Epidermal cells supply neurons with oxygen
1.c. Light and electron microscopic structure of the peripheral nerves (axon, axon sheath and terminals). - Judit Horváth [Translator: Andrea Pethőné Lubics, German proofreading: Judit Horváth]
The most important component of the peripheral nerve is the nerve fiber, which consists of the axon and the adjacent myelin sheath. The axon is the longest extension of the nerve cell, the length of which can be over 1 meter. The axon is not able to exist without the cell body of the nerve cell, because the whole nerve cell forms an anatomical-morphological, embryological, trophic, functional and pathological unit. It is therefore impossible to speak of the peripheral nerves as a whole without mentioning the nerve cell. Knowledge of the structure of the nerve cell and the contact between cell body and axon are essential for understanding normal function and regeneration.
The main parts of the nerve cell are the following: cell body (perikaryon, soma), dendrites, axon and axon terminals (telodendron, end bulb). According to their shape, they are most often multipolar (around 95%, many dendrites, one axon), but 4% of the cells are pseudounipolar (the only process of the cell divides into two: a peripheral and a central process), or they can be bipolar (1 dendrite, 1 axon, or peripheral and central process). The soma of the nerve cells is either in the central nervous system or in a peripheral ganglion. Its presence is not typical in the peripheral nerves.
The somatomotor nerve cells are multipolar, their perikaryen are always located in the central nervous system (in the anterior horn of the spinal cord or in the motor cranial nerve nuclei). The motor fibers emerge from them and establish contact with the striated muscles via the motor end plates.
The nerve fibers that perform the sensitive tasks are formed by the peripheral processes of the pseudounipolar nerve cells. The peripheral endings of these processes are either embedded in the various tissues and organs in receptors particularly suitable for receiving stimuli, or they form free nerve endings. The cell bodies of the pseudounipolar neurons are located in the sensitive ganglia, which are always close to the central nervous system (spinal ganglia along the spinal cord, sensitive ganglia of the cranial nerves). The ganglia of the auditory and equilibrium organs are exceptions to the sensitive ganglia, as their cells have retained their original bipolar shape. The central processes of these pseudounipolar or bipolar nerve cells grow into the central nervous system and form synapses with the multipolar neurons located there (e.g. in the dorsal horn of the spinal cord or in the sensitive cranial nerve nuclei). In this way, they make further processing of the information coming from the periphery possible.
The preganglionic nerve cells of the autonomic nervous system are always located within the central nervous system. They are multipolar nerve cells whose axons form synapses with peripheral, multipolar ganglion cells. The ganglion cells belonging to the sympathetic nervous system are closer to the spine than those belonging to the parasympathetic nervous system. The parasympathetic ganglia are closer to the target organs, they can be embedded in the wall of the target organs, or they build themselves into the nerve that innervates the organ. The axons of the ganglion cells innervate the target organs as postganglionic fibers (synapse en passant).
The sensory and motor roots attached to the spinal cord come to lie next to each other in the spinal nerve. The branches of the spinal nerves (rami ventrales and dorsales) contain sensory and motor fibers mixed. When the fibers leave or enter the spinal cord, their sheath changes: in the central nervous system it is formed by the oligodendrocytes, in the periphery (from 1 mm away from the spinal cord) by the Schwann cells. As long as the nerve fibers run protected from the brain or the spine, the connective tissue sheaths between or around the nerve fibers are much thinner than in the "real" periphery after they have left the skull or the spinal canal through the intervertebral foramina .
In a peripheral nerve, the sheath of the nerve fibers can be the myelin sheath, which is formed by the membrane duplication of the Schwann cells (medullary or myelinated nerve fibers), but there are also axons that run in the lateral folds of the glial cells, and so only with the cytoplasm of the Schwann cells are enveloped. These fibers are unmyelinated or unmyelinated (fibers covered with Schwann sheath). The myelin sheath accompanies the axons throughout their course, they only lose them just before they terminate. The conduction speed of a fiber (axon + myelin sheath!) Depends on the thickness of the axon and the myelin sheath. The thickest nerve fibers are found in the nerves that run to muscles. These are 15-20 micrometers thick, their line speed can reach 100-120 m / s (= 360-430 km / h!). The fibers that only have a Schwann sheath, such as the vegetative postganglionic fibers and the fibers that mediate dull pain, are thinner (0.1-1 micrometers) and conduct more slowly (line speed less than 1 m / s).
The smaller or larger nerve fiber bundles that pull in the periphery are called peripheral nerves. They are also exposed to mechanical effects in an “ordinary” life situation, so their structure must also correspond to this function. If one examines the structure of a larger nerve (e.g. the median nerve, which is about 5 mm in diameter), one sees several “prepackaged” bundles within the nerve, which can then break up into even smaller bundles. The fiber bundles within the nerve are embedded in a connective tissue (epineurium), which also contains fat cells and blood vessels. The surface of the nerve fiber bundles, which can already be seen well under the light microscope with a weak magnification, is covered with the perineurium, which has an inner layer made up of flat cells and an outer layer made up of collagen fibers. The inner cellular layers are linked by basement membrane and cell connections (zonula occludens), which protect the nerve not only from mechanical, but also from chemical effects. The nerve fibers are located within the perineural ring, and are separated from each other only by a minimal amount of connective tissue (endoneurium) containing smaller vessels. With a light microscope, if the section is thick enough, swirling structures can often be seen with the movement of the micrometer screw. This picture indicates the spiral course of the nerve fibers. The connective tissue covering system, which is organized in three layers, and the spiral course of the nerve fibers give the peripheral nerve great resistance to all sorts of damaging effects (compare it with the conventional telephone cord!).
