The development of the human nervous system (NS) is based on a sequential program and is governed by pre-programmed, clear and well-defined principles. The organization and formation of the nervous system is the product of genetic instructions, however, the interaction of the child with the outside world will be decisive in the later maturation of neural networks and structures.
The correct formation and development of each of the structures and connections that form our nervous system will be essential for prenatal development. When any of these processes is interrupted or develops in an abnormal way due to genetic mutations, pathological processes or exposure to chemicals, there may be important congenital defects at the brain level.
Development of the human nervous system
From the macro-anatomical point of view, the nervous system of human beings consists of the central nervous system (CNS), formed by the brain and spinal cord and on the other side by the peripheral nervous system (PNS), constituted by The cranial and spinal nerves.
In development of this complex system two main processes are distinguished: neurogenesis (each part of the NS conforms) and maturation.
Stages of nervous system development
From the moment fertilization takes place, a cascade of molecular events begins. Around 18 days after fertilization, the embryo consists of three germ layers: epiblast, hypoblast (or primitive endoderm) and amines (which will form the amniotic cavity). These layers organize into a bilaminar disk (epiblast and hypoblast) and form a primitive streak or primary groove.
At this moment, a process called gastrulating takes place, resulting in the formation of three primitive layers: ectoderm (outermost layer, consisting of epiblast remains), mesoderm (intermediate layer that brings together primitive cells that extend from epiblast and hypoblast Which is invaginated forming the midline) and endoderm (inner layer, formed with some cells of the hypoblast). The invagination of the mesodermal layer will be defined as a cylinder of cells along the entire midline, notochord.
The notochord will serve as a longitudinal support and will be central in the processes of formation of embryonic cells that will later specialize in tissues and organs. The outer layer (ectoderm) when it is located above the notochord will be called neuroectoderm and will lead to the formation of the nervous system.
In a second developmental process called neurulation, the ectoderm becomes thicker and forms a cylindrical structure, called neural plaque. The lateral ends will fold inwards and with the developed it will be transformed into the neural tube, approximately at 24 days of gestation. The caudal area of the neural tube will give rise to the spine; The rostral part will form the brain and the cavity will constitute the ventricular system.
Near day 28 of gestation, it is already possible to distinguish the most primitive divisions. The anterior portion of the neural tube is derived in: the forebrain or forebrain, the midbrain or midbrain and the posterior or rhomboencephalus brain. On the other hand, the remaining portion of the neural tube is transformed into the spinal cord.
- Prosoencephalus : the optic vesicles arise and at approximately 36 days of gestation, it will derive in the telencephalon and diencephalon . The telencephalon will form the cerebral cortex (approximately 45 days of gestation), basal ganglia , limbic system , rostral hypothalamus, lateral ventricles and third ventricle.
- Mesencephalon will give rise to tectum, quadrimeric lamina, tegmentum, cerebral peduncles and cerebral aqueduct.
- Rhombocephalus : divided into two parts: metencephalon and myelencephalon. Of these approximately 36 days of gestation pons, arise cerebellum and medulla oblongata.
Later, on the seventh week of gestation the cerebral hemispheres will begin to grow and to form the cerebral fissures and convolutions. Around 3 months of gestation, the cerebral hemispheres will differentiate.
Once the main structures of the NS have been formed, the occurrence of a process of brain maturation is essential. In this process, neuronal growth, synaptogenesis, programmed neuronal death or myelination will be essential events.
Even in the pre-natal stage a maturation process occurs, however, this does not end with the birth. This process culminates towards adulthood, when the axonal myelination process ends.
After birth, after about 280 days of gestation, the development of the nervous system of the newborn should be observed both in the motor behaviors and in the reflexes that it expresses. The maturation and development of cortical structures will be the basis for the subsequent development of complex cognitive behaviors.
After birth, the brain undergoes rapid growth, due to the complexity of the cortical structure. At this stage, dendritic and myelinizing processes will be essential. Myelinating processes will allow rapid and precise axon conduction, allowing efficient neural communication.
The process of myelination begins to be observed at 3 months of fertilization and occurs progressively at different times according to the region of development of the nervous system, not occurring in all areas alike. However, we can establish that this process occurs mainly in the second childhood, period between 6 and 12 years, adolescence and early adulthood.
As we have said this process is progressive, so it follows a sequential order. It will be initiated by sub cortical structures and will continue with cortical structures, following a vertical axis. On the other hand, within the cortex, the primary zones will be the first to develop this process and later, the regions of association, following a horizontal direction.
The first structures that are completely militated will be in charge of controlling the expression of reflexes, while the cortical areas will complete later.
We can observe the first primitive reflex responses towards the sixth week of gestation in the skin surrounding the mouth in which, when making contact, a contralateral flexion of the neck occurs.
This sensitivity in the skin extends over the next 6 to 8 weeks and reflex responses are observed when it is stimulated from the face to the palms of the hands and the upper region of the thorax. By week 12 the entire surface of the body is sensitive except for the back and crown. Reflex responses also change from more generalized movements to more specific movements.
Between cortical areas, primary sensory and motor areas, myelination will begin in the first place. Projection and commissural areas will continue to form until 5 years of age. Then, the frontal association and parietal will complete its process around 15 years of age.
As myelination develops, that is, the brain matures, each hemisphere will begin a process of specialization and is associated with more refined and specific functions.
Both the development of NS and its maturation have identified the existence of four secular mechanisms with which they are the essential basis of their occurrence: cellular proliferation, migration and diffusion.
- It Proliferació n : production of nerve cells. Nerve cells start as a simple cell layer along the inner surface of the neural tube. Cells divide and give rise to daughter cells. At this stage the nerve cells are neuroblasts, from which neurons and glia are derived.
- Migration: each of the nerve cells has a genetically marked site in which it must be located. There are several mechanisms by which neurons reach their site. Some reach their site through the displacement along the glia cell, others do so through a mechanism called attraction of neurons. Be that as it may, the migration begins in the ventricular zone, until reaching its location. Alterations in this mechanism have been related to learning disorders and dyslexia.
- Differentiation: once their destinies reach the nerve cells begin to acquire distinctive appearance, that is, each nerve cell will be differentiated according to its location and function to perform. Alterations in this cellular mechanism are closely related to mental retardation.
- Cell death: Apoptosis is a programmed cell death or destruction, in order to self-control development and growth. It is triggered by genetically controlled cellular signals.
In conclusion, the formation of the nervous system occurs in precise and coordinated stages, ranging from pre-natal stages and extending into adulthood.