Nervous tissue consists of highly specialized cells. They have the ability to perceive various kinds of stimuli. In response, human nerve cells can form a pulse, as well as transmit it to each other and other working elements of the system. As a result, a reaction is formed that is adequate to the stimulus. The conditions in which certain functions of the nerve cell manifest themselves form glial elements.
Nervous tissue is laid during the third week of the embryonic period. At this time, the plate is formed. From it develop:
In the course of further embryogenesis, the neural plate turns into a tube. In the inner layer of its wall stem ventricular elements are located. They proliferate and move outwards. In this area, some cells continue to divide. As a result, they are divided into spongioblasts (components of microglia), glioblasts and neuroblasts. From the latter, nerve cells form. In the wall of the tube there are 3 layers:
- Internal (ependymal).
- Medium (mantle).
- External (marginal) - represented by white medulla.
At 20-24 weeks in the cranial segment of the tube begins the formation of bubbles, which are the source of the formation of the brain. The remaining sections are used to develop the spinal cord. From the edges of the neural gutter, the cells involved in the formation of the ridge. It is located between the ectoderm and the tube. Ganglion plates form the basis of myelocytes (pigmented skin elements), peripheral ganglions, melanocytes of the skin, and components of the APUD system.
There are 5-10 times more gliocytes in the system than nerve cells. They perform various functions: support, protective, trophic, stromal, excretory, suction. In addition, glyocytes have the ability to proliferate. Ependymocytes have a prismatic shape. They make up the first layer, lining the brain cavity and the central spinal cord. Cells are involved in the production of cerebrospinal fluid and have the ability to absorb it. The basal part of ependymocytes has a conical truncated shape. It passes into a long thin process, penetrating the medulla. On its surface, it forms a glial marginal membrane. Astrocytes are represented by multislice cells. They are:
- Protoplasmic. They are located in the gray medulla. These elements are distinguished by the presence of numerous short ramifications, wide endings. Part of the latter surrounds the blood capillary vessels, is involved in the formation of the blood-brain barrier. Other processes are directed to the neural bodies and they carry the transfer of nutrients from the blood. They also provide protection and isolate synapses.
- Fibrous (fibrous). These cells are in white matter. Their endings are weakly branching, long and thin. At the ends they have branching and form the boundary membranes.
Oliodendrocytes are small elements with short tails extending around neurons and their endings. They form a glial sheath. Through it transmitted impulses. At the periphery, these cells are called mantle (lemmocytes). Microglia is part of the macrophage system. It is presented in the form of small mobile cells with little branched short processes. The elements contain a bright core. They can be formed from blood monocytes. Microglia restores the structure of the damaged nerve cell.
The main component of the central nervous system
It represents the nerve cell - the neuron. In total, there are about 50 billion. Depending on the size, giant, large, medium, small nerve cells secrete. In their form, they can be:
There is also a classification by the number of endings. Thus, only one process of the nerve cell can be present. This phenomenon is characteristic of the embryonic period. In this case, the nerve cells are called unipolar. Bipolar elements are found in the retina. They are extremely rare. Such nerve cells have 2 endings. There are also pseudo-unipolar. From the body of these elements departs cytoplasmic long growth, which is divided into two processes. Multipolar structures are primarily detected directly in the CNS.
Nerve cell structure
In the element distinguish the body. It has a large bright core with one or two nucleoli. The cytoplasm contains all the organelles, especially the canaliculi from granular EPS. Across the cytoplasmic surface are clusters of basophilic substances. They are formed by ribosomes. In these clusters, there is a process of synthesis of all the necessary substances transported from the body to the processes. Due to the voltage, destruction of these clumps occurs. Due to intracellular regeneration, the process of recovery-destruction is constantly taking place.
Impulse formation and reflex activity
Dendrites are common among processes. Branching, they form a dendritic tree. Due to them, synapses are formed with other nerve cells and information is transmitted. The more dendrites there are, the more powerful and extensive the receptor field and, accordingly, more information. On them is the spread of impulses to the body of the element. Nerve cells contain only one axon. A new impulse is formed at its base. It departs from the body along the axon. The process of the nerve cell can be from a few microns to one and a half meters long. There is another category of elements. They are called neurosecretory cells. They can produce and secrete hormones into the blood. The cells of the nervous tissue are arranged in chains. They, in turn, form the so-called arcs. They determine the reflex activity of man.
