |
Bone Marrow |
Central Nervous System |
Macrophagic activity |
Nerve tissue has only a very small amount of extracellular matrix, and glial cells furnish a microenvironment suitable for neuronal activity.
Glial cells are classifying by their size in larger macroglial cells and smaller microglial cells. The first group includes astrocytes, oligodendrocytes and ependymal cells. Glial macrophages belong to the second group.
Oligodendrocytes
Oligodendrocytes are large glial cells, which have few processes and 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.
Silver-stained oligodendrocyte in the brain
Schwann cells have the same function as oligodendrocytes but they are located around axons in the peripheral nervous system. One Schwann cell forms myelin around one axon, in contrast to the ability of oligodendrocytes to branch and serve more than one neuron and its processes in central nerve system.
Another type of oligodendrocytes – mantial cells surround the perikaryons of pseudounipolar cells of dorsalroot ganglia thus producing continuous protective layer over them.
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 of central nerve system; 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.
Protoplasmic astrocytes stained with gold. This stain adheres to cell plasma membranes and obscures internal cell structure. The cells shown here are protoplasmic astrocytes (arrow), found in gray matter. Astrocytes send many processes which entwine blood vessels; some are shown here by the arrowheads.
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, making a continuous layer. Furthermore, when the central nervous system is damaged, astrocytes proliferate to form cellular scar tissue.
Silver-stained fibrous astrocyte in the brain
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.
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 embryogenesis ependymocytes originate from the innermost (ependymal) layer of nerve tube. 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.
Iron hematoxylin-stained microglial cell (with processes) in the brain
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 (disseminated 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.
Schwann cells are responsible for the myelination of axons in the PNS. A Schwann cell wraps itself, jelly roll-fashion, in a spiral around a short segment of an axon. During the wrapping, cytoplasm is squeezed out of the Schwann cell and the leaflets of plasma membrance of the concentric layers of the Schwann cell fuse, forming the layers of the myelin sheath. An axon’s myelin sheath is segmented because it is formed by numerous Schwann cells arrayed in sequence along the axon. The junction where two Schwann cells meet has no myelin and is called (as you know) the node of Ranvier (the areas covered by Schwann cells being the internodal regions).
The lack of Schwann cell cytoplasm in the concentric rings of the myelin sheath is what makes it lipid-rich. Schwann cell cytoplasm is however found in several locations. There is an inner collar of Schwann cell cytoplasm between the axon and the myelin, and an outer collar around the myelin. The outer collar is also called the sheath of Schwann or neurilemma, and contains the nucleus and most of the organelles of the Schwann cell. The node of Ranvier is also covered with Schwann cell cytoplasm, and this is the area where the plasma membranes of adjacent Schwann cells meet. (These adjacent plasma membranes are not tightly apposed at the node, so that extracellular fluid has free acess to the neuronal plasma membrane.) Finally, small islands of Schwann cell cytoplasm persist within successive layers of the myelin sheath, these islands are called Schmidt-Lanterman clefts. The process of myelination is described in greater detail on pg 264-268 of Ross et al.
Not all nerve fibres is the PNS are covered in myelin, some axons are unmyelinated. In contrast to the situation in the CNS, unmyelinated fibres in the PNS are not completely bare, but are enveloped in Schwann cell cytoplasm. The Schwann cells are elongated in parallel to the long axis of the axons, which fit into grooves on the surface of the Schwann cell. One axon or a group of axons may be enclosed in a single groove. Schwann cells may have only one or up to twenty grooves. Single grooves are more common in the autonomic nervous system.
An EM section of a node of Ranvier.
Identify the axon, neurofilaments, plasma membrane, Schwann cell processes etc.
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
Osmicated peripheral myelinated nerve
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.
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.
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.
NONMYELINATED NERVE FIBRES
Stained with haematoxylin and eosin
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.
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.
Nerve endings
Specialized structures of nerve system, which lie at the beginning and the end of reflex arc.
