HOW DOES HUMAN NERVOUS SYSTEM WORK?
The human nervous system consists of central (CNS) and peripheral (PNS) systems. The CNS consists of the brain and spinal cord, and the PNS consists of nerves "supervising" the work of each cell in the body. The main task of the nervous system is to combine the work of all body organs, tissues, and cells into a harmonious "mechanism," as well as to provide a rapid and adequate reaction to changes in the internal and external environment. The nervous system works in close tandem with the endocrine (hormonal) system, forming a neuroendocrine system that regulates the entire body with nerve impulses and special chemical matters hormones.
BRAIN
The brain is the supreme commander of the nervous system, controlling the entire body. In addition, its unique structure of it is the basis for the higher mental functions that underlie consciousness. The brain weighs only 1.21.4 kg, representing an average of about 2 % of the human body weight. The male brain generally is 1012 % heavier and 10 % bulkier than the female brain[48].
The brain consists of several divisions.
The largest part of the human brain is the forebrain. The cerebral cortex is a part of it. Both hemispheres (right and left) are divided into four lobes: frontal, occipital, temporal, and parietal. The cerebral cortex is responsible for the perception of all information coming from the external and internal environment: visual, auditory, olfactory, gustatory, and somesthetic cortices are located here. The cortex is also responsible for the human higher nervous activity, including thinking and speech.
The midbrain contains the visual and auditory centers, which are responsible for processing impulses from the corresponding analyzers. In addition, the midbrain has a tremendous impact on the cerebral cortex. If the cortex is the "consciousness realm," then the midbrain is the "kingdom of the subconscious." The processes occurring in the midbrain can stimulate or inhibit the processes occurring in the cortex. There is also dopamine, a neurotransmitter that plays a key role in the formation of motivation, healthy habits, and addiction, is synthesized.
The diencephalon mediates the transfer of stimuli to the hemispheres and helps adapt adequately to environmental modifications. Regulates the work of metabolic processes and endocrine glands. Manages the cardiovascular and digestive systems. Regulates sleep and wake, water, and food intake.
The cerebellum is a brain region that is primarily responsible for maintaining balance and load distribution between muscles, unconscious body skills, and bodily memory.
The medulla oblongata is an extension of the spinal cord. Nerve pathways that carry information from the whole body and back pass through it, as well as breathing and blood flow control centers. Damage to the medulla oblongata leads to quick death. It is connected to the overlying parts of the brain by the pons, part of the brain stem.
NERVE CELLS
The main structural unit of the central nervous system is the nerve cell or neuron. Neurons consist of a body and several projections. Nerve cell bodies form the gray matter of the brain and spinal cord, and the long myelin-covered projections form the white matter. Gray matter forms the cerebral cortex and the underlying nuclei, and the white matter forms the nerve pathways a kind of "wires" through which different parts of the brain and other structures "communicate."
At last count, the number of neurons in the cerebral cortex is 1416 billion, while in the cerebellum is 5570 billion[49].
The neuron body contains many fibers that form the cytoskeleton: it helps the nerve cell to keep its shape. It also forms some sort of "tracks" where vesicles with neurotransmitters are delivered to the ends of the projections. There are two types of projections short (dendrites) and long (axons). Most often, neurons have many dendrites and only one axon.
Axons can transmit nerve impulses over long distances to other brain structures, to the spinal cord and to target organs. As a rule, several neurons located above or below the "author" of the impulse are involved in impulse delivery from the brain to the "distant regions" and back.
Axon terminals approach the body of the next chain member, releasing a neurotransmitter into the gap between the terminal and the body (or projection) of the other neuron. These "meeting places" are called synapses[50]: this is where the electrical impulse is converted into a chemical. The next neuron "receives" the chemical signal and converts it again into an electrical one, sending an impulse to its destination.
Sometimes there are several such "stop-overs" on the way of "agents" to the destination this system makes it possible to maintain a high intensity of the impulse, not allowing it to fade away. Axons are covered with myelin sheath, which is like an electrical insulator. Myelin consists of glial cells, as will be discussed later. Short projections of neurons dendrites help to set "local communication" between neurons. They are myelin-free. Each neuron is connected to thousands and tens of thousands of other neurons and target cells with short and long projections.
NEUROGLIA: THE BRAIN'S LIFE SUPPORT AND DEFENSE SYSTEM
Besides neurons, the "base" cells of the brain, the nervous system includes auxiliary structures glial cells. There are several types of glial cells: for example, astrocytes, oligodendrocytes, microglia, and microglia. For many years the number of glial cells was believed to be exceeding the number of neurons by 810 times, but nowadays, it is proved that the ratio of neurons and glial cells is approximately the same[51].
Glial cells are involved in the formation of the blood-brain barrier, a filter protecting the brain from microbes, some cells, and toxins. Glial cells also form the microenvironment around neurons, transport nutrients to nerve cells, excrete waste, form myelin sheaths, etc.
FUN FACT
HOW THE "INTESTINAL BRAIN" AFFECTS THE ENTIRE BODY
The human intestine contains a unique cluster of nerve cells the enteric nervous system (ENS). Sometimes it is also called the intestinal or "abdominal" brain. The ENS uniqueness, its difference from the nerve clusters in other organs, is associated with several characteristics. Firstly, it includes about half a billion neurons! Secondly, the ENS is very similar in structure to the brain. Several types of neurons can receive and send signals and provide the motor function of muscles. The enteric nervous system has its glial cells, which, as in the brain, nourish neurons and activate immune mechanisms. Thirdly, a huge number of neurotransmitters are synthesized in the intestine. The spectrum of the internal neurotransmitters in the ENS is as wide as in the central nervous system. Over 90 % of serotonin and 50 % of dopamine are synthesized in the body being produced in the intestine. Almost all ENS neurotransmitters are of bacterial origin.
All these factors underlie the fourth key feature of the ENS: its autonomy. Unlike other parts of the nervous system, the intestinal brain can function without control by the central and peripheral nervous system, even with extensive brain injury. At the same time, the brain, and the intestine nervous system are closely linked, forming the "gut-brain" axis[52]. Therefore, the ENS not only regulates the digestive tract but also affects the entire body.
Studies show that some diseases affecting the brain, such as Parkinson's disease, are associated with certain changes in the intestinal microbiota.