The dominant position of the nervous system within the body is established early during development. It is the first tissue to differentiate, it maintains the fastest rate of growth of any embryonic tissue, and it is the largest organ during the gestational period of development. Embryonic neurons and their precursors synthesize and release trophic factors that affect the viability of surrounding cells, as well as tropic factors, which influence the direction of neural growth, thus influencing subsequent maturation of neuronal pathways as well as their targets, including muscles and glands.
The steps of neuronal development include (1) formation of the neural plate, (2) folding of the plate to form the neural tube, and (3) bulging and bending of the tube to form a curved configuration of five brain vesicles attached to a straight tubular spinal cord (Figs. 1 and 2). The neural plate, from which the nervous system develops, is formed by a thickening of the ectoderm during the third week of gestation in humans. Through a process called neural induction, regions of the neural plate become genetically programmed to form particular regions of the nervous system. Growth of the neural plate is accelerated along its lateral edges, causing the edges to curve toward each other, eventually fuse, and form an open neural tube with both cranial and caudal openings called neuropores. Closure of both neuropores is normally complete by the fourth week.
Preferential growth at three nodes along the cranial portion of the sealed neural tube causes intermittent bulging of tissue, formation of three brain vesicles (prosencephalon, mesencephalon, and rhombencephalon), and bending of the tube at flexion points generally located between the vesicles. Subsequent growth and division of the three vesicles leads to the formation of five major brain regions: (1) telencephalon, (2) diencephalon (regions 1 and 2 are collectively called the forebrain), (3) mesencephalon (midbrain), (4) the metencephalon (pons and the overlying cerebellum), and (5) myelencephalon (medulla). The spinal cord retains its original configuration as a relatively uniform tubular structure with the generation of slight enlargements at cervical and lumbar levels.
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