2 CHAPTER II THE PHYSIOLOGY AND PATHOLOGY OF  MOTOR FUNCTION 2.1 Main Principles of Structure and Functioning of Nervous System. Neuron – the Functional Unit of Nervous System. Reflexes.

2 CHAPTER II THE PHYSIOLOGY AND PATHOLOGY OF  MOTOR FUNCTION 2.1 Main Principles of Structure and Functioning of Nervous System. Neuron – the Functional Unit of Nervous System. Reflexes.

The elements of the (NS) of the person develop from embryonal ectoderm (neurons and neuralgia) and mesoderm (meninges, vessels, mesoglia). By the end of the 3rd week of the development of human embryo the neural plate, which is located longitudinal on the back side of embryo, is formed from ectoderm. On the neural plate a neural groove appears which turns to the neural tube then. In the generated neural tube three layers are distinguished: the inner ependymal layer; the middle layer of the mantle and the outer layer. The cells of the inner layer turn into glia cells, the cells of the mantle layer give neuroblasts, which turn to mature nervous cells and spongioblasts, giving rise to various kinds of neuroglia (astrocyte, oligodendrocyte).

The NS develops from the (initially) longitudinally oriented neural tube, which consists of a solid wall and a central fluid-filled cavity. The cranial portion of the neural tube grows more extensively than the rest to form three distinct brain vesicles, the rhombencephalon (hindbrain), the mesencephalon (midbrain), and the prosencephalon (forebrain).

The NS is composed of cells, called neurons that are specialized for information processing and transmission. Neurons make contact with each other at junctions called synapses, at which information is transferred from one neuron to the next by means of chemical messenger substances called neurotransmitters. In general, neurons can be divided into two classes: excitatory and inhibitory. The structure of neurons is various. There are numerous classifications of nervous cells based on the form of their body, extent and the form of dendrite and other features. On functional value these cells are subdivided into motor, sensory and interneuron.

A nervous cell carries out two basic functions: 1) specific – processing the information coming to the neuron and transferring of a nerve impulse; 2) biosynthetic, directed on the maintenance of the ability to live. It is reflected in the ultra structure of the nervous cell.

The structure of a nervous cell includes: mitochondrion, determining its energy metabolism; nucleus, nucleolus, granular and not granular endoplasmic reticulum, Golgi complex, polysomes and ribosomes, basically providing synthesis of protein; lysosomes and phagosomes (the basic organelles of «endocellular digestive system»); axons, dendrites, synapses, providing morphofunctional connection of separate cells. The polymorphism of the cell structure is due to the various role of separate neurons in system activity of the brain as a whole.

The axons are exposed to myelinization and in such a way myelinated nerve fibers are formed. The fascicles of nerve fibers which can include separate non-myelinated fibers form the white substance of the brain, the cranial and peripheral nerves. The dendrites and the processes of glia cells form complex and unique pictures of the neuropile. However it is the allocation of the axons and the dendrites, their interposition, afferent-efferent interrelation and the regularity of architectonics of the synapses that are determining in mechanisms of the integral function of the brain.

The synapses are divided into axosomatic, axoaxonic and axodendritic synapses; dendrodendritic synapses are observed less often. In the synapses they distinguish presynaptic part, which contains the vesicles and postsynaptic part. The active zone of synaptic contact where the release of mediator and nerve impulse transmission take place is characterized by the increase of the electronic density of presynaptic and postsynaptic membranes. Different systems of interneuronic connections use various mediators. It is the essential moment in the synaptic transmission. Now about 30 chemically active substances (acetylcholine, dopamine, noradrenaline, serotonin, GABA (gamma aminobutyric acid) and etc.), which take part in synaptic transmission from one cell to another are known. Recently numerous neuropeptides such as enkephalins, endorphines and P-substance are actively studied as mediator in synaptic transmission.

The reflex principle of the work of the NS underlies the functioning of the motor system of the person. The complex of nervous mechanisms takes part in the realization of motor reflex activity: peripheral motor neuron (the cells of anterior horns of the spinal cord and nuclei of the cranial nerves), central motor neuron - or the pyramidal tract, and also numerous structures of the brainstem and the extrapyramidal system, which provide the automatism of stereotyped movements, smooth regulation of muscular tone, the realization of consensual movements. All these movements give individual character to the motor activity of a person. The cerebellum with its system of afferent and efferent tracts plays an important role in the development of automatic coordination of movements, the maintenance of equilibrium of a body.

Thus motor activity is the result of integral activity of the NS. They distinguish voluntary movement which is realized with the help of the cerebral cortex, and involuntary, which is constructed as simple and complex reflex acts. Voluntary movements are controlled by the pyramidal tract, involuntary – the extrapyramidal system, the reticular formation and the segmental apparatus of the spinal cord.

The NS is divided into central and peripheral. Roots, plexuses and nerves belong to peripheral NS. The central nervous system consists of the brain and the spinal cord.