According to the function of the nerve cell, the following types of elements are distinguished:
- Afferent (sensitive). They form 1 link in the reflex arc (spinal nodes). A long dendrite passes to the periphery. There it ends with an ending. In this case, a short axon enters the reflex somatic arch in the region of the spinal cord. He is the first to react to a stimulus, as a result of which a nerve impulse is formed.
- Conductors (insert). These are nerve cells in the brain. They form a 2 link arc. These elements are also present in the spinal cord. Information from them is obtained by motor effector cells of the nervous tissue, branched short dendrites, and a long axon reaching the skeletal muscular fiber. Through the neuromuscular synapse impulse is transmitted. Also isolated and effector (efferent) elements.
In humans, they are predominantly complex. In a simple reflex arc, there are three neurons and three links. Their complication occurs due to the increase in the number of inserts. The leading role in the formation and subsequent conduct of the impulse belongs to the cytolemma. Under the influence of a stimulus in the area of impact depolarization is performed - charge inversion. In this form, the impulse spreads further along the cytolemma.
Around the nerve processes are independently located glial membranes. In the complex, they form nerve fibers. Branches in them are called axial cylinders. There are amyelinic and myelinated fibers. They differ in the structure of the glial sheath. Non-myelinated fibers have a fairly simple device. An axial cylinder suitable for the glial cell bends its cytolemma. Cytoplasm closes over it and forms a mezaxon - a double fold. A single glial cell may contain several axial cylinders. These are “cable” fibers. Their branches can move to adjacent glial cells. The impulse passes with a speed of 1-5 m / s. Fibers of this type are found during embryogenesis and in the postganglionic areas of the vegetative system. Myelin segments are thick. They are located in the somatic system that innervates the muscles of the skeleton. Lemmocytes (glial cells) pass sequentially, in chains. They form a burden. In the center passes the axial cylinder. In the glial membrane are present:
- The inner layer of nerve cells (myelin). It is considered the main one. In some areas between the layers of the cytolemma there are extensions that form myelin notches.
- Pperipheral layer. It contains organelles and a nucleus - neyrylemma.
- Thick basement membrane.
In areas where adjacent lemmocytes border, nerve fiber thinning occurs and the myelin layer is absent. These are places of hypersensitivity. They are considered the most vulnerable. The part of the fiber between adjacent node intercepts is called the inter-node segment. Here the pulse passes at a speed of 5-120 m / s.
With their help, cells of the nervous system are interconnected. There are different synapses: axo-somatic, -dendritic, -axonal (mainly of the inhibitory type). They also secrete electrical and chemical (the first ones are detected quite rarely in the body). In the synapse, there are post- and presynaptic parts. The first one contains a membrane in which highly specific protein (protein) receptors are present. They only respond to certain mediators. Between the pre - and postsynaptic parts is a gap. The nerve impulse reaches first and activates special bubbles. They go to the presynaptic membrane and fall into the gap. From there, they affect the receptor of the postsynaptic film. This provokes its depolarization, which is transmitted, in turn, through the central process of the next nerve cell. In a chemical synapse, information is transmitted only in one direction.
Synapses are divided into:
- Braking, containing inhibitory neurotransmitters (gamma-aminobutyric acid, glycine).
- Exciting, in which there are relevant components (adrenaline, acetylcholine, glutamine to-that, norepinephrine).
- Effector, ending on the working cells.
Neuromuscular synapses form in the skeletal muscle fiber. In them there is a presynaptic part formed by the terminal end section of the axon from the motor neuron. It is embedded in the fiber. The adjacent site forms the postsynaptic part. There are no myofibrils in it, but mitochondria and nuclei are present in large numbers. The postsynaptic membrane is formed by a sarcolemma.
They differ a great variety:
- Loose are found exclusively in the epidermis. The fiber, passing through the basement membrane and discarding the myelin sheath, interacts freely with epithelial cells. These are pain and temperature receptors.
- Non-encapsulated non-free endings are present in connective tissue. Glia accompanies ramifications in the axial cylinder. These are tactile receptors.
- Encapsulated endings are branches from an axial cylinder, accompanied by a glial inner bulb and an outer connective tissue sheath. These are also tactile receptors.