Development Of The Nervous System: Stages And Factors

The first stage of development of the embryonic nervous system occurs in the first weeks of gestation.

1. After fertilization, the initial cell or zygote divides successively, to give rise to a structure made up of 16 cells called morula
2. The morula It can be observed about 4 days after fertilization.
3. Subsequently, these cells continue their division and begin to differentiate, giving rise to a structure formed by a outer spherical shell surrounding a liquid-filled space
4. From this moment, a series of very rapid changes will take place, such as the elongation of this structure and the displacement of some cells of the sphere towards the interior, which leads to the formation of the germ layers (ectoderm, mesoderm and endoderm) which are the precursors of the different tissues of the embryo.
5. These transformations occur between the second and third week and give rise to the trilaminar embryo From this moment on, the embryo can also distinguish a region (rostral) and a caudal region (tail), a left and a right side, and a dorsal (upper) and ventral (lower) surface.

This moment is very important in the development of the nervous system, since around the third week of gestation its development begins.

As we have seen, an elongated structure with three layers has been formed. In the outermost layer (ectoderm) and in a dorsal position, a well-defined central longitudinal band called neural plate, which extends from the cephalic to the caudal region. This plate is the precursor of the nervous system, its cells will give rise to neurons and form nervous tissue.

In the next phase of development of the embryonic nervous system, the neural plate sinks or invaginates inward, forming a neural canal or groove along the longitudinal axis. Both sides of the sulcus are delimited by the so-called neural folds; These surround the canal until they join together, which transforms the neural groove into a hollow neural tube This process takes place approximately in the fourth week of development and begins in the central area of ​​the embryo, progressively extending to the rostral and caudal regions.

Initially, the neural tube is open at both ends, but these openings disappear when the neural tube is completely formed (at the end of the fourth week). This closure of the two ends of the neural tube is very important because if it does not take place, important anomalies occur: for example, if the caudal end is not closed correctly, malformations occur in the spinal cord that give rise to spina bifida. If the rostral end is closed incorrectly, malformations occur in the brain (anencephaly) and in the skull, which is split. In relation to these processes, the importance of folic acid is known.

Between the dorsal ectoderm and the neural tube there is a group of pluripotent cells that form the so-called neural crest which extends from the rostral to the caudal area along the axis of the body. The crest cells migrate in different directions and differentiate to give gives rise to neurons and support cells of the peripheral nervous system Their final differentiation is determined by the environment in which they will finally develop after migration.

From the moment the rostral end of the neural tube closes, a series of rapid transformations of the neural tube begin in this area, leading to the formation of dilations of the neural tube. These protuberances constitute sketches of the different divisions of the central nervous system and their limits Thus, in the fourth week of embryonic development, three bulges or vesicles can be differentiated:

• The forebrain in the most previous part. During the fifth week, the forebrain will give rise to two new vesicles called the telencephalon and diencephalon.
• The midbrain (middle part).
• The rhombencephalon (rhombus shape) that is continuous with the neural tube that will finally give rise to the spinal cord. During the fifth week, the hindbrain will give rise to the metencephalon and the myelencephalon.

At the same time, a dorso-ventral pattern is established in a large part of the neural tube, which differentiates and separates two types of cells:

• The cells that will carry out functions sensory (dorsal region).
• The cells that will intervene in the motor coordination (ventral region).

Likewise, during weeks 4 and 5, spinal ganglia begin to form in the caudal area of ​​the neural tube.

From the 6th week onwards, other structures of the central nervous system will arise from the non-homogeneous differential growth of the aforementioned structures:

• The telencephalon will form the brain hemispheres
• From the diencephalon, they will originate the thalamus, hypothalamus, subthalamus and epithalamus
• From the metencephalon, the pons and the cerebellum
• From the myelencephalon, the medulla oblongata

Towards the 6th week of development, the union between the peripheral ganglia formed from the migration of neural crest cells, and the spinal cord, beginning to acquire its definitive organization.

Throughout all these processes, the hollow interior of the neural tube expands in the Central Nervous System, causing the vesicles cephalic and maintains a smaller diameter in the middle and caudal area, giving rise to the spinal cord canal.

From the ninth week the cerebral convolutions

But the formation of the nervous system does not end during the prenatal stage, since, after birth, a series of crucial events occur for the development of the nervous system.

The processes of development nervous system of the baby and child are:

• During the first four years after birth, the number of csynaptic contacts in the brain in response to neuronal activity.
• From that period onwards, and until puberty, there is a great synaptic reorganization (new synapses are established and those that are erroneous or not used are modified or eliminated). In these periods, the experiences and sensory stimuli that each individual has will determine the destiny and configuration that their synaptic contacts will have later.
• When the axons have finished their growth period, have emitted their collaterals and have consolidated their connections, the process of myelination (formation of myelin sheaths around axons). The process begins in the prenatal period and ends in adulthood (not before the age of 30, and may reach 50).

Myelination is a fundamental process in the development of the SN because myelin increases the speed of conduction of neural signals through the axon. It has been shown that the amount of myelin varies between individuals based on experience and the changing environment in which they develop. Therefore, certain skills, such as learning a language at a native level, can be acquired during the myelination phase of the neuronal circuits involved. Once the process is finished, these processes become difficult since myelin prevents the axons from branching and establishing new connections However, the nervous system of an adult continues to maintain the capacity for change and some neural plasticity is preserved.

This article is merely informative, at PsychologyFor we do not have the power to make a diagnosis or recommend a treatment. We invite you to go to a psychologist to treat your particular case.

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