The most important component of the peripheral nerves is the axon, the extension of the nerve cell (neuron). The Spanish neurohistologist, Ramon y Cajal, developed the neuron concept at the end of the 19th century. Century created. The neuron theory developed by Cajal was opposed to the network theory of Golgi, the other great scientist of his time. The Hungarian anatomist Mihály Lenhossék also agreed with Cajal's theory, and time has confirmed them both. Waldeyer developed this theory further; he used the term neuron for the first time. The main parts of the neuron are the cell body (perikaryon, soma), the dendrite, the axon (neurite), of which all nerve cells only ever have one, and at the end of the axon the axon end button (telodendron, end bulb), through which the neuron attaches another nerve cell or a peripheral successor organ connects.
According to the theory of neurons, each individual nerve cell forms:
an embryological unit because it arises with all its processes from a cell of ectodermal origin;
an anatomical-morphological unit: the whole cell, including the appendages, are surrounded by a continuous cell membrane;
a functional unit: if the stimuli reaching the neuron can trigger the depolarization via the synapses, the excitation wave runs through the whole cell. This happens according to the “all or nothing law”, there are no stages;
a trophic unit: the processes are kept alive by the cytoplasmic components that come from the cell body and are constantly being replaced. The processes themselves are not viable. If they detach from the perikaryon after an injury, they die. Under optimal conditions, the axon is capable of regeneration, but even in this case the components are synthesized in the cell body, which then reach the damaged area with axonal transport, where the process is rebuilt. (see later *** 6.).
Similar to the epithelial cells, the neurons are polarized cells. The principle of the so-called histodinamic polarity comes from Cajal. This means that under physiological circumstances the excitation is directed from the cell body in the direction of the terminal button (this is also true in most cases according to the current state of science), the terminal button makes contact with the dendrite or the cell body of another nerve cell through synapses. The chemical synapses only allow excitation to pass in one direction.
3. About the nervous tissue in general
The nerve tissue consists of nerve cells (neurons) and glial cells. The neurons come into contact with other nerve cells or - in the periphery (in the skin, muscles, tendons, etc.) and the intestines - with receptors or effectors via their extensions. The cell membrane of the neurons is excited via synapses by another nerve cell or - in the peripheral or visceral receptors - by mechanical, chemical or thermal stimuli. The excitation is transmitted in the afferent fibers in the direction of the central nervous system, in the efferent fibers away from it via action potential.
The other cell type in nerve tissue is the glial cell. There are several types of glial cells, the distribution of which is determined by the various areas of the nervous system. Their presence is essential for the proper functioning of the nervous system. They perform many tasks: they envelop axons, nourish and isolate the nerve cells, and close the inner and outer surfaces of the nervous system.
3.1.1. Main parts of the neuron
The Cell body (Perikaryon or Soma). Its size can be different, it varies between 5-6 and 100-150 micrometers. The nerve cells with a longer axon usually have a larger perikaryon (e.g. the motor anterior horn cells).
The Dendrites (The multipolar nerve cells have many dendrites): They grow like branches from the perikaryon, branch out at an acute angle and become thinner and thinner in their course. Most of the time, the dendrites pick up excitations from the axon terminals of other neurons via synapses. Depending on the size of the dendrite tree, there may be several hundred or even 200,000 synapses (such as in the case of the Purkinje cells of the cerebellum) on the dendrites.
The Axon (Neurite): the nerve impulses are directed away from the perikaryon via the axon. It is usually covered with a myelin sheath, the covered axon is called a nerve fiber. (It must be mentioned, however, that the terms axon and nerve fiber are not always used consistently in the literature). The axons are evenly thick in their course, their secondary branches are called collaterals, which typically arise at right angles from the axon.
Both the collaterals and the axon itself end in the telodendron (in axon terminals): near their end they divide into several fine branches. At the end button of the axon terminal, the excitation is transmitted via synapses to another nerve cell or to success organs (muscle or glandular cell).
In the synapses, the electrical signal is chemically transmitted to the postsynaptic structure via messenger substances (transmitters), but the downstream cell can only react to it with depolarization if the stimulus is strong enough.
When axons of several other nerve cells synapse with a neuron, it is called convergence. The motor anterior horn cells, from whose axons the motor fibers in the peripheral nerve arise, have an average of 10,000 synapses. The summation of the stimuli arriving via the synapses determines whether the neuron is discharged or not, i.e. whether the axon emanating from it can pass this impulse on to the striated muscle via the motor endplate. An axon can also transmit signals in several directions via its collaterals. This is called divergence. The interplay of convergence and divergence makes it possible for the neurons to function in a network and for the relevant response to an external stimulus to come about.
3.1.2. Classification of neurons
The neurons can be classified according to several aspects: because of their connections, the nerve cells whose axons contact with distant areas are called projection neurons (Golgi type I). The neurons with a short axon are Golgi type II neurons, they come into contact with their immediate surroundings. Most neurons belong to the latter group.
The neurons can be excitatory or inhibitory. Both types can be divided into further groups according to their typical transmitter substances: the most common excitatory messenger substances are the following: acetylcholine, glutamate, aspartate, dopamine, noradrenaline, serotonin, neuropeptides, adenosine, nitric oxide, the typical inhibitory substances are gamma-aminobutyric acid and glycine .(The point of attack of the drugs acting on the nervous system is related to the transmitters.)
Most often, however, the neurons are divided according to their shape
22.214.171.124. Morphological classification of neurons
Apolar neuroblasts without appendage are only present in the course of the differentiation of the nerve cells, in the early phase of development.
Unipolar nerve cells, which only have a single process, are typical of lower-ranking living beings; they are rarely found in the human nervous system. Special cells with an appendage are the amacrine cells of the retina, which do not have an axon, but only a strongly branched dendrite.
The bipolar cells have a dendrite (peripheral process) and an axon (central process). Such cells are typically found in the retina and in the spiral and vestibular ganglion, in the inner ear. Compared to the other types of nerve cells, their number is negligible.
