Nervous tissue consists of highly specialized cells. They have the ability to perceive all sorts of irritants. In response, human nerve cells can form a pulse, and also transmit it to each other and other working elements of the system. As a result, a reaction is formed that is adequate to the effect of the stimulus. The conditions in which certain functions of the nerve cell manifest themselves are formed by glial elements.
The laying of the nervous tissue occurs in the third week of the embryonic period. At this time, a plate is formed. From it develop:
During further embryogenesis, the neural plate becomes a tube. In the inner layer of its wall are located the stem ventricular elements. They proliferate and go outside. In this area, some cells continue to divide. As a result, they are divided into spongioblast (microglia components), glioblasts and neuroblasts. Of the latter, nerve cells are formed. In the wall of the tube are 3 layers:
- Internal (ependymic).
- Medium (raincoat).
- The outer (marginal) is represented by a white brain substance.
At 20-24 weeks in the cranial segment of the tube begins the formation of blisters, which are the source of brain formation. The remaining departments serve for the development of the spinal cord. The cells participating in the formation of the crest emerge from the edges of the nerve trough. It is located between the ectoderm and the tube. Ganglionic plates forming the basis for myelocytes (pigmentary cutaneous elements), peripheral nerve nodes, melanocyte cover, components of APUD-system are formed from these cells.
Gliocytes in the system are 5-10 times larger than nerve cells. They perform different functions: supporting, protective, trophic, stromal, excretory, sucking. In addition, gliocytes have the ability to proliferate. Ependymocytes differ prismatic form. They form the first layer, lining the cerebral cavity and the central spinal cord. Cells are involved in the production of cerebrospinal fluid and have the ability to suck it. The basal part of the ependymocytes has a conical, truncated form. It passes into a long, thin process that pierces the brain substance. On its surface it forms a glial boundary membrane. Astrocytes are represented by multibeam cells. They are:
- Protoplasmic. They are located in the gray brain substance. These elements are distinguished by the presence of numerous short branches, wide endings. Some of the latter surround the blood capillary vessels, participate in the formation of the blood-brain barrier. Other outgrowths are directed toward the neuronal bodies and they transfer nutrients from the blood. They also provide protection and isolate synapses.
- Fibrous (fibrous). These cells are in white matter. Their endings are weakly branched, long and thin. At the ends, they have branching and delimiting membranes are formed.
Oliodendrocytes are small elements with outgoing short tails, located around neurons and their endings. They form a glial shell. Through it, impulses are transmitted. On the periphery these cells are called mantle (lemocytes). Microglia is part of the macrophage system. It is represented in the form of small mobile cells with slightly branched short processes. Elements contain a bright core. They can be formed from blood monocytes. Microglia restores the structure of a nerve cell that has been damaged.
The main component of the central nervous system
It is represented by a neural cell - a neuron. In total, there are about 50 billion. Depending on the size, giant, large, medium, small nerve cells are isolated. In their form they can be:
There is also a classification by the number of endings. So, there can be only one process of the nerve cell. This phenomenon is typical for the embryonic period. In this case, the nerve cells are called unipolar. Bipolar elements are found in the retina of the eye. They are extremely rare. Such nerve cells have 2 endings. There are also pseudo-unipolar ones. From the body of these elements, the cytoplasmic long outgrowth extends, which is divided into two processes. Multipolar structures are found primarily directly in the central nervous system.
Structure of the nerve cell
In the element, the body is distinguished. It has a large bright nucleus with one or two nucleoli. The cytoplasm contains all the organelles, especially the tubules from the granular EPS. Clusters of basophilic substance are distributed throughout the cytoplasmic surface. 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 stress, these clumps are destroyed. Thanks to intracellular regeneration, the process of recovery-destruction constantly takes place.
Impulse formation and reflex activity
Dendrites are common among the processes. Branched, 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, correspondingly, more information. They spread the impulses to the body of the element. Nerve cells contain only one axon. At the base of it a new impulse is formed. He departs from the body along the axon. The outgrowth of a nerve cell can have a length of a few microns to one and a half meters. There is one more category of elements. They are called neurosecretory cells. They can produce and release hormones into the bloodstream. Cells of the nervous tissue are arranged in chains. They, in turn, form the so-called arcs. They determine the reflex activity of a person.
