Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Nervous System (General Structural and Developmental Plan of the Nervous System)

The Nervous system coordinates and regulates The activity of all Organs and systems, ensuring the functioning of the body as a single entity. It provides the most effective adaptation of the Organism to environmental changes and maintains the constancy of its internal environment (Homeostasis). In humans, the nervous system serves as the material basis for mental activity (thinking, speech, and complex forms of social behavior). These highly complex and vital tasks are accomplished through Nerve Cells (Neurons), which receive, process, store, and transmit information.

All Organs of the nervous system are formed from Nervous Tissue, which exhibits The properties of excitability, the generation of nerve impulses, and the conduction of these impulses toward the Brain or to effector organs in the periphery. Topographically, the human nervous system is divided into the central and peripheral nervous systems.

The Central Nervous System includes the Spinal Cord and the brain.

The Peripheral Nervous System consists of the spinal and Cranial Nerves along with their roots, nerve branches, nerve endings, plexuses, and ganglia located throughout all parts of The Human Body.

According to the anatomical and functional Classification, the unified nervous system is conventionally divided into two parts: somatic and autonomic. The somatic nervous system innervates primarily the body surface—the Skin and skeletal Muscles. This (somatic) division of the nervous system establishes relationships with the external environment by perceiving its stimuli (Touch, sensation, pain, Temperature) and generating conscious (voluntary) contractions of skeletal muscles (protective and other movements).

The Autonomic nervous system regulates metabolic processes in all organs and tissues, as well as GROWTH AND REPRODUCTION. It innervates all Internal Organs (digestive, respiratory, and urogenital systems), glands (including Endocrine glands), Cytology/cytology/32.html">Smooth Muscle tissue of organs (including Blood Vessels), and The Heart. The autonomic nervous system also provides trophic innervation to skeletal muscles, skin, other organs and tissues, and the nervous system itself.

Despite its conventional nature, this division of the nervous system has developed traditionally and proves quite convenient for studying the nervous system as a whole and its individual parts.

Development of the nervous system. In vertebrates, the nervous system develops from the outer germ layer, the ectoderm. In the posterior, dorsal region of the embryo, ectodermal cells divide rapidly to form a thickened area known as the neural plate. As Cell mass increases, this strip deepens and transforms into the neural groove. Subsequently, the edges of the neural groove fuse to form the neural tube, while on either side of the neural tube, a paired ganglionic plate forms, giving rise to the sensory ganglia of the Cranial and Spinal Nerves.

In humans, by the end of the 3rd week of embryonic development, the anterior end of the neural tube expands to form three primary brain vesicles. At this stage, the appearance (primordium) of the brain can already be observed: these are the Forebrain, Midbrain, and rhombencephalon (Hindbrain).

By the end of the 4th week of intrauterine life, As a result of further Differentiation of the forebrain and hindbrain, five secondary brain vesicles emerge: the Telencephalon (cerebrum), Diencephalon, mesencephalon (midbrain), metencephalon, and myelencephalon (Medulla Oblongata).

As a result of further growth and differentiation, the fifth brain vesicle develops into the medulla oblongata, which transitions directly into the spinal cord. The ventral part of the fourth vesicle forms the Pons, while its dorsal part forms the Cerebellum. The anterior portion of the hindbrain is distinguished as the rhombencephalic isthmus. The third brain vesicle lags in growth; its ventral part develops into the cerebral peduncles, and its dorsal part forms the roof of the midbrain (corpora quadrigemina). The second vesicle gives rise to paired outgrowths (optic vesicles) and the nuclei of the diencephalon. The first paired brain vesicle gives rise to the cerebral hemispheres. The Cerebral Cortex develops on their surface, beneath which lies the White matter. Accumulations of Gray matter form basal nuclei (or subcortical ganglia) within the white matter.

The thin anterior wall of the first vesicle gives rise to the corpus callosum and the anterior (white) commissure of the brain. Due to the rapid growth of the cerebral hemispheres, sulci appear on their surface, dividing the hemisphere surface into lobes and subsequently into gyri. All major sulci and gyri are formed in the fetus by the time of birth. Postnatal Changes in the human nervous system are associated with myelination and the growth of its individual structures. The portion of the neural tube posterior to the brain vesicles develops into the spinal cord. Cells from the ganglionic plate form not only the sensory ganglia of the cranial and spinal nerves, but also the peripheral ganglia of the autonomic nervous system and the chromaffin (secretory) cells of The adrenal medulla.

Initially, the walls of the neural tube throughout its entire length consist of a single layer of germinative (embryonic) neuroepithelial cells. Cells of this layer divide intensively, causing the wall of the neural tube to thicken. The inner cellular layer of the neural tube gives rise to the ependyma (ependymal layer), which lines the cavities of the brain vesicles. The outer cellular layer of the tube wall is termed the mantle layer. The uneven accumulation of cells in the mantle layer plays a significant role in The formation of the neural tube. As a result, the dorsal and ventral walls of the neural tube lag considerably in growth compared to its lateral walls. The rapidly developing lateral walls are divided by a clearly defined longitudinal sulcus limitans—visible during ontogeny—into a ventrally located basal plate and a dorsally located alar plate. The basal plate subsequently develops into the ventral horns of the spinal cord and the motor nuclei of the cranial nerves, whereas the alar plate gives rise to the dorsal horns of the spinal cord and the sensory nuclei of the cranial nerves. In the region of the sulcus limitans itself—that is, the area situated between the basal and alar plates—the Reticular Formation and the nuclei of the autonomic nervous system develop. The basal and alar plates extend unequally in the rostral (cephalic) direction. The sulcus limitans reaches only to the level of the future forebrain, terminating the basal plate there. The forebrain represents a derivative of the alar plate, which is of great importance for understanding the true nature of its Functions.