The nervous system is the most complex system of the body and one of the most vulnerable. To understand how different diseases affect nerve cells, why certain symptoms develop, you need to start with the basics - to study the histology of the nervous system.
What is histology?
Histology is the science of the structure of organs and tissues at the microscopic, that is, at the cellular, level. After all, any body system consists of organs, an organ of tissues, and tissue of millions of cells.
The process of tissue formation from the moment of fertilization to the birth of the baby is called histogenesis, and the formation of organs (some of them continue to develop after birth) - organogenesis.
Bodies are divided into two types:
- Tubular, received this name due to its shape. Tissues in such organs are arranged in layers, they are distinguished by three - the mucous membrane, muscle layer and serous membrane. This, for example, the gastrointestinal tract, blood vessels.
- parenchymal - consist of two main parts: parenchyma (a part of an organ that has a specific function that is characteristic only for a given formation), stroma (a site that provides nutrition and normal functioning of the parenchyma).
Classification of the nervous system
Before proceeding to a brief histology of the nervous system, it is necessary to find out how it is classified at the macroscopic (organismal) level. In anatomy, two types of the nervous system are distinguished:
- central (CNS), which include the large brain, brain stem, spinal cord ;
- peripheral (PNS), including nerves, ganglia.
The organs that make up the nervous system belong to the parenchymal, where the parenchyma is represented by nervous tissue and the stroma by connective tissue cells.
There is another functional classification. Depending on the functions performed by the nervous system in the body, the following parts are distinguished:
- autonomic nervous system, which regulates the functioning of organs without the influence of consciousness. So, for example, a person cannot force the intestinal motility to work more actively or regulate the synthesis of bile by the liver by force of will. The ANS is divided into two subsections: sympathetic, which is activated by stress hormones (adrenaline, cortisone, norepinephrine) and parasympathetic, whose function is provided by the vagus nerve, or n.vagus, which is most active at night.
- somatic nervous system, which ensures the work of skeletal muscles and is regulated by human consciousness.
Nervous system functions
The histology of the nervous system primarily determines its functions. The main ones are:
- correction and maintenance of the constancy of the internal environment of the body;
- the interaction of the body with the environment;
- the formation of conscious, mental activity;
- body survival by creating protective reactions and mechanisms.
Nervous system development
The development of all organs and systems of the body in the prenatal period is called embryogenesis, and the formation and development of tissues is called histogenesis.
The histology of the development of the organs of the nervous system originates from the neuroectoderm, which is subsequently divided into two parts: the neural tube, from which the brain and spinal cord subsequently develop, and the neural crest, from which the cells of the nervous (neurocytes) and endocrine systems (adrenal cortex are formed) diffuse endocrine system), pigment cells - melanocytes.
During the formation of the spinal cord from the lumen in the middle of the neural tube, the spinal cord channel is formed, while the front of the neural tube forms brain bubbles, from which the forebrain hemispheres will subsequently form. There are three such brain bladders: the anterior, middle and hindbrain. In Latin, they are called prosencephalon, mesencephalon and rhombencephalon, respectively. It is worth noting that the brain is formed for a longer time than the spinal cord.
At the next stage of histology of the development of the nervous system, the prosencephalon (forebrain) is divided into two more parts: the final and intermediate. In Latin - telencephalon and diencephalon, respectively. From the diencephalon, two more small cerebral bladders are further separated, which subsequently become the retinas of two eyes. That is, the retina is also part of the nervous system!
Two more are also formed from the posterior cerebral bladder, one of which will give rise to the cerebellum and the bridge (it is called metencephalon), and the second medulla is formed from the second (this cerebral bladder is called myelencephalon).
Only the midbrain bubble does not give any further branches; the midbrain is formed from it.
Thus, all parts of the nervous system are formed, at the next stages only the growth of various parts occurs, their shape changes with the formation of bulges and furrows.
Nerve cell: histology
The main structural as well as functional unit of the structure of the nervous system is a nerve cell, also called a neuron. The main elements of the neuron: the body and processes. The long process is called the axon, and the short ones are dendrites.
Nerve cells are divided into afferent, that is, those that transmit momentum from peripheral structures (skin, muscles) to the central nervous system, efferent, which conduct excitation from the nervous system to internal organs and skeletal muscles, as well as insertion, through which the impulse passes from afferent to efferent neurons.
Depending on the number of processes, nerve cells are divided into:
- unipolar - have only one process;
- bipolar - with two processes;
- psvedounipolar - one process, which after a certain interval is divided into two more;
- multipolar - have more than two processes.