About 4-5% of the nerve cells make up the pseudounipolar cells. They occur in the spinal ganglion (Fig. 1.) and in the majority of the sensitive ganglia of the cranial nerves. During development, these cells are initially bipolar, but then the two processes come closer and closer to each other and finally they unite. As a result, the large, round nerve cells develop, the specialty of which is that there are no synapses on their perikaryon. Because of their special structure and their special function, the processes cannot be called dendrite and axon, but are called peripheral and central processes. A single process emerges from the perikaryon and runs several times around the soma, then it splits into two in a T-shape. The peripheral process directs the impulses away from the receptors towards the center, the depolarization wave is transmitted to the central process, which passes them on to the multipolar neurons of the central nervous system. Both processes are myelinated, their structure is similar to the axons. The peculiarity of this cell is that the excitation is passed on from the peripheral end of the process towards the cell body.
Figure 1.13. Figure 1 .: A. pseudounipolar cells of a sensitive ganglion, HE staining. The satellite cells are arranged in a ring around the large nerve cells. B. pseudounipolar cells of a sensitive ganglion, silver impregantion. Only one process emerges from the cell body
In humans, 95% of the nerve cells make up the multipolar neurons, which have numerous dendrites and an axon (Fig. 2.). Their shape and size can be different. Specialized neurons are e.g. pyramidal cells (Fig. 3), stellate cells, basket cells, Purkinje cells (Fig. 4), granule cells. The excitation reaches the dendrites or soma through synapses, and when the cell depolarizes, it is carried away by the axon away from the cell body.
Figure 1.14. Figure 2 .: Vegetative ganglion with multipolar nerve cells, silver impregnation. The branching, gradually tapering dendrites emerging from the cell body are clearly visible. Each cell has only one axon, which, however, rarely comes into the cutting plane
Figure 1.15. Figure 3 .: Pyramidal cells of the cerebral cortex, impregnation according to Golgi
Figure 1.16. Figure 4 .: Purkinje cell, cerebellum, impregnation according to Golgi. On the dendrite tree, where the dendrites branch out in one plane, the dendrite spines can also be easily recognized with a light microscope
There are about 10 times as many glial cells as there are neurons in the human nervous system. The higher the level of evolution the living being, the more glial cells help the neurons with their function. Their role is in the nourishment of neurons, re-absorption of released neurotransmitters, maintenance of the ion balance of the extracellular space, elimination of destroyed elements and formation of the myelin sheath. Research over the past decade supports the view that the function of glial cells is much more important than they should be considered as the connective tissue of the nervous system. With the exception of the microglia, they are of ectodermal origin.
3.2.1. Central nervous system glial cells
The astrocytes (macroglia) are cells with a large number of processes. A distinction is made between two forms: the fibrillar astrocytes (Fig. 5), which are found mainly in the white matter, and the protoplasmic, which can be found in the gray matter. They primarily fulfill the following functions: formation of the blood-brain barrier, nutrition, electrical insulation, support function, phagocytosis, scarring after neural injuries. You are GFAP positive.
(The GFAP / glial fibrillary acidic protein / is an intermediate filament protein that occurs in many cells of the nervous tissue, but also in peripheral cells.)
Figure 1.17. Figure 5 .: Fibrillar astrocytes, gold chloride impregnation. The processes of the glial cells end around vessels
The oligodendroglial cells have a small cell body. With their flattened appendages they envelop about 5-50 axons in their environment, thereby electrically isolating the axons from the surrounding tissue. You are GFAP negative.
The ependymal cells line the ventricular system. Since they cover the choroid plexus, they are also involved in the formation of the cerebrospinal fluid. With their cilia they also contribute to the liquor circulation. They are also likely to function as neural stem cells. You are GFAP positive.
The radial glial cells in their original form play a role in the development of the nervous system. The Bergmann glial cells of the cerebellum and the Müller cells of the retina have retained their characteristic shape best. You are GFAP positive.
The microglial cells (Fig. 6) are the only glial cells that arise from the mesoderm. In the resting stage, they monitor a certain area of the brain with their fine process system. When injured, they are activated, they phagocytize the destroyed tissue components. Their appendages become shorter and thicker. You are GFAP negative.
Figure 1.18. Figure 6 .: A. Resting microglial cells B. Activated microglial cells. (C3 receptor immunohistochemistry) (images by Dr. Ábrahám Hajnalka, PTE ÁOK Central Electron Microscope Laboratory)
3.2.2. Glial cells of the peripheral nervous system
The Schwann cells envelop the peripheral nerve fibers (form myelin sheath, or just Schwann sheath). They are also capable of phagocytosis.
The satellite cells are tiny cells that surround the neurons of the ganglia. In their function they are similar to the astrocytes.
The enteral glial cells of the intestinal wall enclose the neurons of the enteric nervous system. Together with the neurons, they act on gastrointestinal motility.
4. The light and electron microscopic structure of the neurons
The perikaryon (cell body, soma) is that part of the cell in which all the elements required for the maintenance of the cell are synthesized. The cell nucleus is poor in heterochromatin, the nucleolus is easily recognizable. Ribosomes appear free in the cytoplasm, in the form of rosettes and in the rough endoplasmic reticulum. In some of the nerve cells the rough endoplasmic reticulum appears in the form of Nissl substance, where the lamellae are in large numbers, parallel to each other. Under the light microscope, these look like Nissl clods in the cell (Mihály Lenhossék suggested the name tigroid granulation), the granulation of which can be fine or coarse under the light microscope (Fig. 7). With an electron microscope you can still observe Golgi apparatus, mitochondria and also lysosomes. Brown pigmentation is visible in some cells. This can be lipofuscin, an "age pigment" (Fig. 8), which corresponds to the end product of the lysosomal digestion process and is not capable of further remodeling. It belongs to the group of lipochromes. In addition, neuromelanin can be present in the cytoplasm of the cells in circumscribed groups of neurons (nucleus coeruleus, substantia nigra), the cells of which synthesize dopamine or noradrenaline.
