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  3. The structure of the nervous system

The structure of the nervous system

The nervous system controls the operation of all systems of the body, ensures the coordination of their activities in any conditions. Individual features of the nervous system makes each person unique personality. The nervous system performs its functions because the nerve cells – neurons – have special properties – excitability. The membrane of excitable cells, which are in addition to the nerve and muscle receptor cells, dormant inside is negatively charged relative to its outer surface, ie the membrane has a membrane potential or the potential of peace. In the case of external impact of the stimulus on the cell, the force reaches a certain threshold, the neuron is excited. Upon excitation of the cell membrane temporarily changes its potential, and the outer surface of the outer membrane becomes positively charged relative to the inner surface – a so-called action potentials.
Another feature of excitable cells – conductivity, t. E. The ability to transmit the action potential of their process to the other cells.

Nervous tissue, as stated above, is formed by neurons and glial cells. A neuron consists of a body, short processes – dendrites – and one long process – the axon, which can reach a length of 1 m. By the excitation of the axon from the cell body in the form of the action potential is transmitted to other nerve or muscle cells. Resting potential in nerve cells due to the fact that in the cytoplasm of neurons over potassium ion and protein anions (negatively charged ions) than the surrounding medium neuron. The outer membrane of a neuron has semipermeability properties: it can pass out of the neuron to the external environment cations (positively charged ions) of potassium, but does not transmit large anions (negatively charged ions). Therefore, part of potassium exits the neuron and the excess of negatively charged particles remaining on the inner side of the membrane of a neuron, charges it negatively. Released outside of the potassium cations interfere with their positive charges the entry of new potassium ions, and this equilibrium is established when the membrane of the neuron recorded resting potential of approximately -70 mV equal.

In the external environment surrounding the neuron contains many times more sodium cations and chloride, than in its cytoplasm. If a neuron action potential approaches another neuron or the experimenter artificially irritating the neuron membrane in its open special channels, skipping into the neuron positively charged sodium ions, but do not let the potassium ions from the neuron. Excess sodium doing inside the neuron, it recharges the membrane, and instead of -70 mV on it, you can register 40 mV. This brief depolarization of the membrane of the neuron is called an action potential, or nerve impulse. The action potential is able to move with great speed on the membrane of the axon of a neuron excited, reaching other nerve or muscle cells. The velocity of propagation of an action potential along an axon depends on its size: the larger the diameter, the higher the speed. The maximum speed of the action potential reaches 120 m / s.

The contact of the axon of a neuron body or dendrites of another cell (nerve or muscle) is called a synapse. Synapses can be electrical, and chemical. In an electrical synapse, action potential that came along the axon, it leads directly to the electrical response of the cell to another. In humans, the more common chemical synapses. They excitation transfer from one cell (presynaptic) to another (postsynaptic) occurs by means of a special chemical messenger – a neurotransmitter. Under the influence of the action potential of the end of the axon, which is located, for example, skeletal muscle, is released neurotransmitter – acetylcholine. It affects receptors of the muscle fiber membranes, it opens the ion channels, and there is agitation, with sufficient force that can lead to a reduction of the muscle fibers. Within the central nervous system excitation transfer from neuron to neuron also occurs with the help of mediator release from nerve endings. And some mediators of excitatory neurons, ie, force them to generate action potentials, and others – inhibit, ie inhibit neuronal excitation, the membrane which they stand out from the endings of other neurons. The most widely distributed in the central nervous system excitatory neurotransmitters: acetylcholine, norepinephrine, dopamine, serotonin, glutamate. From Brake most common mediators of gamma-aminobutyric acid (GABA) and glycine.

In higher animals nervous system consists of individual elements – of the nerve cells, or neurons, which are in contact with each other. Contacts between neighboring neurons called synapses. Neurons are varied in structure depending upon the location and function. Their similarity is that each neuron consists of the cell body, which houses the core and the typical organelles. The dimensions of the body of the neuron 4-130 microns. The number of synapses on a single neuron body 100 or more, and at its dendrites – many thousands. The body of the neuron has one or several long processes – neurite or axon, and many shorter branching processes – dendrites. The length of the axon in the adult may reach 1-1.5 m, and its thickness is less than 0.025 mm. Neurons are supported by connective tissue cells, glia, which surround the axon like a pouch. Some axons are located by glial cells, Schwann cells containing myelin and constitute a meat cut sheath. Each Schwann cell forms a segment of the shell and the meat is separated from the neighboring nodes of Ranvier Schwann cells. Also pulpy, non-fleshy and have axons. The body of the neuron’s metabolism provides its appendix, which are carried out due Biotok nerve impulses to synapses. The excitation of synapses transmitted to the subsequent neuron through a mediator. On the surface of the body and internal organs axons associated with receptors – special formations, perceiving irritation. The resulting stimulation of the nerve impulses are transmitted along axons centripetal or afferent neurons in the central nervous system and of the nervous system axons centrifugal or efferent, neurons nerve impulses arrive in the working bodies.

Higher and lower nervous activity

The activity of the nervous system that leads to normal relations of the animal organism with the outside world, or behavior, IL Pavlov suggested naming the higher nervous activity. The implementation of the first, the main activity of the nervous system to ensure the unity of the body and the conditions of his life (see. Article “The body as a whole”), associated with a second, lower nervous activity, which integrates and controls the operation of all internal organs. It is clear that without concerted action of all the organs of the body corresponding to the conditions of life, inconceivable the existence of the body.

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