Although neurons are the main element of the histology of the peripheral nervous system (PNS) and, equally, of the central nervous system (CNS), they cannot exist without trophism and blood supply. Supports the vital activity of neurons neuroglia, which randomly surrounds nerve cells.
Nerve fiber: structure
This image shows the preparation of a nerve; histologists often use them for a more detailed study of tissues.
There are two types of nerve fibers: myelin and non-myelin. Myelin is a membrane that covers the nerve and provides faster transmission of impulse from organs to the nervous system and vice versa. Therefore, myelin fibers provide faster transmission of a nerve impulse than nerves lacking this sheath.
To study the histology of the structure of the nerve, it is necessary to understand that myelin does not cover the nerve with a continuous case, but there are areas where this nerve sheath is absent. Such sites are called Ranvier interceptions (the drug is shown in the image below). It is thanks to the presence of these sections that the impulse passes not slowly moving along the fiber, but in an abrupt manner, from interception to interception.
In addition, the nerve itself, which is the parenchyma, covers the stroma - membranes. These include:
- endoneuria;
- perineurium containing microscopic gaps - perineural vaginas, which play a role in the circulation of cerebrospinal fluid;
- epineuria;
- paranevium.
Types of Nerve Centers
The nerve center is a complex of nerve cells located in approximately the same area of the nervous system and performing one common function. The following types of nerve centers of the organs of the nervous system (according to histology) are distinguished:
- nuclear type - nerve cells are located randomly, without a specific order (nuclei of the brain and spinal cord);
- screen type - nerve cells are arranged in an orderly, layer-by-layer manner, resembling screens, which is what the name came from (cortex of the forebrain, cerebellum).
Histology and types of nerve centers of the organs of the nervous system determine the main processes that occur in these centers: convergence and divergence.
Convergence - narrowing and reducing the number of pathways from many cells on the periphery to a smaller number of them.
Divergence is the opposite of convergence, when connections from one neuron spread to many other nerve cells.
These two processes are always in interaction and balance, providing a connection of the nervous system with the whole body through the nerve centers.
Arc reflexes: structure
When considering the histology of the nervous system, it is also necessary to consider such a thing as a reflex arc. It is a sequence of nerve cells that provide an impulse from the sensitive receptor to the end in the working organ. The reflex arches of the somatic and autonomic nervous systems have some differences.
Somatic arc consists of the following departments:
- I neuron is sensitive, its short process (dendrite) ends on the periphery of the receptor, and the long process - axon passes through the posterior horns of the spinal cord into the anterior horns, where it binds to the next neuron;
- II neuron is a motor neuron, with its long process it passes to the skeletal muscles, forming a neuromuscular synapse.
Such a reflex arc is called a two-neural. A vegetative arc consists of at least three nerve cells, that is, it is trineuron. It includes the following elements:
- I neuron is also sensitive, its dendrite departs to the periphery, ending with the receptor, and the axon, passing through the posterior horns, forms a connection (synapse) in the lateral horns with the second neuron.
- II neuron is preganglionic, its body and dendrites are located in the lateral horns, and the axons go through the front horns to the nerve ganglia (sympathetic or parasympathetic), where they switch to the third neuron;
- III neuron - postganglionic, the body and dendrites of which lie in the nerve ganglion, and the axon stretches to the organ, which is innervated by this nerve.
Nerve plexus
The part of the histology of the autonomic nervous system, namely, reflex arcs, has already been described above. However, the main part of the ANS neurons is located not in the arcs, but in the nerve plexuses. The nerve plexuses are divided into intramural (located in the walls of the internal organs) and intramuscular, which are located between the fibers of the skeletal muscles. The nerve plexuses receive blood supply through their own network of small vessels - capillaries, but they only surround the plexus capsule without penetrating the nerve cell itself. The plexus capsule consists of several layers of connective tissue fibers delimited by flat cells.
Central nervous system: brain
Before analyzing the histology of the peripheral nervous system, it is worth studying the structure of the central nervous system. After all, the nerves and ganglia, which are part of the PNS, continuously receive impulses from the central nervous system.
Macroscopically, the brain is composed of:
- the trunk, including the medulla oblongata (medulla), the midbrain, the bridge (pons), which is also called the warolium bridge. Sometimes the cerebellum (cerebellum) and diencephalon are included in the trunk.
- the front, or large, brain, which consists of two hemispheres.
Now we will consider the microscopic structure, that is, the histology of the central nervous system. As in any parenchymal organ, there are two main parts in the brain: the parenchyma, characterized by the presence of gray and white matter, and the stroma, which includes the membrane.
