The neurodevelopment It is the name given to the natural process of formation of the nervous system from birth to adulthood. It is an exceptional morphological and functional construction, perfectly designed by two fundamental architects: genes and experience..
Thanks to them, neural connections will develop. These will be organized in a complex network that will be responsible for cognitive functions, such as attention, memory, motor skills, etc..
Genes and the environment in which the individual develops, often interact with each other and influence development together. However, the degree of participation of each one seems to vary according to the stage of development in which we find ourselves.
Thus, during embryonic development, the main influence comes from genetics. In this period, the genes will determine the proper formation and organization of brain circuits. Both those associated with vital functions (brain stem, thalamus, hypothalamus ...), as well as those that constitute the cerebral cortical areas (sensory, motor or association areas).
Through numerous studies it is known that neurodevelopment continues until the end of adolescence or early adulthood. However, the baby is already born with a surprisingly developed brain in his organization.
With the exception of some specific neuronal nuclei, almost all neurons are created before birth. Furthermore, they arise in a part of the brain other than their final residence..
Later, neurons must travel through the brain to get into their rightful place. This process is called migration, and it is genetically programmed.
If there are failures in this period, neurodevelopmental disorders such as agenesis of the corpus callosum or lissencephaly may arise. Although it has also been associated with disorders such as schizophrenia or autism.
Once located, neurons establish a multitude of connections between them. Through these connections, the cognitive, socio-emotional and behavioral functions that will constitute the identity of each person will emerge..
The environment begins to exert its effects once the baby is born. From that moment on, the individual will be exposed to a demanding environment that will modify part of their neural networks.
In addition, new connections will emerge to adapt to the historical and cultural context in which you find yourself. These brain plastic changes are the result of the interaction between neuronal genes and the environment, which is known as epigenetics..
This statement by Sandra Aamodt and Sam Wang (2008) will help you understand the idea:
“Babies are not sponges waiting to soak up everything that happens to them. They come into the world with brains that are ready to seek certain experiences at certain stages of development. "
In general, two specific phases of neurodevelopment can be defined. These are neurogenesis or formation of the nervous system, and brain maturation.
As mentioned, this process appears to end in early adulthood, with the maturation of the prefrontal areas of the brain..
The most primitive and basic parts of the nervous system develop first. Progressively those of greater complexity and evolution are formed, such as the cerebral cortex.
The human nervous system begins to develop approximately 18 days after fertilization. At that time the embryo has three layers: the epiblast, the hypoblast and the amnion..
The epiblast and hypoblast gradually give rise to a disc composed of three cell layers: the mesoderm, the ectoderm and the endoderm..
Around 3 or 4 weeks of gestation, the neural tube begins to form. For this, two thickenings are developed that join together to form the tube.
One of its ends will give rise to the spinal cord, while the brain will emerge from the other. The hollow of the tube will become the brain ventricles.
On the 32nd day of gestation, 6 vesicles will have formed that will originate the nervous system as we know it. These are:
- Spinal cord
- The myelencephalon, which will give rise to the medulla oblongata.
- The metancephalon, which will give rise to the cerebellum and the bridge.
- The midbrain, which will become the tegmentum, the quadrigeminal lamina, and the cerebral peduncles.
- The diencephalon, which will evolve into the thalamus and hypothalamus.
- The telencephalon. From which will arise part of the hypothalamus, the limbic system, the striatum, the basal ganglia and the cerebral cortex.
About 7 weeks, the cerebral hemispheres grow and the sulci and convolutions begin to develop.
At three months of gestation, these hemispheres can be clearly differentiated. The olfactory bulb, hippocampus, limbic system, basal ganglia, and cerebral cortex will emerge.
As for the lobes, first the cortex expands rostrally to form the frontal lobes, then the parietal ones. Next, the occipital and temporal bones will develop.
On the other hand, brain maturation will depend on cellular processes such as axon and dendrite growth, synaptogenesis, programmed cell death, and myelination. They are explained at the end of the next section.
There are four main cellular mechanisms responsible for the formation and maturation of the nervous system:
It is about the birth of nerve cells. These arise in the neural tube, and are called neuroblasts. Later they will differentiate into neurons and glial cells. The maximum level of cell proliferation occurs at 2 to 4 months of gestation.
Unlike neurons, glial (support) cells continue to proliferate after birth.
Once the nerve cell is formed, it is always in motion, and has information about its definitive location in the nervous system..
Migration starts from the brain ventricles and all cells that migrate are still neuroblasts.
Through different mechanisms, neurons reach their corresponding place. One of them is through the radial glia. It is a type of glial cell that helps the neuron to migrate through support “wires”. Neurons can also move by attraction to other neurons.
Maximum migration occurs between 3 and 5 months of intrauterine life.
Once it reaches its destination, the nerve cell begins to take on a distinctive appearance. Neuroblasts can develop into different types of nerve cells.
In which type they transform will depend on the information that the cell has, as well as the influence of neighboring cells. In this way, some have an intrinsic self-organization, while others need the influence of the neural environment to differentiate themselves..
Programmed cell death or apoptosis is a genetically marked natural mechanism in which unnecessary cells and connections are destroyed.
