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December 03, 2016 00:03

nervous tissue

Nervous tissue is the main structural element of the nervous system organs - brain, spinal cord, nerves, ganglia (ganglia) and nerve endings.Nerve tissue is composed of neuronal cells (neurocytes or neurons) and associated anatomically and functionally supporting glial cells.

neurocytes (neurons) from off of their processes are structural and functional units of the nervous system organs.Nerve cells are able to perceive stimuli, becomes excited, generate and transmit information encoded in the form of electrical and chemical signals (nerve impulses).Nerve cells are also involved in the processing, storing and retrieving information from memory.

Each nerve cell has a body and appendages.Outside the nerve cell is surrounded by a plasma membrane (tsitolemmy), able to carry out the excitement, as well as to ensure the exchange of substances between the cell and its environment.nerve cell body contains the nucleus and its surrounding cytoplasm, which is also called perikaryonic (from the Greek rap -. arou

nd, karyon - core).In the cytoplasm of the cell organelles are: granular endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, etc. For neurons characterized by the presence in their cytoplasm chromatophilic substance (Nissl substance) and neurofibrillary..Chromatophilic substance is detected in the form of basophilic clumps (clusters of structures granular endoplasmic reticulum), the presence of which indicates a high level of protein synthesis.

nerve cell cytoskeleton presented microtubules (neural tube) and intermediate filaments, which are involved in the transport of various substances.Dimensions (diameter) of bodies of neurons are from 4-5 to 135 micron.The form of nerve cell bodies is also different - from round, ovoid to pyramidal.From the body of the nerve cell depart surrounded by a shell of different lengths thin cytoplasmic processes.Mature nerve cells have two types of processes.One or more tree-like branching processes by which the nervous impulse reaches the body of the neuron, called deydritom.This so-called dendritic transport substances.Most dendritic cell length of about 0.2 microns.The direction of the long axis of the dendrite are many neyrotrubochek and a small amount of neurofilament.In the cytoplasm of dendrites are elongated mitochondria, and a small number of tanks nezernistoy endoplasmic reticulum.The final sections of dendrites often kolboobrazno expanded.Only, usually long process, in which a nerve impulse travels from the nerve cell body - this is the axon or neurite.The axon extends from the end of the axon hillock in the nerve cell body.It ends with a variety of axon terminal branches that form synapses with other nerve cells or working body tissues.Surface tsitolemmy axon (axolemma) is smooth.In axoplasm (cytoplasm) are thin elongated mitochondria, and a large number of neyrotrubochek neurofilament, vesicles and tubules nezernistoy endoplasmic reticulum.Ribosomes and elements of granular endoplasmic reticulum in axoplasm absent.They are just the axon hillock in the cytoplasm, where the beams neyrotrubochek, while neurofilaments are small amount.

Depending on the speed of nerve impulses are two types of axonal transport;slow transport speed from 1-3 mm per day and fast, at a speed of 10.5 mm per hour.

Nerve cells dynamically polarized, ieable to conduct nerve impulses in only one direction - from the body of the dendrites of nerve cells.

Nerve fibers are nerve cell processes (dendrites neurite) coated membranes.Each nerve fiber outgrowth is an axial cylinder and surrounding lemmotsity (Schwann cells) relevant to neuroglia fiber form membranes.

Given the structure of shells of nerve fibers are divided into non-fleshy (unmyelinated) and meat (myelin).

unmyelinated (non-fleshy) nerve fibers are mainly in autonomic neurons.The shell of the fiber is thin, structured in such a manner that the cylinder axis as if pressed into Schwann cells, formed in her a deep groove.Closed, dual membrane of axons called neyrolemmotsita mezaksonom.Often within the shell is not one axial cylinder, and several (5 to 20), forming a type of nerve fiber cable.During the process of nerve cells form its shell many Schwann cells, which are located one behind the other.Between axolemma each nerve fiber and Schwann cells have a narrow space (10-15 nm), filled with tissue fluid, involved in conducting nerve impulses.