Classification of nerve endings
I SENSORY (receptors)
A. Disposition: 1. Interoceptors
2. Proprioceptors
3. Exteroceptors
B. Feelings:
1. Pain
2. Pressure
3. Temperature
C. Structure: 1. Simple (free)
2. Compound (nonfree): encapsulated, and noncapsulated
II SYNAPSES (chemical and electrical)
Structure
Functions: excitatory, inhibiting
Mediator: acetylcholine, adrenalin, bombesin
III. EFFECTORY (effectors)
Motor
Secretory
Sensory endings have dendrite inside. Reflex arch begins with receptor.
Effectory endings are the last compound of reflex arch and consist of axon, which is connected with muscle (motor ending) or glandular cell (secretory ending).
Synapses are specialized intercellular junctions of communicative type.
Detail of a motor nerve ending upon a skeletal muscle cell (voluntary muscle). The axon terminal is highly branched to form an oval motor end plate.
Myoneural junction
The cell body which sends out this axon is a multipolar motor neuron, such as those in the anterior horn of the spinal cord.
Electronmicrograph of axo-muscular synapse in skeletal muscle. X 33000.
Diagram of motor end plate (myoneural junction) as seen with electron microscopy. This drawing shows a detail of one knob of an end plate as it rests in a trough on the surface of a muscle cell. The “subneural clefts” labelled here are also called “gutters” in the sarcolemmal membrane. The label “glycoprotein” indicates the position of the basal lamina of the muscle cell.
EM detail of neuro-neural synapse in the brain or spinal cord. The axon terminal contains many seed-like synpatic vesicles containing transmitter substances. The intercellular cleft between the axon and the contacted dendrite can be seen. Just below the dendritic cell membrane is a dark, filamentous post-synaptic density. Other profiles in this field, most of them very irregular in outline, belong to both neuronal processes and glial processes. There is one large and one small mitochondrion just left of the synaptic vesicles.
Muscle spindle – a specialized sensory receptor for muscle stretch and position sense, as related particularly to unconscious maintenance of skeletal muscle tone and proper balance of postural muscle activity. The spindle is the encapsulated group of muscle fibers lying in the center of the field of regular skeletal muscle fibers, all cut in cross-section.
The sensory nerve endings themselves (not visible here) wrap around the muscle fibers within the spindle. Such endings relay sensory information along dendrites within peripheral nerves, back to pseudounipolar cell bodies in a spinal ganglion, and thence to the spinal cord.
Pacinian corpuscle – another specialized sensory ending, this time for deep pressure. This particular view is from a whole mount of mesentery, so you are seeing the corpuscle three-dimensionally. They are also found in subcutaneous tissue, deep to skin. Notice the onion-like layers of specialized connective tissue surrounding a dark pink dendritic terminal. Again, the cell body for this dendrite lies in a spinal ganglion, and the axon of that same cell then proceeds into the spinal cord.
Paccinian corpuscle
Ruffini’s body (mechanoreceptor)
Bulb of Crause (thermoreceptor)
Meisner’s body (scheme)
A Meissner’s corpuscle. Seen bracketed, these are found unevenly distributed in the CT of the dermal papillae of the integument. This slide is from thick skin. These structures function in mediating light touch and tactile feeling. The arrow shows the dendritic nerve fiber which spirals into a spindle-shaped structure in contact with the epidermis. The entire structure is ensheathed by a connective tissue capsule.
Student’s Practical Activities
Task No 1. Students must know and illustrate such a histologic specimens.
Specimen 1 Chromatophilic substance (Nissl substance) in the multipolar neurons (spinal cord).
Stained with methylene blue.
Under a low magnification one must find the multipolar neurons in the spinal cord grey matter. Watch them carefully under high magnification. Special attention should be paid to the blue granules and globules (basophilic Nissl substance) in the perikaryon and dendrites, that are absent in axon hillock. Pay attention to the light nucleus with euchromatine and nucleolus.
Illustrate and indicate: 1. Cell body (perikaryon). 2. Chromatophilic substance. 3. Nucleus. 4. Dendrites. 5. Axon.
Specimen 2. Neurofibrils in the neurons (the spinal cord).
Stained with silver impregnation.
In the spinal cord grey matter one must find the neurons with processes under a low magnification and watch them carefully under a high magnification. Pay attention to the fibrillar structures – neurofibrill, that form a network in cell body and lie parallely in the processes.