Figure 1.19. Figure 7 .: A.Nissl clods in a pyramidal cell of the human cerebral cortex. The arrow points to a Betz giant cell (Nissl stain) B. Electron microscope image of the Nissl clods (images by Dr. Ábrahám Hajnalka, PTE ÁOK electron microscope laboratory)
Figure 1.20. Figure 8 .: Electron microscope image of lipofuscin. (Photos by Dr. Ábrahám Hajnalka, PTE ÁOK electron microscope laboratory)
The cytoskeleton of the cell body consists of microtubules (neurotubules made up of tubulin, the diameter of which is approx. 20 nm), intermediate filaments (diameter: approx. 10 nm) and actin filaments (diameter: approx. 5 nm). The task of the cytoskeleton is the mechanical stabilization of the cell body and the processes and the transport of the organelles and substances within the cell or the processes. The components of the cytoskeleton are linked together by the microtubule associated proteins (MAPs).
There are synapses on the surface of the cell body too, but the majority of the synapses are found on the dendrites.
The dendrites grow out of the cell body with a broad base. As they are further and further away from the soma, they become thinner and thinner and they can branch out finely at an acute angle. In their area of origin, their structure is the same as that of the cell body (unlike the axons), but as they rejuvenate they contain fewer and fewer cell organelles. On their surface there are often button-shaped protuberances called dendrite thorns (Fig. 9). Most synapses can be found on the dendrites or the thorns. The polarity of the microtubules is variable, usually their minus end points towards the soma, but it can also be the other way around. In the dendrites, the components of the cytoskeleton are typically connected to one another by MAP2.
Figure 1.21. Figure 9 .: Electron microscope image of the dendrite and the dendrite spike (d = dendrite, the red arrow points to the dendrite spike). Arrowhead: asymmetrical (excitatory) synapse, black arrows: symmetrical (inhibitory) synapses. (Photos by Dr. Ábrahám Hajnalka, PTE ÁOK electron microscope laboratory)
Each nerve cell has an axon (Fig. 10), the length of which is variable: it can range from 10 micrometers to over 1 meter. The motor fibers that run in the peripheral nerves run from the anterior horn of the corresponding spinal cord segment to the innervated muscle. For example, the abductor hallucis muscle is innervated by the medial plantar nerve. The motor fibers that pull in the nerve arise from the motor neurons, which are located in spinal cord segments L4-5. Since the spinal cord is shorter than the vertebral canal, these segments can be found at the level of the 12th thoracic vertebra. This axon is about 120 cm long in an average person. Accordingly, the nerve fiber that innervates the skin in this area of the sole of the foot is just as long. The cell body of this pseudounipolar neuron sits in the spinal ganglion corresponding to the segment; its central process ends at interneurons in the spinal cord segments L4-5. The proprioceptive afferents, which originate in the muscle spindles of the abductor hallucis muscle, are longer: the central process of these pseudounipolar neurons rises about 50 cm in the posterior cord of the spinal cord until it joins the neuron in the medulla oblongata in the gracilis nucleus forms a synapse. This neuron can even be 150-180 cm long, depending on the body size!
Figure 1.22. Figure 10 .: Pyramidal cell, impregnation according to Golgi. The red arrows point to the axon. On the opposite side the fork-shaped branched main dendrite and on the basal side of the cell the basal dendrites are visible
The axon emerges from the axon hillock, which does not show a rough endoplasmic reticulum, neither with a light nor with an electron microscope. The "initial segment" connects to the axon hillock. This is where action potentials arise, which are passed on via the axon. The structural background for this is the high density of sodium ion channels.
The cytoplasm of the axons is called the axoplasm. Besides the cytoskeleton, it contains mitochondria, lysosomes, vesicles, but no ribosomes, so there is no protein synthesis here. The plus end of the microtubules is always oriented distally. The connections between the microtubules are secured by the so-called tau proteins.This is also neuropathologically important because in some neurodegenerative diseases the neurotubule system disintegrates due to the changed chemical structure of the tau proteins, pathological aggregates (“neurofibrillary tangles” - balls of neurofibrils) are formed and they can no longer fulfill their tasks (e.g. in Alzheimer's disease -Illness)
The following link takes you to a video showing this process and gives a good idea of how neurons and axonal transport work: http://www.youtube.com/watch?v=NjgBnx1jVIU
All axoplasmic components and all necessary components of the cell membrane (axolemm) are synthesized in the cell body. They are then transported to the point of use by axoplasmic transport. Mitochondria, vesicles containing transmitters, membrane proteins, enzymes and other membrane-packed substances reach their target area with fast anterograde transport (40 cm / day). This requires a protein, the so-called kinesin, to which the substances mentioned bind and are carried away on the microtubules. The following video shows how the kinesin works:
The more soluble components of the cytoskeleton are replaced by the slow anterograde transport (1-6 mm / day).
The membrane parts and cell organelles worn at the axon terminals are packed in vacuoles, which the dynein protein returns along the microtubules back to the cell body, where they are broken down (retrograde transport, 20 cm / day).
The pathological significance of retrograde transport is that toxins can gain access to the nerve cells in this way. The following viruses also get into the neurons in this way: herpes simplex, poliovirus (pathogen causing polio) and rabies (rabies).
4.4. Sheaths of nerve fibers
4.4.1. Sheaths of the nerve fibers in the central nervous system
Within the nervous system, the myelin sheath is formed by the flattened processes of the oligodendrocytes that wrap around the axon that runs next to them several times (Fig. 11). An oligodendrocyte envelops a segment of 10-50 axons running in its vicinity. The stretches enclosed by the various oligodendrocytes are separated from one another by Ranvier cord rings (4.2.2. ***), which are covered with appendages of the astrocytes. There is no basal lamina on the surface of the fiber.
Figure 1.23. Figure 11 .: Myelinated nerve fibers in the central nervous system. (Photos by Dr. Ábrahám Hajnalka, PTE ÁOK electron microscope laboratory)
The axons which are free or surrounded by the axons of the astrocytes form the neuropil, which occurs in the gray matter of the central nervous system.
The denaturation of the nerve fibers in the central nervous system is the cause of multiple sclerosis.