myelinated nerve fibers have a thickness of up to 20 microns.They formed quite a thick axon cells - axial cylinders around which there is a shell, consisting of two layers: the thicker the inner - and outer myelin - a thin layer formed neyrolemmotsitami.Myelinated nerve fiber layer has a complex structure as Schwann cells in the development spirally wound onto the axons of nerve cells (axial cylinders).Dendrites are known to have the myelin sheath.Each lemmocyte envelops only a small portion of the cylinder axis.Therefore myelin layer consisting of lipid, has only within Schwann cells, it is not continuous, but intermittent.Every 0.3-1.5 mm are the so-called nerve fiber nodes (nodes of Ranvier) where the myelin layer is absent (interrupted) and neighboring lemmotsity their ends fit directly to the axons.Schwann cells covering the basal membrane solid, it passes without interruption through the nodes of Ranvier.These traps are considered as locations permeability for ions and Na + depolarization current (nerve impulse).This depolarization (only in the nodes of Ranvier) contributes to the rapid passage of nerve impulses along myelinated nerve fibers.Nerve impulses are conducted on myelin fibers as it leaps - from one node of Ranvier to the next.In unmyelinated nerve fiber depolarization occurs throughout the fiber, and nerve impulses in such fibers are slow.Thus, the rate of nerve impulses by unmyelinated fibers is 1-2 m / s, and for meat (myelin) - 5-120 m / s.

Classification of nerve cells.Depending on the number of processes are distinguished unipolar or odnootrostchatye, and bipolar neurons or dvuhotrostchatye.Neurons with more appendages called multipolar or mnogootrostchatymi.By bipolar neurons are such lozhnounipolyarnye (psevdounipolyarnye) neurons, which are the cells of the spinal ganglia (nodes).These neurons are called psevdounipolyarnymi because of the body's cells depart next two appendages, but the space between the spikes is not detected by light microscopy.Therefore, these two process are taken under the light microscope for one.The number of dendrites, the degree of branching vary widely depending on the location and neuronal function they perform.Multipolar neurons of the spinal cord have a body of irregular shape, a lot of slabovetvyaschihsya dendrites extending in different directions, and a long axon, which depart lateral branches - collaterals.From large triangular bodies of pyramidal neurons in the brain (large) cortex leaves a large number of short horizontal slabovetvyaschihsya dendrites, axon extends from the base of the cell.And dendrites and axon end with nerve endings.At dendrite is sensitive nerve endings in the neurite - effector.

By functionality nerve cells divided into receptor, effector and associative.

receptor (sensory) neurons by their endings perceive different kinds of feelings and suffer arising in the nerve endings (receptors) impulses to the brain.Therefore, sensory neurons are also called bringing (afferent) nerve cells.Effector neurons (causing effects, the effect) carried out nerve impulses from the brain to the body of the worker.These nerve cells are also called efferent (efferent) neurons.Associative, or intercalary, of conductor neurons transmit nerve impulses from the neuron-bearing outflow.

There are large neurons whose function is to produce secretions.These cells are called neurosecretory neurons.Secret (neurosecretion) containing protein and lipids, polysaccharides released in the form of pellets and is transported in blood.Neurosecretion involved in the interactions of the nervous and cardiovascular (humoral) systems.

Depending on the location of the following types of nerve endings - receptors:

  1. exteroreceptors perceive irritation environmental factors.They are located in the outer integument, the skin and the mucous membranes, in the sense organs;
  2. interoreceptors get irritated mainly by changing the chemical composition of the internal environment (chemoreceptors), the pressure in the tissues and organs (barortseptory, mechanoreceptors);
  3. proprioceptors or proprioceptors, perceive irritation in the tissues of the body itself.They are available in the muscles, tendons, ligaments, fascia, joint capsules.

In accordance with the function of isolated thermoreceptors, mechanoreceptors and nociceptors.The first perceive the temperature change, the second - the various types of mechanical effects (touching the skin, its compression), and others - painful irritation.

Among the distinguished free nerve endings, devoid of glial cells, and non-free, in which the nerves have a shell - a capsule formed by glial cells, or connective tissue elements.

free nerve endings in the skin there.Approaching the epidermis, nerve fiber loses its myelin, penetrate the basement membrane into the epithelial layer, which divides between the epithelial cells until the granular layer.Terminal branches less than 0.2 microns in diameter at their ends kolboobrazno expanded.Similar nerve endings are in the epithelium of the mucous membranes and cornea.End the free receptor nerves feel pain, heat and cold.Other nerve fibers penetrating the same manner and in the epidermis with end contacts tactile cells (Merkel cells).Nerve endings expands and forms a cell Merkel sinapsopodobny contact.These endings are mechanoreceptors, perceiving pressure.