Illustrate and indicate: 1. Perikaryon. 2. Nucleus. 3. Processes. 4. Neurofibrils
Specimen 3. Pseudounipolar neurons (spinal ganglion).
Stained with haematoxylin and eosin.
Under a low magnification find a group of round-shaped nervous cells with a vesicular nuclei under the ganglion sheath. Under a high magnification one must watch the pseudounipolar neurons surrounding cells (mantial cells) with a small dark nuclei. These are the satellite-cells (oligodendrocytes).
Illustrate and indicate: 1. Ganglion cell body. 2. Nucleus. 3. Oligodendrocytes (satellite cells).
Specimen 4. Non-nmyelinated nerve fibers
Stained with haematoxylin and eosin.
Under a low magnification one must find the bands of nerve fibers stained in a pink color. Under a high magnification they look like cords with an oval shaped violet nuclei of neurolemmocytes along them. Axons lie inside the Shwann’s cells that’s why they are invisible.
Illustrate and indicate: 1. Unmyelinated nerve fibers. 2. Tunica. 3. Shwann’s cells nuclei. 4. Axons.
Speciment 5. Myelinated nerve fibers
Stained with osmic acid.
Under a low magnification one must find myelinated fibers disposed in different turns and choose isolated one. Under a high magnification one must watch a pale stained axon in the middle of the nerve fiber.
A black colored myelin surrounds it. Find thin light lines in the myelin sheath that have oblique turns – Schmidt-Lanterman clefts and the other portion without myelin – Ranvier node. Neurolemma is a thin light tunica around the nerve fiber.
Illustrate and indicate: 1. Axon. 2. Myelin. 3. Ranvier nodes. 4. Myelin incisures. 5. Neurolemma.
Specimen 6. Lamellar (Fatter-Paccinian) corpuscle.
Stained with haematoxylin and eosin.
Under a low magnification one must find in derma a round or an oval-shaped Paccinian body with terminal branches of axon in the middle portion that is surrounded by changed lemmocytes (inner bulb). There are concentrically disposed connective tissue lamellae with fibroblasts arround it (external bulb).
Illustrate and indicate: Inner bulb. 2. Axons branches. 3. Outer lamellae of the corpuscle. 4. Connective tissue lamellae. 5. Fibroblasts nuclei.
Task No 2. Students must know and illustrate such a scheme.
Synapse scheme.
Illustrate and indicate:
Presynaptic pole: a) synaptic vesicles; b) mitochondria; c) presynaptic membrane.
Synaptic fissure. 3. Postsynaptic pole: a) postsynaptic membrane; b) mitochondria
Task No 3. Students should be able to indicate elements in the electron micrographs:
1. Fragment of the neuron
2. Neurosecretory cell
3. Oligodendrocyte
4. Astrocyte (cerebral cortex)
5. Ependimal cells
6. Myelinated nerve fiber
7. Non-myelinated nerve fiber
8. Synapse. Interneuronal junction
9. Axo-vasal synapse in neurohypophisis
10. Motor nerve ending (myoneural junction)
References:
A-Basic:
1. Practical classes materials
http://intranet.tdmu.edu.ua/data/kafedra/internal/histolog/classes_stud/English/medical/II%20term/10%20Nerve%20tissue.%20Nerve%20cells.%20Glial%20cells.%20Nerve%20fibers.%20Nerve%20endings.htm
http://intranet.tdmu.edu.ua/ukr/kafedra/index.php?kafid=hist&lengid=eng&fakultid=m&kurs=1&discid=Histology, cytology and embryology
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P.77-82
4. Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 122-150
5. Inderbir Singh Textbook of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002. – P. 135-167
6. Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P. 352-400
B – Additional:
1. Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. – Lippincott Williams and Wilkins, 2008. – P. 135-156
2. Junqueira L. Basic Histology / L. Junqueira, J. Carneiro, R. Kelley. – [seventh edition]. –
3. Charts:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#inf3
4. Disk:
http://intranet.tdmu.edu.ua/data/teacher/video/hist/
5. Volkov K. S. Ultrastructure of cells and tissues / K.
http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
http://en.wikipedia.org/wiki/Circulatory
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.udel.edu/biology/Wags/histopage/histopage.htm