4.4.2. Sheaths of nerve fibers in the peripheral nervous system
In the peripheral nervous system, the Schwann cells form the myelin sheath. When a nerve fiber leaves the central nervous system, the myelin sheath formed by oligodendrocytes changes. In the case of the cranial nerves, this borderline can be several cm long; in the case of the fila radicularia, it can be found immediately next to the spinal cord (Fig. 12).
Figure 1.24. Figure 12 .: Filum radicular (FR) protruding from the spinal cord, HE staining. The yellow arrow points to the Redlich-Obersteiner line. Above this, the myelin sheath is formed by oligodendrocytes, including Schwann cells
The Schwann cells can form two types of myelin sheath:
126.96.36.199. In the non-medullary fibers several thin axons are enclosed by the cytoplasm of the Schwann cells. The length of the Schwann cell can be 500 micrometers. The membrane of the Schwann cell hugs the axon and forms a membrane duplication towards the surface, but the axolemma remains in contact with the extracellular space. The whole fiber is surrounded by a basal lamina, which is also a product of the Schwann cell.
188.8.131.52. With the myelinated fibers only one axon is deposited in the Schwann cell, which rotates several times around the axon. As a result, the double membrane mesaxon wraps around the axon. The rolled up membrane lamellae are covered by a thin cytoplasmic layer of the Schwann cell, where the cell nucleus is also located. The place between myelinated sections formed by two Schwann cells is called the Ranvier ring. The distance between two rings is the internode, the length of which can vary between 200 and 2000 micrometers. The thicker axons have a thicker myelin sheath, a longer internode, and they conduct faster. The length of the internode increases as the body grows.
In the Ranvier lacing rings, the axolemma is rich in sodium ion channels, which make the saltatory conduction of excitation possible. The nodal axolemm is covered by the microvillus-like processes of the Schwann cells.
70% of the myelin sheath consists of lipids, 30% of proteins (e.g. myelin basic protein / MBP /, myelin-associated glycoprotein / MAG /, connexins, E-cadherin). The compact myelin shows a pattern of electron-dense lines (main and intermediate lines). The reason for this is that the membrane layers have laid against each other. At those places where the cytoplasm of the Schwann cells was preserved in strips, gaps are visible in the compact myelin ((Schmidt-Lanterman notches).
The nerve fibers are divided according to their thickness and function. The classification of Erlanger / Gasser and that of Lloyd / Hunt has become widespread in the specialist literature. Both can be used in describing the nervous system (Table 1).
The conduction speed of a nerve fiber is related to its thickness. Roughly, you get the conduction velocity in m / s if you multiply the thickness of the nerve fiber given in the micrometer by five. The thicker fibers are needed for the quick reactions. If, for example, the knife reaches your finger while cutting bread, it would not be good if you responded a second later. After a mosquito bite or appendicitis it is not so bad if you start to feel the itching or pain a few seconds later.
Table 1.5. Table 1.
|Erlanger / Gasser||Lloyd / Hunt||Diameter, micrometer||Speed, m / sec|
|A-alfa||I.a, I.b||10-22||60-120||Afferents from muscle spindles (I.a) and tendon organs (I.b), efferents to the skeletal muscles|
|A-béta||II||7-15||40-90||Afferents from muscle spindles and the mechanoreceptors of the skin|
|A-gamma||4-9||30-45||Efferents to the muscle spindles|
|A-delta||III||3-5||5-25||Afferents from the skin (pain, temperature)|
|B.||1-3||3-15||preganglionic vegetative fibers|
|Unmyelinated fibers, only surrounded by Schwann sheath|
|C.||IV||0,1-1||0,5-2||Pain afferents (dull pain conducting), postganglionic vegetative fibers|
There are no perikaryons in the peripheral nerve. The most important component of the peripheral nerve is the nerve fiber, which consists of the axon and its sheath. The sheath can either be a myelin sheath (184.108.40.206. ***) or a Schwann sheath, which is typical for the unmyelinated fibers. The latter are surrounded by the cytoplasm of the Schwann cells (220.127.116.11. ***). The axon leaves its shell just before it terminates.
(The terms nerve fiber and axon are not always used consistently in textbooks!)
The following fibers can run in the peripheral nerve:
afferent (sensitive) nerve fibers: these are the peripheral processes of the pseudounipolar cells that sit in the sensitive ganglia, and
efferent (motor) nerve fibers: these can belong to both the somatic and the vegetative nervous system. The somatomotor fibers correspond to the myelinated axons of the motor neurons in the anterior horn that innervate skeletal muscles. The vegetative motor fibers can be either preganglionic, thinly myelinated, or postganglionic unmyelinated nerve fibers.
If a nerve is described as sensitive in the anatomical sense, it does not mean that the nerve contains exclusively afferent fibers: in addition to the nerve fibers innervating the skin receptors, there are also efferent fibers in the nerve that innervate the vessels, glands and the arrector pili muscle of the skin . The motor nerves also contain not only motor but also sensory fibers that innervate the muscle spindles and the receptors in the tendon organs and fascia (7.1.3, 7.1.4. ***).
5.1. Light microscopic structure of the peripheral nerve
In a peripheral nerve (Fig. 13) the nerve fibers of different types form bundles (fascicles) (4.5. ***). During their course, the nerve fibers can redistribute themselves within the bundle: in the proximal section of a nerve there are few, but thicker, fascicles, distally the number of bundles increases, but their diameter becomes smaller.
Figure 1.25. Figure 13 .: Peripheral nerve (e.g. median nerve), HE staining. A: weak magnification, Ep = epineurium, P = perineurium (around the nerve fiber bundles) B: part of a fiber bundle. P = perineurium, En = endoneurium (between the nerve fibers of different thickness) C: nerve fibers with great magnification. En = endoneurium, A = axon, around which the myelin sheath, which embraces the nucleus of a Schwann cell in a crescent shape, is clearly visible. Thinly myelinated and unmyelinated fibers can be found in the marked area
Each individual bundle is enclosed in a ring by the perineurium. Within this, the individual nerve fibers are separated from each other with a fine-fiber connective tissue, the endoneurium. The fascicles are embedded in a loose connective tissue, the epineurium, which contains fat cells as well as thick collagen fiber bundles. The amount of connective tissue components changes depending on the traumatic effects a particular section of the nerve can be exposed to. This can also be seen well during dissection. The intracranial sections of the cranial nerves or the spinal roots are already peripheral nerves after their sheaths have been built up, but the sheaths are thin, they are poor in connective tissue components, which is why they are very vulnerable. The nerves that really run in the periphery are protected by their connective tissue elements not only from chemical, but also from mechanical influences from their environment. Thanks to the spiral course of their nerve fibers, the nerves are also slightly elastic, so histological sections show a vortex-like arrangement of the nerve fibers, especially if you follow their course in a thicker section with the rotation of the fine drive.