At first, our body creates many more neurons and connections than it should. At this stage, the leftovers are discarded. In fact, the vast majority of neurons in the spinal cord and some areas of the brain die before we are born..
Some criteria that our body has to eliminate neurons and connections are: the existence of incorrect connections, the size of the body surface area, competence when establishing synapses, levels of chemical substances, etc..
On the other hand, the brain maturation It is mainly aimed at continuing with the organization, differentiation and cellular connectivity. Specifically, these processes are:
Axons are extensions of neurons, similar to wires, that allow connections between distant areas of the brain.
These recognize their path by a chemical affinity with the target neuron. They have chemical markers in specific phases of development that disappear once they have connected with the desired neuron. Axons grow very rapidly, which can already be seen in the migration stage..
While dendrites, the small branches of neurons, grow more slowly. They begin to develop at 7 months of gestation, when the nerve cells have already settled in their corresponding place. This development continues after birth and changes according to the environmental stimulation received..
Synaptogenesis is about the formation of synapses, which is the contact between two neurons to exchange information.
The first synapses can be observed around the fifth month of intrauterine development. At the beginning, many more synapses are established than necessary, which are later eliminated if they are not necessary.
Interestingly, the number of synapses decreases with age. Thus, a lower synaptic density is related to more developed and efficient cognitive abilities..
It is a process characterized by the myelin coating of the axons. Glial cells are the ones that produce this substance, which serves so that electrical impulses travel faster through the axons and less energy is spent.
Myelination is a slow process that begins three months after fertilization. Then it occurs at different periods depending on the area of the nervous system that is under development.
One of the first areas to myelinate is the brain stem, while the last is the prefrontal area.
The myelination of a part of the brain corresponds to an improvement of the cognitive function that that area has.
For example, it has been observed that when the brain areas of language are being covered with myelin, there is a refinement and advance in the linguistic abilities of the child.
As our neurodevelopment advances, our abilities progress. Thus, our repertoire of behaviors is becoming broader..
The first 3 years of life will be essential to achieve mastery of voluntary motor skills.
Movement is so important that the cells that regulate it are widely distributed throughout the nervous system. In fact, about half of the nerve cells in a developed brain are dedicated to planning and coordinating movements..
A newborn will only present motor reflexes of sucking, searching, grasping, moro, etc. At 6 weeks, the baby will be able to follow objects with his eyes.
By 3 months he can hold his head, voluntarily control grasping and sucking. Whereas, at 9 months, he will be able to sit alone, crawl and pick up objects.
By the age of 3, the child will be able to walk alone, run, jump, and go up and down stairs. They will also be able to control their bowels, and express their first words. In addition, manual preference is already beginning to be observed. That is, if you are right-handed or left-handed.
After such accelerated development from birth to age 3, progress begins to slow until age 10. Meanwhile, new neural circuits continue to be created and more areas are being myelinated..
During those years, the language begins to develop to understand the outside world and build thinking and relating to others..
From 3 to 6 years there is a significant expansion of vocabulary. In these years, it goes from about 100 words to about 2000. While from 6 to 10, formal thinking develops.
Although environmental stimulation is essential for proper language development, language acquisition is mainly due to brain maturation.
From the age of 10 to 20, major changes occur in the body. As well as psychological changes, autonomy and social relationships.
The bases of this process are in adolescence, which is characterized mainly by sexual maturation caused by the hypothalamus. Sex hormones will begin to secrete, influencing the development of sexual characteristics.
At the same time, personality and identity are gradually being defined. Something that can continue practically for a lifetime.
During these years, neural networks reorganize and many continue to myelinate. The brain area that is finished developing in this phase is the prefrontal region. This is what helps us make good decisions, plan, analyze, reflect and stop inappropriate impulses or emotions..
When there is any alteration in the development or growth of the nervous system, it is common for various disorders to appear.
These disorders can affect the ability to learn, attention, memory, self-control ... which become visible as the child grows.
Each disorder is very different depending on what failure has occurred and in what stage and process of neurodevelopment it has occurred.
For example, there are diseases that occur in stages of embryonic development. For example, those due to a poor closure of the neural tube. Usually the baby rarely survives. Some of them are anencephaly and encephalocele.
They usually involve severe neurological and neuropsychological alterations, usually with seizures.
Other disorders correspond to failures in the migration process. This stage is sensitive to genetic problems, infections and vascular disorders.
If the neuroblasts are not placed in their proper place, abnormalities may appear in the sulci or gyrus of the brain, leading to micro-polygyria. These abnormalities are also associated with agenesis of the corpus callosum, learning disorders such as dyslexia, autism, ADHD or schizophrenia.
Whereas, problems in neuronal differentiation can cause alterations in the formation of the cerebral cortex. This would lead to intellectual disability.
Also, early brain damage can impair brain development. When a child's brain tissue is injured, there is no new neuronal proliferation to compensate for the loss. However, in children the brain is very plastic and with the appropriate treatment their cells will reorganize to alleviate the deficits.
While, abnormalities in myelination have also been associated with certain pathologies such as leukodystrophy.
Other neurodevelopmental disorders include motor disorders, tic disorders, cerebral palsy, language disorders, genetic syndromes, or fetal alcohol disorder..
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