DEVELOPMENT OF THE NERVOUS SYSTEM IN HUMANS (2 STAGES) - NEUROPSYCHOLOGY - 2025

The development of the 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 child's interaction with the outside world will be decisive in the subsequent maturation of neural networks and structures.

The correct formation and development of each of the structures and connections that make up 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, important congenital defects can appear at the brain level.

From a macro-anatomical point of view, the nervous system of human beings is made up of the central nervous system (CNS), made up of the brain and spinal cord, and on the other hand, the peripheral nervous system (PNS), made up of the cranial and spinal nerves.

In the development of this complex system, two main processes are distinguished: neurogenesis (each of the parts of the NS are made up) and maturation.

Stages of nervous system development

Prenatal stage

From the moment fertilization occurs, a cascade of molecular events begins to occur. Around 18 days after fertilization, the embryo is made up of three germ layers: epiblast, hypoblast (or primitive endoderm) and amines (which will form the amniotic cavity). These layers are organized into a bilaminar disk (epiblast and hypoblast) and a primitive streak or primary groove is formed.

At this time, a process called gastrulation takes place, which results in the formation of three primitive layers:

• Ectoderm: outermost layer, made up of remains of the epiblast.
• Mesoderm: intermediate layer that gathers the primitive cells that extend from the epiblast and hypoblast that invaginates forming the midline.
• 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 function as longitudinal support and will be central in the processes of embryonic cell formation that will later specialize in tissues and organs. The outermost layer (ectoderm) when located above the notochord, will receive the name of neuroectoderm and will give rise to the formation of the nervous system.

In a second developmental process called neurulation, the ectoderm thickens and forms a cylindrical structure, called the neural plate.

The lateral ends will fold inwards and with development it will transform into the neural tube, approximately 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.

Around the 28th day of gestation, it is already possible to distinguish the most primitive divisions. The anterior portion of the neural tube is derived into: the forebrain or forebrain, the midbrain or midbrain, and the hindbrain or rhombus. On the other hand, the remaining portion of the neural tube becomes the spinal cord.

• Prosoencephalon: the optic vesicles arise and at approximately 36 days of gestation, they 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.
• Midbrain will give rise to the tectum, lamina quadrigémina, tegmentum, cerebral peduncles and cerebral aqueduct.
• Rhomboencephalon: it is divided into two parts: the metancephalon and the myelencephalon. The pons, cerebellum and medulla oblongata emerge from these at approximately 36 days of gestation.

Later, around the seventh week of gestation, the cerebral hemispheres will begin to grow and form the cerebral fissures and convolutions. Around 3 months of gestation, the cerebral hemispheres will differentiate.

Once the main structures of the nervous system have been formed, the occurrence of a brain maturation process is essential. In this process, neuronal growth, synaptogenesis, programmed neuronal death or myelination will be essential events.

Already in the pre-natal stage there is a maturation process, however, it does not end with the birth. This process culminates in adulthood, when the axonal myelination process ends.

Postnatal stage

Once birth occurs, after approximately 280 days of gestation, the development of the newborn's nervous system must be observed both in motor behaviors and in the reflexes it expresses. The maturation and development of cortical structures will be the basis for the subsequent development of complex behaviors at the cognitive level.

After birth, the brain undergoes rapid growth, due to the complexity of the cortical structure. At this stage, dendritic and myelinating processes will be essential. The myelinating processes will allow fast and precise axonal conduction, allowing efficient neuronal communication.

The myelination process begins to be observed 3 months after fertilization and occurs progressively at different times according to the region of development of the nervous system, not occurring in all areas equally.

However, we can establish that this process occurs mainly in second childhood, a 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 begin with subcortical structures and 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 association regions, following a horizontal direction.

The first structures that are completely myelinated will be in charge of controlling the expression of reflexes, while the cortical areas will complete it later.

We can observe the first primitive reflex responses around the sixth week of gestation in the skin surrounding the mouth in which, on contact, a contralateral flexion of the neck occurs.

This skin sensitivity extends over the next 6 to 8 weeks and reflex responses are observed when stimulated from the face to the palms of the hands and the upper chest region.

By week 12 the entire surface of the body is tender, except the back and crown. Reflex responses also change from more generalized to more specific movements.

Between the cortical areas, the primary sensory and motor areas, myelination will begin first. The projection and commissural areas will continue to form up to 5 years of age. Then, those of frontal and parietal association will complete their process around 15 years of age.

As myelination develops, that is, the brain matures, each hemisphere will begin a process of specialization and become associated with more refined and specific functions.

Cell mechanisms

Both in the development of the nervous system and in its maturation, the existence of four secular mechanisms have been identified with the essential basis of its occurrence: cell proliferation, migration and differentiation.

Proliferation

Production of nerve cells. Nerve cells start as a single cell layer along the inner surface of the neural tube. Cells divide and give rise to daughter cells. In this stage, 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 various mechanisms by which neurons reach their site.

Some reach their site through movement along the glia cell, others do so through a mechanism called neuron attraction.

Be that as it may, the migration begins in the ventricular zone, until reaching its location. Alterations in this mechanism have been linked to learning disorders and dyslexia.

Differentiation

Once their destinations have been reached, the nerve cells begin to acquire a distinctive appearance, that is, each nerve cell is going to differentiate itself based on its location and function to perform. Alterations in this cellular mechanism are closely related to mental retardation.

Cell death

Apoptosis is a programmed cell destruction or death, 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, which range from prenatal stages and extend into adulthood.

References

1. Jhonson, MH, & de Hann, M. (2015). Language. In MH Jhonson, & M. de Hann, Developmental Cognitive Neuroscience (Fourth Edition ed.,

   Pp. 166-182). Wiley Blackwell.

2. Purves, D. (2012). In Neuroscience. Pan American.
3. Roselli, Monica; Hooch, Esmeralda; Alfredo, Ardila;. (2010). Neuropsychology of Child Development. Mexico: The Modern Manual.