M ORPHOFUNCTIONAL CHARACTERISTICS OF NERVOUS SYSTEM IN CHILDREN Prepared by: Toktassyn A. 302GM Checked by: Kasenkhan B. JSC Astana Medical University Department of hystology

B RAIN : - Pyramidal cells have no inherent shape, they lack pigment; - Underdevelopment of dendrites in nerve cells; - Centers not formed crust, the crust becomes cytoarchitectonical that belongs to adults, 1-2 years of age; - In infants hemisphere poorly developed; main grooves formed only having a small height and depth; the birth of the best-developed temporal lobe; - Brain tissue is very rich in water, easily develop swelling; - Gray matter poorly differentiated from white.

Spinal cord: - More mature than the head, with age only increases the number of nerve cells; - Relatively longer; - Completely fills the spinal canal to the 5th month of intra-uterine development. Peripheral nerves: - Little miyelinisated fibers (first miyelinisated afferent fibers, efferent then); - Intracranial nerves miyelinisated to 3 months of age; - Most of the peripheral nerves miyelinisated nunder 3 years, cord white matter - up to 4-7 years.

Analyzers: - Eye - to 2-3 months of age there is a physiological photophobia, nystagmus physiological, physiological hyperopia, greater latitude accommodation; 6 month old baby distinguish between colors; - Hearing - sound perception in newborns reduced because tympanic cavity filled with air; with a 2-month old baby differentiate sounds; from 7-8 months of life is to coordinate the auditory and visual analyzers; - Olfactory - in newborns lowered threshold, sees only the strong smells; starting from 4-month old child differentiates several odors; - Taste - in the neonatal receptor field wider and higher threshold; 3 months of life the child differentiates several taste; thin taste improved in the early school years.

Sensitivity: - Tactile - determined from the 7th month of fetal period, better developed on the face, soles, brushes; - The temperature - higher threshold; better child perceives cold; - Pain - well developed; formed to 6th day after birth, has the highest threshold; - Deep (vibration, musculo-articular sensitivity, sensation of pressure, weight) - formed to 2 years of life. The autonomic nervous system: - Both units are functioning at birth; - In neonates predominant norepinephrine; - With the age of the transition from generalized autonomic responses to local, specialized reactions; - From 3 to 7 months of age dominated by the parasympathetic nervous system.

T HE MAIN NERVOUS SYSTEM S FUNCTIONS ARE : -psychological (intelligence) activity; -emotions -locomotion activity (function); -neural regulation of vegetation functions. - the nervous system realizes coordination of psychological and metabolic processes in different tissues, organs and systems and realizes connection between the organism and environment.

T HE B RAIN D URING D EVELOPMENT The nervous system develops from embryonic tissue called the ectoderm. The first sign of the nervous system developing is the neural plate that can be seen at about the 16th day of development. Over the next few days, a "trench" is formed in the neural plate - this creates a neural groove. By the 21st day of development, a neural tube is formed when the edges of the neural groove meet. The rostral (front) part of the neural tubes goes on to develop into the brain and the rest of the neural tube develops into the spinal cord. Neural crest cells become the peripheral nervous system.

T HE NERVOUS SYSTEM IS ORIGINATED Forepart of medullar tube growths and gives the origin of brain bubbles during 5-6 weeks of intrauterine period.

T HE NERVOUS SYSTEM IS ORIGINATED Five parts of brain – 2 hemispheria, connected by telencephalon, diencephalon, mesencephalon, metencephalon, myencephalon, are formed from back part of madullar tube.

T HE B RAIN D URING D EVELOPMENT Almost all the convolutions and fissures (gyre and sulcus) the brain is to possess in later life are present at birth, but they are extremely underdeveloped. The layers of the cerebral cortex are already almost completely formed at term, but differentiation of all the nerve cells occurs predominantly in the postnatal period. At the same time the spinal cord, the optic thalamus, and the corpus striatum are of a more mature structure. The cerebellum is elongated and its sulci are weakly pronounced. It is only towards the end of the first year of life that macroscopic structure of babys brain approaches that of an adult. Differentiation of the brain matter (nerve tissue, ganglionic cells and nerve fibers) is slower.

D EVELOPMENT OF THE SPINAL CORD

C RITICAL PERIOD The most intensive fission of neural cells occurs between 6 and 18 weeks of intrauterine period. We call this period the critical period of CNS development due to possibility of the influence of various teratogenic factors to fetus nervous system, formation the defects of development of the nervous system is possible.

D EVELOPMENT OF THE BRAIN

D EVELOPMENT Since the cortical layers of the brain of the newborn infant are little differentiated the formation of the cortical centers is also immature. An intensive development of the cerebral cortex takes place during the first three month after delivery. A most characteristic point is the absence of dendrites (arboraceous branches) in the nerve cells (neurons). Principal differentiation of the nerve cells is completed by the age of 3 years, although the final termination of the process occurs only at 8 years. The conduction routes (with the exception of the pyramidal pathways) are sufficiently developed at birth; the pyramidal routes myelinize by the time the baby is 5 or 6 month old. According the existing date the first neuron pathways to become myelinized in the cerebral hemispheres are the efferent routes, while pyramidal myelinization (the efferent pathway) occurs later.

DEVELOPMENT OF MYELIN SHEATHS A significant aspect of brain development is the continued growth of myelin sheaths around the axons of the cerebral cortex. Myelin is a fatty substance which is deposited around many (though not all) axons as an insulating sheath. Its presence allows conduction of nerve impulses to occur from ten to one hundred times as rapidly as would occur along a non-myelinated axon. Since this obviously increases the efficiency of the axon system (just as increased computing speed enhances the efficiency of a computer), the development of axonal sheaths are taken as a measure of increasing maturity of the neural system involved. Myelin sheath development, or myelinization as it is called, has a rather well recognized time table in the cerebral hemispheres. Fibers serving the primary sensory (touch, vision, audition etc.) and motor areas are myelinated shortly after birth while those which are involved with more complex associative and cognitive functions myelinate later. It is generally believed that fiber systems of the prefrontal lobes (executive functions, intentions, future planning, etc.) are among the latest to myelinate, a process that may go on into young adulthood.

T HE SPINAL CORD is almost prepared to function at birth. Its weight at that time is 2 to 6g and subsequent gains are less intensive than those of the brain. The growth of the spinal cord runs parallel to the development of motor function – its initial weight triples by the age of 5 years; however, in distinction from brain the structure of the spinal cord approaches adult structure already in the second year of life. The only subsequent increase is in number of motor cells in the ventral (anterior) horns. By puberty the spinal cord increases four to fivefold. The intracranial nerves myelinize by the three month after birth, the peripheral nerves by three years, but growth of the myelin sheath and even of the axis cylinder continues. The autonomic nerves system functions from the moment of birth. So, the morphological features most characteristic of the nervous system in early childhood are immaturity of cerebral cortex, meager differentiation of the nerve cells, and insufficient myelinization of the nerve fibers.

D EVELOPMENT OF THE SENSE ORGANS Hearing – the receptive abilities of the auditory analyzers, develops in the child from the time he is two weeks old. Vision – as a reflex to object is defined from the third month of life; transient strabismus and absence of coordinated movement of the eyeballs are physiological phenomena in the newborn. The olfactory sense (smell) develops gradually in the form of reflex to odors after the age of six months, but the perception of taste appears much earlier.