The following types of elements are distinguished according to the functions of the nerve cell:
- Afferent (sensitive). They form one link in the reflex arc (spinal nodes). On the periphery there is a long dendrite. There it ends with an ending. In this case, a short axon enters the reflex somatic artery into the region of the spinal cord. He is the first to react to the stimulus, resulting in the formation of a nerve impulse.
- Conductor (interlaced). These are the nerve cells of the brain. They form two links of the arc. These elements are also present in the spinal cord. From them information is obtained by motor effector cells of the nervous tissue, branched short dendrites and a long axon reaching the skeletal muscular fiber. An impulse is transmitted through the neuromuscular synapse. Also, effector (efferent) elements are also isolated.
In humans, they are mostly complex. In a simple reflex arc, there are three neurons and three links. Their complication is due to the increase in the number of intercalating elements. The leading role in the formation and subsequent carrying out of the impulse belongs to the cytolemma. Under the influence of the stimulus, a depolarization-inversion of the charge-is performed in the region of the action. In this form, the pulse propagates further along the cytolemma.
Around the nerve processes, glial membranes are located independently. In a complex they form nerve fibers. Branching in them is called axial cylinders. There are bezmielinovye and myelin fibers. They differ in the structure of the glial shell. Bezmielin fibers have a fairly simple device. The axial cylinder, which is suitable for the glial cell, deflects its cytolemma. The cytoplasm closes over it and forms a mezakson - a double fold. One glial cell may contain several axial cylinders. These are "cable" fibers. Their branches can pass into adjacent glial cells. The pulse travels at a speed of 1-5 m / s. Fibers of this type are found during embryogenesis and in postganglionic areas of the vegetative system. Myelin segments are thick. They are located in the somatic system innervating the musculature of the skeleton. Lemmatocytes (glial cells) pass successively, by a chain. They form a burden. In the center there is an axial cylinder. In the glial shell there are:
- The inner layer of nerve cells (myelin). It is considered basic. In some areas between the layers of the cytomegma, there are extensions that form myelin incisions.
- Pan eripheral layer. There are organelles and a nucleus - the neurilemma.
- Thick basal membrane.
Places of heightened sensitivity
In areas where adjacent lemmocytes border, thinning of the nerve fiber occurs and the myelin layer is absent. This is a place of hypersensitivity. They are considered the most vulnerable. The part of the fiber located between adjacent nodal intercepts is called the inter-node segment. Here, the pulse travels at a speed of 5-120 m / s.
With their help, the cells of the nervous system are connected together. There are different synapses: axo-somatic, -dendritic, -axonal (mostly of the inhibitory type). Also isolated electrical and chemical (the first are identified quite rarely in the body). Synapses distinguish post- and presynaptic parts. The first contains a membrane in which highly specific protein (protein) receptors are present. They react only to certain mediators. Between the pre- and postsynaptic parts there is a gap. The nervous impulse reaches the first and activates the special vesicles. They pass to the presynaptic membrane and enter the gap. From there, they affect the receptor of the postsynaptic film. This provokes its depolarization, which, in turn, is transmitted through the central process of the next nerve cell. In the chemical synapse, information is transmitted only in one direction.
Synapses are divided into:
- Brake, containing retarding neurotransmitters (gamma-aminobutyric acid, glycine).
- Excitatory, in which the relevant components are present (adrenaline, acetylcholine, glutamine, norepinephrine).
- Effector, ending in working cells.
Neuromuscular synapses are formed in the skeletal muscle fiber. They contain a presynaptic part, formed by the terminal terminal section of the axon from the motor neuron. It is embedded in the fiber. The adjacent site forms the postsynaptic portion. There are no myofibrils in it, but they are present in a large number of mitochondria and nuclei. The postsynaptic membrane is formed by a sarcolemma.
They are very diverse:
- Free found only in the epidermis. The fiber, passing through the basal membrane and discarding the myelin sheath, interacts freely with the epithelial cells. These are painful and temperature receptors.
- Unencapsulated non-free endings are present in the connective tissue. Glia accompanies the branches in the axial cylinder. These are tactile receptors.
- Encapsulated endings are branches from an axial cylinder, accompanied by a glial inner bulb and an external connective tissue envelope. These are also tactile receptors.