Neuron

Neuron is a morphologically and functionally independent unit. With the help of processes (axon and dendrites) it makes contacts with other neurons, forming reflex arcs - links from which the nervous system is built. 

Depending on the functions in the reflex arc, afferent (sensitive), associative and efferent (effector) neurons are distinguished. Afferent neurons perceive impulses, efferent transmit them to the tissues of working organs, inducing them to action, and associative neurons provide inter-neural connections. The reflex arc is a chain of neurons connected to each other by synapses and providing a nerve impulse from the receptor of the sensory neuron to the efferent termination in the working organ.

Neurons are distinguished by a wide variety of shapes and sizes. The diameter of the bodies of the granular cells of the cerebellar cortex is about 10 μm, and the giant pyramidal neurons of the motor cortex of the cerebral cortex are 130-150 μm.

The main difference of nerve cells from other cells of the body is the presence of a long axon and several shorter dendrites. The terms "dendrite" and "axon" are applied to processes on which incoming fibers form contacts that receive information about excitation or inhibition. The long process of the cell, through which the impulse is transmitted from the body of the cell and forming contact with the target cell, is called the axon.

Axon and his collaterals branch into several branches, called telodendrons, the latter terminating in terminal thickenings. Axon contains mitochondria, neurotubules and neurofilaments, as well as agranular endoplasmic reticulum.

The three-dimensional region in which the dendrites of a single neuron branch is called the dendritic field. Dendrites are the true protrusions of the cell body. They contain the same organelles as the body of the cell: chromaphilic substance (granular endoplasmic reticulum and polysomes), mitochondria, a large number of microtubules (neurotubules) and neurofilaments. Due to dendrites, the receptor surface of a neuron increases by 1000 or more times. Thus, the dendrites of pear-shaped neurons (Purkinje cells) of the cerebellar cortex increase the area of the receptor surface from 250 to 27 LLC μm2; On the surface of these cells, up to 200,000 synaptic endings are found.

 

Types of nerve cells: a - unipolar neuron; b - pseudo-unipolar neuron; c - bipolar neuron; r - multipolar neuron

[1], [2]

Structure of the neuron

Not all neurons correspond to the simple cell structure shown in the figure. Some neurons lack axons. There are cells whose dendrites can carry out impulses and form bonds with target cells. The ganglion cell of the retina corresponds to the pattern of a standard neuron with dendrites, a body and an axon, and in photoreceptor cells there are no obvious dendrites and an axon, since they are activated not by other neurons, but by external stimuli (quanta of light).

The body of the neuron contains the nucleus and other intracellular organelles common to all cells. The vast majority of human neurons have one nucleus, located more often in the center, less often - eccentric. Dual-core and, moreover, multi-core neurons are extremely rare. An exception is the neurons of some ganglia of the autonomic nervous system. The nuclei of neurons are round in shape. In accordance with the high metabolic activity of neurons, chromatin in their nuclei is dispersed. In the nucleus there is one, sometimes two or three large nucleoli. Strengthening the functional activity of neurons is usually accompanied by an increase in the volume (and number) of nucleoli.

Plasmalemma (plasma membrane) of the neuron has the ability to generate and conduct a pulse, its structural components are proteins that function as selective ion channels, as well as receptor proteins that provide neuronal responses to specific stimuli. In the resting neuron, the transmembrane potential is 60-80 mV.

When staining the nervous tissue with aniline dyes in the cytoplasm of neurons, a chromophilic substance is detected, which is found in the form of basophilic grains of various sizes and forms. Basophilic grains are localized in the pericarion and dendrites of neurons, but they are never found in axons and their conical bases - axonal hillocks. Their color is explained by the high content of ribonucleotides. Electron microscopy showed that the chromophilic substance includes cisterns of the eudoplasmic reticulum, free ribosomes and polysomes. The granular eudoplasmic reticulum synthesizes neurosecretory and lysosomal proteins, as well as integral proteins of the plasma membrane. Free ribosomes and polysomes synthesize proteins of the cytosol (hyaloplasm) and nonintegral membrane proteins.

To maintain integrity and perform specific functions, neurons require a variety of proteins. For axons that do not have organelles that synthesize a protein, a constant cytoplasm current from the pericarion to the terminals is characteristic with a speed of 1-3 mm per day. The Golgi apparatus in neurons is well developed. When light microscopy, it is revealed in the form of different in shape granules, crimped filaments, rings. Its ultrastructure is common. Vesicles, budding from the Golgi apparatus, transport proteins synthesized in the granular endoplasmic reticulum, either to the plasma membrane (integral membrane proteins), or to terminals (neuropeptides, neurosecret), or to lysosomes (lysosomal hydrolases).

Mitochondria provide energy with a variety of cellular functions, including processes such as ion transport and protein synthesis. Neurons need a constant inflow of glucose and oxygen with blood, and the cessation of blood supply to the brain is detrimental to nerve cells.

Lysosomes participate in the enzymatic cleavage of a variety of cell components, including receptor proteins.

From the elements of the cytoskeleton in the cytoplasm of neurons there are neurofilaments (diameter 12 nm) and a neurotube (diameter 24-27 nm). Bunches of neurofilaments (neurofibrils) form a network in the body of the neuron, in their processes they are located in parallel. Neurotubules and neurofilaments are involved in maintaining the shape of neuronal cells, in the growth of processes and in the implementation of axonal transport.

The ability to synthesize and secrete biologically active substances, in particular mediators (acetylcholine, norepinephrine, serotonin, etc.), is common to all neurons. There are neurons that specialize primarily in performing this function, for example, cells of the neurosecretory nuclei of the hypothalamic region of the brain.

Secretory neurons have a number of specific morphological features. They are large; The chromophilic substance is located mainly on the periphery of the body of such neurons. In the cytoplasm of the nerve cells themselves and in the axons, there are various sizes of neurocellular granules containing proteins, and in some cases lipids and polysaccharides. The granules of the neurosecretion are excreted into the blood or into the cerebrospinal fluid. Many secretory neurons have irregularly shaped nuclei, which indicates their high functional activity. Secretory granules contain neuroregulators, which ensure the interaction of the nervous and humoral systems of the body.

Neurons are highly specialized cells that exist and function in a strictly defined environment. Such a medium is provided by the neuroglia, which performs the following functions: supporting, trophic, demarcating, protective, secretory, and also maintains the constancy of the environment around the neurons. There are glial cells of the central and peripheral nervous system.