5.1.1. The structure of the endoneurium
The endoneurium consists of collagen fibers arranged in fine bundles, the course of which is parallel to that of the nerve fibers. Its structure is denser around the endoneural vessels. The majority of the cell nuclei visible within the fascicle belong to the Schwann cells, only 4% of the cells make up the fibroblasts. Endothelial cells, macrophages and mast cells are also found here. The muscular layer of the vessels running in the endoneurium is poorly developed and does not respond particularly well to external innervation. The pressure of the endoneural interstitial fluid is slightly higher than that in its vicinity.
5.1.2. The structure of the perineurium
The inner layer of the perineurium (pars epitheloidea) continuously envelops the fascicles of the peripheral nerve from the point of exit from the central nervous system (where the myelin sheath formed by the oligodendrocytes also merges into the sheath formed by the Schwann cells) to the end apparatus, where the cells of the perineurium intertwine with the capsule of the terminal apparatus. If it does not have a capsule, the perineurium ends freely in the periphery. The inner layer of the perineurium consists of flattened cells that can form as many as 15-20 layers. The cells are strongly connected to one another by tight junctions and are covered with a (0.5 micrometer) thick basal lamina, between which there are also collagen fibers. The cells of the perineurium contain bundles of microfilaments and pinocytotic vesicles. They play an important role in the demarcation of the nerve fibers from the surrounding tissue (diffusion barrier, blood-nerve barrier), in maintaining the higher fluid pressure within the endoneurium and in the osmotic conditions. The flattened cells of the perineurium are believed to be descendants of fibrocytes, but according to other opinions they correspond to special glial cells of ectodermal origin.
The outer layer of the perineurium (pars fibrosa) is a network of collagen fibers that narrows or disappears in the thinner fascicles.
(If the peripheral nerves are injured, the nerve is divided into fascicles and the perineurium sutured, thus creating the opportunity for the nerve to regenerate. (6. ***)
5.1.3. The structure of the epineurium
The epineurium is the continuation of the dura mater, but it descends from the mesoderm. It consists of loose connective tissue, but it also contains numerous fat cells and thick, longitudinal bundles of collagen fibers. It also extends between the fascicles. The epineurium is also where the blood vessels that supply the nerve and also the lymphatic vessels run. It makes up 30-70% of the weight of the nerve.
The larger nerves are surrounded on the outside with a circular layer of connective tissue (paraneurium), which secures the displacement of the nerve against the environment.
6. The degeneration and regeneration of the nerve fiber
When the axon close to the perikaryon is injured, the nerve cell usually dies. If the injury is further away from the soma, the axon and myelin sheath degrade distal from the site of the injury as Waller's degeneration progresses. The axon stump degenerates proximally up to the first Ranvier cord, the soma swells, and “chromatolysis” takes place in the cytoplasm. In the course of this process, the ribosomes detach from the rough endoplasmic reticulum, the nucleus becomes eccentric, and the number of dendrites and synapses decreases. Then comes the restitution phase, during which the perikaryon tries to regenerate the process.
In the central nervous system, the chances of regeneration are quite small because the process that tries to regenerate does not find the right path without a leading structure. The processes of the oligodendrocytes die distally from the injury with Waller's degeneration.
Waller's degeneration also occurs in the peripheral nerves distal to the injury, the axon swells and then it perishes. The myelin sheath also disappears, but the Schwann cells remain. They are arranged in rows within the preserved basal lamina, multiply and form the so-called Büngner bands. This tubular structure makes it possible that - albeit if the separated nerve stumps are not far apart - the newly emerging axon finds its innervation area again. Processes grow out of the proximal axon stump, of which the central conical zone grows in the direction where it finds a corresponding extracellular environment. The optimal proportions are mainly ensured by the Schwann cells of the Büngner bundles, and by growth factors such as the nerve growth factor (NGF) produced by the endoneural fibroblasts, the fibroblast growth factor (FGF), the insulin-like growth factors (IGF), etc.
The reunification of the injured stumps requires a precise surgical procedure during which the bruised part of the nerve is to be removed and the perineural sheaths must be tied together without tension or gaps. The axon grows 2-5 mm / day. There are greater chances of successful regeneration at a younger age.
7. Nerve endings (terminals)
Part of the endings of neurons are receptors (in the skin, sensory organs, intestines) whose task is to perceive stimuli from inside the body or the environment and to conduct them into the central nervous system. In the central nervous system, the answer is worked out through complex connections, which are then carried out like a command by the terminals of the motor neurons (effectors which, in this case, can contact skeletal muscle fibers, smooth muscle cells or glands).
Primary or secondary sensory (epithelial) cells or the endings of the afferent nerve fibers can act as receptors. The latter can be free or surrounded by a capsule. Since the primary and secondary sensory cells are very similar to epithelial cells, they are classified as epithelial tissue, which is the sensory epithelium.
The receptors can be classified according to their position: the exteroceptors transmit the stimuli coming from the outside world, the proprioceptors give information about the body position, the interoceptors are located in the wall of the intestines. Depending on the modality, a distinction is made between mechanoreceptors, thermoreceptors, nociceptors (react to damaging effects), as well as chemo- and baroreceptors.
Basically, the receptors consist of 3 components: the peripheral process of the sensitive neurons, glial and connective tissue components.
