The anatomical features of nervous system in children - Реферат

Picture 1.(b) second step hypothalamus). Source: Picture 1.(c) neural tube organization-final The neural tube nears by 3-4 weeks of gestation (Picture 2) /brain-development-embryos.html and is the basis for all further nervous system development. Birth defects such as spina bifida and meningomyelocele arise from abnormalities in neural tube formation (Victor et al., 2001). From 4 to 12 weeks the neural tube differentiates into what will become various components of the nervous system. The forebrain and facial structures develop at one end, and the spinal cord at the other. Picture 2. Sequence of events in brain maturation. The hollow center of the tube in the region that will become brain will eventually form the ventricles. Regions called proliferative zones form near the ventricles and give rise to young neurons. From 12 weeks to 20 weeks these neurons multiply and migrate from their origins to destinations in the cortex, moving along a scaffolding of glial cells (Rakic, 1990). After this migration, a period of rapid cell death occurs, reducing the neural number by half from 24 weeks of gestation to 4 weeks after birth. The cell bodies of the neurons are primarily found in the gray matter of the brain. Their myelinated axons form white matter. Myelination occurs regionally beginning with the brain stem at 29 weeks (Inder and Huppi, 2000) and generally proceeds from inferior to superior and posterior to anterior. Proximal pathways tend to myelinate before distal, sensory before motor, and projection before association (Volpe, 2000). Although most major tracts are significantly myelinated by early childhood, axons within the cortex and in some regions such as the arcuate fasciculus, a white matter bundle near the temporal lobe, continue to myelinate into the second and third decades of life (Yakovlev and Lecours, 1967). A third major developmental process is the proliferation and organization of synapses, which begins slightly later, around the 20th week of gestation. Synaptic density increases rapidly after birth, reaching by 2-years of age a level approximately 50% greater than that typically seen in adults (Huttenlocher, 1979). This is followed by a regionally specific loss of synaptic connections. For example, maximum synaptic density occurs in the visual cortex at 4 months postnatally, but it does not typically peak in the prefrontal cortex until 4 years of age. Beginning at approximately 15 weeks the surface of the growing brain begins to fold into sulci and gyri (Levine and Barnes, 1999) (picture 3) The major sulci, except for the occipital lobe, are in place by 28 weeks of gestation, after which secondary and tertiary sulci are elaborated, with nearly all gyri present by birth. The sulcal and gyral patterns continue to increase in complexity after birth, likely related to changes in cellpacking density and maturation of subcortical tracts. Picture 3. Embryo. The dynamic interplay between progressive and regressive events results in relatively rapid brain growth in the first 2 years of life, by which time it has achieved 80% of its adult weight. By age 5 years brain size is approximately 90% of adult size (Dekaban and Sadowsky, 1978). Picture 4.Graph. The formation of a tube from the flat layer of ectodermal cells known as the neural plate. This will give rise to the central nervous system. Another classification concerns trimester of pregnancy stages: In the first trimester of pregnancy is such stages of development of the nervous system of the fetus (picture 5): Picture 5: The main events in the development of the CNS and motor development during the first 8 weeks of pregnancy 1) The dorsal induction or primary neurulation (Picture 6) - the period of 3-4 weeks of gestation; 2) Ventral induction - a period of 5-6 weeks of gestation; 3) Partial neuronal proliferation - a period of 2-4 months of gestation; Picture 6. primary neurulation Dorsal induction or primary neurulation - in connection with the development of individual characteristics may vary over time, but always adheres to 3-4 weeks (18-27 days after conception) of gestation.