SEASONS OF THE BRAIN: DEVELOPING BRAIN
The first season, the season of development, is when the main cognitive abilities and skills are formed, which is characterized by dramatic changes in the brain. This season begins before we are born and extends into the third decade of our life. Brain development is a complex and multifaceted process. It starts with neurogenesis, the birth of neurons, which are the brain cells most directly involved in information processing, and their migration, finding their proper places in the complex organization of the brain. For the most part, neurogenesis occurs during gestation, at somewhat different times for different brain structures. It was thought until recently that neurogenesis ran its course and ground to a complete halt sometime during gestation and the first few years of life. By that time, most brain structures have acquired their recognizable shape. Today we know, however, that neurogenesis continues throughout the lifetime, albeit not as vigorously as during the early period.
As the neurons are born and migrate to their proper locations in the brain, connections between neurons begin to develop. These connections, formed as protrusions emanating from the neuron bodies, are called axons and dendrites. They begin to develop during gestation, and the dendrites begin to sprout through the process called arborization. This process culminates during the first years of life.
Synapses, the tiny interfaces between the dendrites and axons emanating from different neurons, are critical for communication between neurons. Their formation is called synaptogenesis and its time course varies considerably for different parts of the brain. In the visual cortex, for instance, most of synaptogenesis is complete by the end of the first few years of life. By contrast, the synaptogenesis of the prefrontal cortex extends well into late adolescence and early adulthood.
Production of neural structures is complemented by the elimination of excessive neurons, dendrites, and synapses. This process, known as pruning or apoptosis, occurs after birth and also unfolds at different time courses for different parts of the brain, the frontal cortex being the last. Pruning is akin to sculpting, a process that the great sculptor Auguste Rodin described as “eliminating everything that does not belong.” Pruning is not random, but rather is a consequence of reinforcing heavily used neural structures and letting go of those underused or not used at all. These competitive processes of the brain molding itself are somewhat akin to natural selection, which was captured in the term “neural Darwinism,” coined by Gerald Edelman.
Neurons are not the only type of cells found in the brain. In fact, they account for only about one-third of all the brain cells. The remaining two-thirds are taken up by the glial cells, which serve various supporting functions and come in two kinds: astrocytes and oligodendrocytes. At a certain point in development the process of myelination begins: Oligodendrocytes begin to wrap around long axons, forming a fatty protective coating called myelin. Myelin is white, which gave rise to the term white matter (composed of all the long pathways covered with myelin), as opposed to gray matter (composed of all the neurons and short local nonmyelinated pathways). Myelin facilitates signal transmission along the axon, greatly enhancing and improving transmission of information within large coordinated neuronal ensembles. Dramatic increase in brain weight during the first years of life is largely due to myelination. The brain structures are not fully functional until the axons connecting them are insulated with myelin, and the time course of myelination varies vastly from structure to structure. As you can probably guess by now, myelination takes the longest in the frontal cortex, extending well into late adolescence and young adulthood, possibly until the age of thirty. The volume of the frontal lobe, and particularly of the prefrontal cortex, continues to grow at least until the age of eighteen and possibly longer, and this growth reflects an ongoing increase in white matter.
If nothing else, this brief review shows that brain development is the interplay among numerous processes unfolding at different time scales. This is a time of great flux in the life of the brain. This is also a time of great flux in the life of the mind— the time of learning, of accumulating the basic fund of mental skills and knowledge, and ultimately the time of forming our identities.
You may have noticed that the frontal lobes, the prefrontal cortex in particular, are the last to complete their biological maturation—only by young adulthood, sometime in the very end of the second decade and possibly even in the third decade of life. Modern society operates on the basis of certain tacit or explicit assumptions about the age of social maturity. This is the age of emergence of the cognitive and personality traits that we associate with social maturity, such as the capacity for impulse control, foresight, and critical self-appraisal. Like the biological maturation of the frontal lobes, these “adult” traits reach their full functionality sometime in the end of the second and the beginning of the third decades of life. Unsurprisingly, this age has been codified in virtually every modern society as the age of transition from social immaturity to social maturity. This is the approximate age (plus or minus a few years) when you are ready to assume a whole range of “mature” rights and responsibilities, such as driving, voting, getting married, buying alcohol, serving in the military, and finally being treated by the legal system as an adult and not a minor. What most people don’t realize is that the emergence of these “adult” traits is most likely caused by the maturation of the frontal lobes, a belief shared by an increasing number of neuroscientists. Thus, many neuroscientists find it useful to think of the completion of the maturation of the frontal lobes, particularly the myelination, as the watershed between the first and the second seasons of the brain: the stage of development and the stage of maturity.
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