• Proliferation = The production of embryonic neural stem cells which later elongate. These stem cells are called Radial Glial cells (RGC)s.

 

• Neurogenesis = The glial cells divide several times to produce the neurons. There are several types of neurons that differ from each other in their shape and structure of the axons and the dendrites. Each type has its own function and location in the brain [47].

• Gliogenesis = This is the formation of non-neuronal cells - Glial precursor cells which derived from some type of the neural stem cells. Glial precursor cells provide multiple functions to the central nervous system. They differentiate into Astrocytes and Oligodendrocytes.  [47]. Astrocytes are part of the Blood Brain Barrier - BBB, which regulates the entrance of different substances from the blood into the brain [48]. The proliferation and migration of glial precursors and differentiation into astrocytes and oligodendrocytes are largely after birth processes.

• Migration = The neurons of a newborn baby migrate long distances to their final destinations and become cortical neurons. The cortical area of the brain (the external layer) is made up of six horizontal cortical layers (Figure 1). Each layer contains different types of neurons. The six layers are organized in columns or pillars, which are perpendicular and passing through all the six cortical layers. The neurons in each column receive impulse from the same area in the body. The cortical neurons grow dendrites and axons by which they connect to each other in the same layer, and between the different cortical layers. They are connected as well to other areas in the brain. An extensive amount of cell migration occurs in the first few postnatal weeks mainly by glial progenitors that differentiate into astrocytes and oligodendrocytes cells in their target places [23].​

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Figure 1: The 6 cortical layers of the brain [modified from p10, p11].

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Synaptogenesis = The creation of synapses which involves the process of Dendritic arborization (Figure 2), also known as dendritic branching. This is a multi-steps process in which neurons form new dendritic trees and branches to create new synapses. The synaptogenesis process rapidly increases immediately after birth and continue at a high rate until the baby is 2 years old [21]. The number of synapses in the prefrontal cortex (the outside layer of the prefrontal) increases by 25% by the end of the first month and by 60% by the end of the first two months [22].

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Figure 2: Dendritic arborization of layer 3 pyramid cell in the prefrontal cortex (PFC) [modified from p12].

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• Myelination = The production of myelin sheets by the oligodendrocytes lasts until the adult stage.  At birth, most of the axons are not yet myelinated and the area which is supposed to be the white matter in the brain will be shown distinctively when the infant is one year old [24]. ​

​T​he regulation control of the neural circuit in the brain checks over-production or defect production. It involves two main processes: apoptosis, which is a natural cell death and pruning, which is the trimming of neurons.

• Apoptosis is a natural programmed cell death occurs throughout life. A process that once began can not be stopped. It occurs as a result of stress such as a change in chemical balance in the cell, heat, toxicity, etc. [49].

• Synaptic pruning is the elimination of synapses whereby both axons and dendrites are completely decay and die off. The development of neural networks or neural circuit requires the formation of precise connections between a specific axon of a neuron to a specific dendrite of another neuron in another target area of the brain. Synaptic pruning is the process of elimination of unnecessarily synapses that will disturb the specific neural circuit (see definition bellow). It occurs between early childhood and the onset of puberty in many mammals, including humans [50, 51].

The timescales of these two sets of events are different. Most cells death in the neuronal populations occurs prenatally (before birth). Cell death in glial cells populations and exuberant production and pruning of connections between axons and dendrites (synapses) are largely postnatal (after birth) events.​

To enable the brain to perform tasks (both simple and complicated), the brain builds a network that connects across different areas in the same or different regions of the brain. That nerves-network or neural circuit integrates a vast amount of information and performs complicated cognitive and regulatory functions. The structures and functions of the neural circuits perpetually change and evolve from the time of first contact between nerve cells. The interplay of inherent genetic programs with a wide range of environmental exposures and experiences determines the birth, death, and cellular characteristics of neurons, as well as the formation and reformation of their axons, dendrites, and synapses [52].

Two synaptic factors are important in the neural circuit.

1. Synaptic plasticity is the ability to strengthen or weaken neuronal connections at the level of the synapse. The strength and pattern of activity at a given synapse produces transient or enduring communication between neurons, which can be either depression or potentiation [53].

2. Synaptic stabilization is crucial in the developing of an adult nervous system and it is considered a result of the late phase of long-term potentiation (LTP). LTP is the persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signals transmission between two neurons [54], which results also in a storage of information called the long-term memory [160].