The nervous system controls, coordinates and regulates the coordinated work of all organ systems, maintaining the constancy of the composition of its internal environment (thanks to this, the human body functions as a whole). With the participation of the nervous system, the body is connected with the external environment.
The nervous system is made up of nerve tissue, which consists of nerve cells – neurons – and small satellite cells (glial cells), which are about 10 times more numerous than neurons.
Neurons provide the basic functions of the nervous system: transmission, processing and storage of information. Nerve impulses are electrical in nature and propagate along the processes of neurons.
Satellite cells perform nutritional, support and protective functions, promoting the growth and development of nerve cells.
The neuron is the basic structural and functional unit of the nervous system.
The structural and functional unit of the nervous system is a nerve cell – a neuron. Its main properties are excitability and conductivity.
A neuron consists of a body and processes.
Short, highly branching processes – dendrites, along which nerve impulses come to the body of the nerve cell. There can be one or several dendrites.
Each nerve cell has one long process – an axon, along which impulses are directed from the cell body. The length of the axon can reach several tens of centimeters. Uniting in bundles, axons form nerves.
The long processes of the nerve cell (axons) are covered with a myelin sheath. Clusters of such processes, covered with myelin (a white fatty substance), in the central nervous system form the white matter of the brain and spinal cord.
The short processes (dendrites) and bodies of neurons do not have a myelin sheath, so they are gray in color. Their clusters form the gray matter of the brain.
Neurons connect to each other in this way: the axon of one neuron connects to the body, dendrites, or axon of another neuron. The place where one neuron contacts another is called a synapse. There are 1200-1800 synapses on the body of one neuron.
A synapse is a space between neighboring cells, in which a chemical transmission of a nerve impulse from one neuron to another is carried out.
Each synapse consists of three sections:
- membrane formed by a nerve ending (presynaptic membrane);
- cell body membranes (postsynaptic membrane);
- synaptic cleft between these membranes
The presynaptic part of the synapse contains a biologically active substance (mediator), which ensures the transmission of a nerve impulse from one neuron to another. Under the influence of a nerve impulse, the mediator enters the synaptic cleft, acts on the postsynaptic membrane and causes excitation of the next neuron in the cell body. So through the synapse, excitation is transmitted from one neuron to another.
The spread of excitement is associated with such a property of the nervous tissue as conduction.
Types of neurons
Neurons vary in shape. Depending on the function performed, the following types of neurons are distinguished:
- neurons that transmit signals from the senses to the central nervous system (spinal cord and brain) are called sensitive. The bodies of such neurons are located outside the central nervous system, in the nerve nodes (ganglia). A nerve node is a collection of nerve cell bodies outside of the central nervous system.
- Neurons that transmit impulses from the spinal cord and brain to muscles and internal organs are called motor neurons. They provide the transmission of impulses from the central nervous system to the working organs.
- Communication between sensory and motor neurons is carried out by intercalary neurons through synaptic contacts in the spinal cord and brain. Intercalary neurons lie within the central nervous system (i.e., the bodies and processes of these neurons do not go outside the brain).
The accumulation of neurons in the central nervous system is called the nucleus (nucleus of the brain, spinal cord).
The spinal cord and brain are connected to all organs by nerves.
Nerves are sheathed structures composed of bundles of nerve fibers formed primarily by the axons of neurons and neuroglia cells.
Nerves provide a connection between the central nervous system and organs, blood vessels and the skin.
There are nerves:
- sensitive, providing impulses from receptors in the central nervous system;
- motor, formed by the axons of motor neurons and conducting nerve impulses from the central nervous system to the executive organs;
- mixed, formed by sensory and motor fibers and capable of conducting impulses both to the central nervous system and from the central nervous system.
Nerve plexuses are represented by reticular accumulations of nerve fibers of various nerves that connect the central nervous system with internal organs, skeletal muscles and skin.
The most famous solar plexus is found in the abdomen.