The disappearing adolescent brain

As one approaches adolescence, the brain undergoes major structural and hormonal changes; Recent studies explain the biology of adolescents in ways that were previously confined to psychology.

An image of the human brain (photo credit: REUTERS)
An image of the human brain
(photo credit: REUTERS)
In many families, puberty arrives without any prior warning – suddenly, young children stop listening to their “old parents” and respond only to their friends. Moreover, children of this age also often exhibit bursts of energy that result in tension within the family.
A good example of such behavior appears in our ancient texts: Rehoboam, the son of King Solomon, rose to the throne at a young age, and during the anointing ceremony, his subjects asked him to reduce their tax burden in return for their loyalty. King Rehoboam did not listen to his temperate elders; rather, he listened to his young comrades: “And the young men that had grown up with him spoke to him” (I Kings 12:10).
These friends advised him to answer: “My father chastised you with whips, but I will chastise you with scorpions.” The result of the aggressive policy was a rebellion led by Jeroboam, the looting of the Temple by the king of Egypt and, eventually, a split kingdom.
THE RATIONAL evaluation ability of children aged 15 is not different from that of adults. Numerous studies have shown they know how to analyze the degree of risk in various situations in a manner similar to that of adults.
However, as happened to Rehoboam, their emotional intelligence causes adolescents to take more risks, be more aggressive and discount the opinion of their elders without “counting to 10” before making a decision.
Social experiments have seen adolescents taking even more risks when their peers are watching.
Until late last century, it was thought that the brain tissue develops solely during infancy, and from then it is fixed. In recent years, it has been found that the brain undergoes major changes during puberty, as well. The changes occur in different areas, and the levels of activity in certain brain parts directly explain the behavioral changes we observe. So what is the difference between the brain of an adolescent and that of an adult? The answer can be divided into two aspects: the structure of the brain, including its internal wiring, and the hormones that affect brain activity.
THE BRAIN undergoes tremendous changes from the beginning of adolescence till one’s early 20s. Changes in brain tissue are concentrated within the so-called “gray matter” (i.e., the region containing the bodies of nerve cells), unlike “white matter,” which contains nerve fibers (axons). The amount of gray matter in the prefrontal cortex, which is responsible for acts of will, creative thinking, management and strategizing, reaches its peak in girls at age 11 to 12, and in boys at age 13. After this, and for the duration of adolescence, the amount of gray matter in this region significantly shrinks, with more than a third of the connections between neurons (synapses) disappearing. It should be seen as a welcomed pruning that calibrates the system, along with the thickening of the nerve cell insulating sheet (myelin), resulting in a higher precision of inter-neuronal communication.
In a series of magnetic resonance imaging (MRI) simulations on a group of subjects aged four to 21, the changes in gray matter were shown to be gradual. The prefrontal cortex areas develop first, while the regions that are more advanced in evolutionary terms, namely those found only in monkeys and humans, develop relatively late, at the age of puberty. The studies also show that environment plays a major part in determining which inter-neuronal connections disappear (prune), and which remain. Therefore, adolescence is a time when the learning environment is especially important as a tool for shaping the brain toward adult life.
According to some theories, problems in the amount of gray matter and in myelination formation, along with high sensitivity to changes in the quantity of sex hormones, can contribute to severe mental illnesses such as schizophrenia, which usually starts to manifest during adolescence.
The cerebral cortex is also connected to the pleasure center of the brain.
Changes in these areas are linked to causing teenagers, as well as other “adolescent” animals (e.g., laboratory rats), to easily become addicted to nicotine and other drugs, in comparison to their adult counterparts.
This is also linked to the a fact that adolescents react more strongly to new experiences, as proven in experiments with functional magnetic resonance imaging (fMRI).
AT THE hormonal level, the adolescent brain undergoes unique processes due to significant changes in the levels of growth hormone, sex hormones and hormones secreted from the adrenal gland – adrenaline and cortisone. This gland communicates with the hypothalamus and the pituitary gland, which is located below it and is responsible for the release and activation of many hormones, including sex hormones.
Adrenaline and cortisol are responsible for the body’s stress response, and changes in their levels impact our ability to cope with stress.
The hormonal changes that happen during puberty are momentous. For example, the levels of the male sex hormone, testosterone, and the female sex hormone, estrogen, increase by a factor of more than 10. Sex hormones also affect the neurotransmitter serotonin, which plays an important role in controlling mood and the level of sexual arousal.
Even levels of the melatonin hormone, which regulates the cycle of day and night, undergo significant changes during adolescence, resulting in many teenagers with difficulties falling asleep in the evening or waking up early in the morning. Equally important, sex hormones affect the neurotransmitter dopamine, forming a closely interacting circuitry with behavioral consequences.
Dopamine, which is released from brain tissues and from the adrenal gland, is responsible for a variety of functions, including perception, learning, higher thinking, self-regulation and attention.
The amount of dopamine receptors in the cerebral cortex increases significantly during adolescence, resulting in an increased capacity to capture the signal that this hormone is trying to send. In contrast, the receptors in other parts of the brain reach their full capacity already by childhood. This phenomenon could explain impulsiveness and the pursuit of pleasure without rational processing, which is characteristic of adolescence.
Numerous studies demonstrate that many changes during adolescence are related to largescale modifications in the brain. These include the volume of different brain tissue, the amount of hormone receptors, the type of connections forming between brain tissue, and the impact of significant hormonal changes that occur during these years. Some of these changes are gradual while others are exhibited in leaps and bounds.
Brain changes throughout adolescence appear differently for each individual, and they are more important than one’s exact chronological age. Knowing that the brain develops so significantly not only during pregnancy and infancy, but also up until the beginning of the third decade of life, requires us to better understand the phenomena of puberty.
During any outburst by a teenager, or even the lack of regard for the opinion of others, it is important to keep in mind that if we take a saliva sample and examine changes in the amount of hormones, it can explain quite a bit about the challenges of growing up. Just as we are delighted when a baby exits the womb and cries upon opening its lungs for a first breath, one should also accept that many adolescent behaviors are a normal part of adolescent brain development.
The writer is a biochemist, an academic adviser at the Weizmann Institute and a writer for the Davidson Institute of Science Education.