Gray matter consists of the bodies of nerve cells - neurons. It forms the cerebral cortex of both the cerebral hemispheres and the cerebellum. But gray matter is present not only in the cortex, but also inside the brain, forming subcortical nuclei there.
White matter consists of processes of neurons - axons and dendrites. They form pathways that both connect different parts of the brain to each other and pass from the brain to the lower parts: the spinal cord and the peripheral nervous system.
But the brain is not entirely composed of nerve tissue; there are also cavities free of nerve cells - the ventricles. Two lateral are located in the cerebral hemispheres, the third is located in the diencephalon, and finally, the fourth is on the posterior wall of the medulla oblongata, between it and the cerebellum. These cavities are lined with cells - ependymogliocytes. All ventricles are interconnected, because in them there is a movement of cerebrospinal fluid - cerebrospinal fluid.
There are three shells of the brain that perform a protective function, namely, they prevent injury to the solid bone structures of the skull:
- solid, which is closest to the skull;
- arachnoid located in the middle;
- soft, tight to the substance of the brain.
As noted above, the nervous system is very vulnerable, and the brain is the most problematic part. That is why there is a blood-brain barrier - a formation that protects the higher nerve centers from the action of toxins and other harmful substances circulating in the blood.
The cerebral cortex is the center of higher nervous activity. It is thanks to the presence of the cortex that we can speak, read, write, even think! Carrying out such complex functions, the cortex has a rather complicated structure. It consists of six main layers:
- molecular;
- external granular;
- pyramidal;
- internal granular;
- ganglionic;
- polymorphic.
All these layers are not isolated from each other, but constantly interact with each other and the underlying nerve centers. They take part in conducting a nerve impulse, both within the cortex and outside it. In addition, each layer performs its specific function. So, the first layer is involved in the propagation of the impulse along the cortex itself, and the nerve cells that divert the impulse to other structures of the nervous system are located in the third and fifth layers of the cortex.
Central nervous system: spinal cord
This image shows the preparation of the nervous system for histology, namely, the spinal cord.
By analogy with the brain, the spinal parenchyma is also represented by gray matter, and the stroma is white. However, there are differences. Looking at a cross section of the brain, gray matter will be located on the periphery, and white - in the center. If we look at the preparation of the spinal cord, we will see a gray substance in the center resembling a butterfly in shape, and a white substance surrounding gray on the periphery. The bodies of nerve cells, grouping, gather in gray matter into the nuclei of the spinal cord.
When studying the histology of the nervous system, and specifically the structure of the spinal cord, it is necessary to remember that gray matter forms specific protrusions - horns. There are three of them:
- front ones that contain motor (motor) nuclei;
- lateral, with their own vegetative nuclei;
- back, including sensitive (sensory) nuclei.
An analogue of the ventricles in the brain is the spinal canal, which runs in the middle of the spinal cord. It is also lined with ependimogliocytes, and its main function is also the circulation of cerebrospinal fluid.
In the white matter, the cords of the spinal cord pass, which provide the connection of peripheral structures (organs, muscles) with the higher nerve centers.
Similarly to the blood-brain barrier that protects the brain, the dorsal has an external border glial membrane, which consists of processes of nerve cells - ependymogliocytes and astrocytes.
The stroma of the spinal cord, just like the brain, consists of the membranes: hard, arachnoid and soft.
Peripheral nervous system
What is the histology and histopathology of the peripheral nervous system? PNS consists of several parts:
- Nerves, or nerve trunks. The main function is the transformation and transmission of a nerve impulse from the periphery to the center and in the opposite direction. In addition, nerves are involved in the circulation of cerebrospinal fluid. Allocate 12 pairs of cranial nerves, and 31 - spinal nerves. The structure of these nerves is fundamentally no different from their general structure described in the corresponding section, as well as the histology of the tertiary nerve - one of the cranial nerves, due to which we can chew food, cry, and experience pain on the face. Thus, this nerve is mixed (sensitive and motor) and is divided into three branches located on the face: nervus ophtalmicus, nervus maxillaris and nervus mandibularis. In Latin, the tertiary nerve is called nervus trigeminus.
- Ganglia, or nerve nodes. They are a group of neuron bodies outside the central nervous system. The stroma of the nerve ganglia is formed by a capsule of connective tissue with vessels passing in it. The parenchyma incorporates nerve cells (neurons) and the surrounding oligodendrocytes. , ( ). , , , , , .
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