7.1.1. Primary sensory cells
The sensory epithelial cells that emerged from the placode and are surrounded by special supporting cells are embedded in the epithelium of the olfactory mucous membrane of the nasal cavity. The sensory epithelial cells are similar to the bipolar neurons, whose peripheral processes receive the olfactory stimulus. The axons of the cells form the fila radicularia, which pass through the lamina cribrosa and form synapses with the mitral cells of the olfactory bulb.
7.1.2. Secondary sensory cells
Secondary sensory cells can be found in the taste buds and in the auditory and equilibrium organs (Fig. 14), which are also surrounded by special supporting cells. In a certain sense, the Merkel cells also belong to this group (these cells are described in more detail under skin receptors 18.104.22.168. ***). Transmitters are released from the secondary sensory cells, which trigger action potential in the nerve fibers ending at these cells via synapse-like structures. This nerve fiber corresponds to the peripheral end of an afferent nerve fiber, the perikaryon of which sits either in a spinal ganglion or in a sensitive nucleus of the cranial nerve. The central process of these neurons leads into the central nervous system.
Figure 1.26. Figure 14 .: Organ of Corti, HE. The arrows point to the nuclei of the secondary sensory cells
The exteroceptors are typically found in the skin, in the subcutaneous connective tissue or in the mucous membrane near body orifices and convey excitations from the outside world (pain, temperature, tactile sensation, chemical stimuli). The fibers ending in the receptors are mostly myelinated. The specialized structures in their environment, which consist of Schwann cells, perineural cells and connective tissue components, serve to amplify and transmit the stimuli.
These are slowly adapting mechanoceptors that are descendants of epidermal cells. There are a particularly large number of Merkel cells in the fingertips of the hand and the soles of the feet, but they are also found in the outer root sheath of the hair follicle. In addition to tactile sensation, they also play an important role in regulating grip strength. They sit in the stratum basale of the epidermis and are linked to the surrounding cells via desmosomes. Since their cytoplasm contains serotonin and also neuropeptides (metencephalin, bombesin), they could also be assigned to neuroendocrine cells. The flat end of the axon contacts the basal side of the cell. The contact between the axon and the Merkel cell resembles a synapse.
22.214.171.124. Meissner corpuscles
These are rapidly adapting, pressure-sensitive egg-shaped receptors that are located under the epidermis, in the connective tissue papillae, very close to the epithelium. Their number is particularly high in the fingertips and lip. They consist of flattened Schwann cells that form irregularly arranged disks. Collagen fibrils also wind between the Schwann cells, anchoring the receptor to the basal lamina, to the displacement of which it reacts just as well. The Meissner corpuscles have a capsule (derivative of the perineurium) only in their basal part, where several (1 to 7) axons, which have already lost their myelin sheath, enter the corpuscle and spiral between the lamellae.
126.96.36.199. Receptors around the hair follicles
The receptors that enclose the outer surface of the hair follicles like cuffs also adapt quickly. They are flattened axon terminals that are rich in mitochondria and coated with Schwann cells. They respond to the displacement of the hair follicle. In addition, there are Merkel cells in the hair follicle, free nerve endings in its vicinity, and receptors that look similar to the lamellar bodies.
188.8.131.52. Father Pacini corpuscles
This egg-shaped, 1-4 mm large, very rapidly adapting receptor with a lamellar structure (Fig. 15), located in the subcutis, receives information from a large area, reacts to pressure and acceleration, and is particularly sensitive to vibration. During the execution of fine movements, the information transmitted by these receptors plays an important role in regulating the strength of muscle contractions. They also occur in the mesentery, pancreas, periosteum, in the urinary bladder, in the intermuscular septa and also in the vagina.
Figure 1.27. Figure 15 .: Vater-Pacini corpuscles, silver impregnation
In the long axis of the corpuscles there is an axon ending (sometimes two) that contains many mitochondria. The axon can also branch, or sometimes spiral. It is enclosed with flattened Schwann cells arranged in several layers, this unit is the inner bulb. The outer, lamellar layers, the number of which can even be more than 50, are formed by flattened perineural cells, all of which are covered with basal lamina. Between the lamellae there are collagen fibrils, proteoglycans, interstitial fluid and tiny vessels. From the outside, the corpuscle is covered with a connective tissue capsule of varying thickness.
184.108.40.206. Ruffini corpuscles
These are slowly adapting receptors that occur in varied forms in the skin, in joint capsules, in the root skin of the teeth. The receptors found in the dermis have a cylindrical capsule made of perineural cells that is open at both ends. The collagen fibers inside the cylinder anchor the receptor to its surroundings, making them look like crackers. The axon enters the side or at the end of the corpuscle, it can also branch. The joint capsules contain similar receptors, but in which the collagen fibers run out in several directions.
220.127.116.11. Free nerve endings
The free nerve endings can be nociceptors (pain sensation), thermoreceptors or mechanoreceptors. They are mainly found in the skin, but they can also be found in the internal organs or the musculoskeletal system. The sharp pain is mediated by A-delta fibers, the unmyelinated C-fibers, which only have a Schwann sheath, conduct the dull pain sensation.
The proprioceptors (proprio = own) inform the central nervous system about the state of tension in the muscles, ligaments, tendons and connective tissue capsules; they are accordingly located at the points listed.
18.104.22.168. Golgi tendon organs
These receptors respond to the tension in the muscle; their typical location is the transition area between muscle and tendon (or aponeurosis). They are spindle-shaped structures, the length of which is 1-1.5 mm, they are 100-120 micrometers thick. On the outside they are surrounded by flattened perineural cells, while inside they are drawn by collagen fibrils that connect the muscle fibers with the capsule of the receptor. The outer surface is covered with the collagen fibers of the tendon. Several nerve fibers (type Ib) enter the capsule, which after their entry lose their myelin sheath and branch out several times. The branching axons are covered with Schwann cells, the basal lamina of which connects to the inner collagen-fibrous network.
This structure makes it possible to monitor muscle tension, and if the muscle contracts too much (risk of muscle rupture), it inhibits the function of the motor neurons via impulses emanating from the receptor.