Non-free nerve endings can be encapsulated (covered by a connective tissue capsule) and non-encapsulated (devoid of capsules).Nonencapsulated nerves are found in connective tissue.They also include a closure hair follicles.Encapsulated nerve endings are palpable calf, the calf plate, bulbous body (Golgi-Mazzoni corpuscles), genital cells.All of these nerve endings - mechanoreceptors.This group includes the terminal and the bulb are thermoreceptors.

lamellar bodies (corpuscles of Vater-Pacini) - the largest of all encapsulated nerve endings.They oval reach 3-4 mm in length and 2 mm in thickness.Located in the connective tissue of internal organs and subcutaneous tissue (dermis, often - on the border of the dermis and hypodermis).A large number of lamellar bodies have in adventitia of large vessels in the peritoneum, tendons and ligaments, along the arteriolovenulyarnyh anastomoses.Taurus outside covered connective tissue capsule having a lamellar structure and rich hemocapillars.Under the connective tissue sheath is outer bulb consisting of concentric plates 10-60, educated flattened hexagonal perineural epithelioid cells.On entering the body, the nerve fiber loses its myelin sheath.Inside the bull is surrounded by lymphocytes, which form the inner bulb.

Tactile corpuscles (Meissner corpuscles) length of 50-160 microns and about 60 microns wide, oval or cylindrical.They are especially numerous in the papillary layer of the skin of fingers.They are also found in the skin of the lips, the edges of the eyelids, external genitalia.Taurus formed a plurality of elongated, flattened or pear-lymphocytes, lying one on the other.Nerve fibers are included in the body lose the myelin.Perineurium passes into the surrounding capsule body formed by several layers of epithelioid perineural cells.Tactile corpuscles are mechanoreceptors, perceiving a touch of skin compression.

genital cells (corpuscles Ruffini) fusiform, located in the skin of hands and toes, joint capsules, and blood vessel walls.Taurus is surrounded by a thin capsule formed perineural cells.Going into the capsule, the nerve fiber loses its myelin and divides into many branches, which end kolboobraznymi swellings surrounded lemmotsitami.End seal tightly to the fibroblasts and collagen fibers that form the basis of a calf.Taurus Ruffini mechanoreceptors are, they also perceive heat and serve proprioceptors.

End flask (flask Krause), spherical in shape, located in the skin, conjunctiva, the mucous membrane of the mouth.The bulb has a thick connective tissue capsule.Going into the capsule, the nerve fiber loses its myelin sheath and branches in the center of the bulb to form a plurality of branches.Flasks Krause perceive coldness;perhaps they are and mechanoreceptors.

in the connective tissue of the papillary layer of the skin of the glans penis and the clitoris are many genital cells, similar to the end of the flask.They are mechanoreceptors.

Proprioreceptors perceive reduce muscle tension tendons and joint capsules, muscle strength needed to perform a particular movement or hold parts of the body in a certain position.By proprioretseptornym nerve endings belong to the neuro-muscular and neuro-tendon spindles, which are located in the abdomen muscles or their tendons.

Golgi tendon organ are places of transition in the muscle tendon.They represent the tendon bundles (collagen) fibers bonded with muscle fibers surrounded by a connective tissue capsule.To fit the spindle is usually thick myelinated nerve fiber, which loses the myelin sheath and forms a terminal branch.These endings are located between the beams of the tendon fibers, which they perceive the contractile effects of muscle.

muscle spindle larger, 3-5 mm long and 0.5 mm thick, surrounded by a connective tissue capsule.Inside the capsule to 10-12 thin short striated muscle fibers with different structure.In some muscle fibers are concentrated in the central core and form part of the 'nuclear bag ".In other core fibers are arranged "nuclear chain" throughout the muscle fiber.On those and other fibers spiral ring-shaped branch (primary) nerve endings that react to changes in the length and speed reductions.Around the muscle fiber with the "nuclear chain" branch also acinar (secondary) nerve endings that perceive only the change in muscle length.

The muscles are effector neuromuscular endings that are located on each muscle fiber.Going to muscle fiber, nerve fiber (axon) myelin is lost and branches.These endings are covered lemmotsitami, their basement membrane, which goes into the basement membrane of the muscle fiber.Axolemma each of these nerve endings in contact with the sarcolemma of muscle fibers, bending the it.The gap between the end and the fiber (20-60 nm width) is an amorphous substance containing, as synaptic gap acetylcholinesterase.Near neuromuscular endings in the muscle fiber is a lot of mitochondria polyribosomes.

effector nerve endings neischerchennoy (smooth) muscle tissue swellings form, which are also in synaptic vesicles and mitochondria containing norepinephrine and dopamine.Most nerve axons and blisters in contact with the basement membrane of myocytes;only a small number of them pierces the basement membrane.In contacts with nerve fibers in muscle cell axolemma separated from tsitolemmy myocyte gap of about 10 nm thick.