The muscle spindles are 1-7 mm long, in their middle 100-200 micrometers thick receptors that register the muscle stretching. They are surrounded by capsules made up of perineural cells. The intrafusal fibers that pull inside the capsule are parallel to the working muscle fibers and attach to the capsule or the perimysium of the surrounding muscle fibers. The nuclei of the intrafusal muscle fibers form a group in the middle of the fibers. If this core group is wider, one speaks of a core bag fiber, if the cores form more of a row, then it is a core chain fiber. The muscle spindles function as receptors as well as effectors. The type Ia fibers that wrap around the fibers form the primary annulospiral endings. The secondary, so-called flower umbel ends, are terminals of type II fibers that occur mainly around core chain fibers.
The adequate stimulus of this receptor is the stretching of the muscle. If, for example, you put a book on your forearm with your elbow bent, after a little swaying it will take up its original position in that the upper arm flexors contract counteracting the stretching caused by the weight (see the proprioceptive reflex arc). The middle section of the intrafusal fibers reacts to the stretching. The two end pieces are contractible, so the muscle spindle can take up the length corresponding to the surrounding working muscle fibers at any moment. These contractile parts are innervated by the A-gamma fibers that originate from the gamma interneurons.The importance of the contractile end pieces of the intrafusal fibers lies not only in the fact that they can follow the changes in length of the working muscle fibers, but also in the fact that the intrafusal fibers also participate in the regulation of muscle tone: they can contract before the working muscle fibers contract As a result, the middle section of the intrafusal fibers, which is responsive to stretching, is stretched, so the contraction of the working muscle fibers is triggered reflexively (stretching reflex) and the desired muscle tone is set.
22.214.171.124. Other proprioceptors
As mentioned earlier, Ruffini (126.96.36.199. ***) and Vater-Pacini corpuscles (188.8.131.52. ***) are present in the joint capsules and ligaments, which are used for proprioception at these points.
7.1.5. Intestinal mechanoreceptors (interoceptors, visceroceptors)
In part, they are similar to the exteroceptors, the structure of which we have already shown earlier. The Golgi-Mazzoni corpuscle is similar to a smaller Vater-Pacini corpuscle (184.108.40.206. ***), and free nerve endings also occur. The latter are capable of registering mechanical, chemical or pressure stimuli. They occur in the lungs, where they react to the stretching of the lung tissue, or in the aortic wall, in the atria of the heart, where they register pressure. In the gastrointestinal tract, they are responsible for setting the peristalsis in motion. You can also respond to pain in the case of cramp-like pain in the bowels, for example. The nociceptors also react to the reduced blood flow with pain (e.g. heart attack). (In general, the receptors of the viscera convey a lot more central stimuli than one would think: 70% of the fibers of the vagus nerve, for example, are afferent fibers!)
220.127.116.11. Carotid body
This is a receptor area that consists of main cells, supporting cells and sensitive nerve endings, which primarily registers chemical stimuli (pH, oxygen and carbon dioxide partial pressure). It is rich in capillaries lined with fenestrated endothelium. Due to the high content of biogenic amines, this tissue also belongs to the “chromaffin” tissues.
7.2.1. Neurohumoral Endings
The simplest effector is the neurohumoral ending. The thickened end of the nerve fiber attaches to the basal lamina of a capillary and empties the neurohormone into the bloodstream. These terminals can also end at the smooth muscle cells of the vessels (e.g. when bradykinin is released, the effect is either vasoconstriction or vasodilation depending on the receptor. There are numerous nerve endings that function according to a similar principle).
7.2.2. Neuroglandular Synapses
The nerve endings invade the gland cell, and the two basal lamina merge with each other at the point of the end.
7.2.3. Nerve-muscle contact
18.104.22.168. Smooth muscle innervation
The nerve ending is separated from the smooth muscle cell membrane by a basal lamina. The excitation can be transmitted to the surrounding smooth muscle cells via communicating cell connections (nexus / gap junction). The smooth muscle cells of the intestinal tract are loaded with both inhibitory and excitatory nerve endings; they often appear en passant like synapses: in this case the varicosities (thickenings) of the axons leave the Schwann envelope and form a relatively wide synaptic gap (100-500 nm) make contact with the smooth muscle cells, then they move away, and in their course they can form other similar synapses.
22.214.171.124. Innervation of the skeletal muscles
In the area of the motor end plate (Fig. 16), the nerve fiber loses its sheath, branches out and grasps the muscle fiber like a finger. Typically, one finds an accumulation of cell nuclei here, partly from the tightly packed cell nuclei of the muscle fiber, partly from the nuclei of the Schwann cells covering the nerve endings. The nerve ending is separated from the folded sarcolemma of the muscle fiber by a basal lamina. The terminal is rich in mitochondria and synaptic vesicles that contain acetylcholine. The duration of the contraction that follows the release of acetylcholine is regulated by an enzyme called acetylcholinesterase. (This enzyme is the point of attack of certain neurotoxins).
Figure 1.28. Figure 16 .: Motor end plates, silver impregnation
Choose the right positions! (B, D)
The multipolar nerve cells can have several axons
The size of the cell body of the nerve cell can even be 100-150 micrometers
The telodendron can be found on the dendrites
The side branches of the axons are called the collaterals
Put the following types of neurons in the correct order according to the frequency of their occurrence! Start with the rarest cell type! (A, B, C, D)
Choose the right positions! (A, C)
The myelin sheath is made by the oligodendrocytes and the Schwann cells.
The umnyelinized fibers never have a shell.
The peripheral unmyelinated fibers are covered by Schwann cells.
The fibrillar glial cells can also form myelin sheaths.
What is the Nissl substance (Nissl clods)? (B)
The substance that appears as the nerve cells age and is no longer broken down by the cell.
Large amount of endoplasmic reticulum with attached ribosomes and a rosette-like arrangement of ribosomes
Large amount of free ribosomes
Large amount of endoplasmic reticulum
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