The Adolescent Brain
The role of genes in synapse reorganization during adolescence
- To bring about these architectural changes, cells turn on many genes, which then synthesize many proteins as the brain rises to the maturational challenge. Genes function in cells not only to convey hereditary information, but also as the master blueprint for guiding the organism's metabolic machinery throughout its life. - Genes related to synapse reorganization are especially active in brain cells during adolescence, probably reflecting genetic programs that make early adolescence a time of dramatic synaptic reorganization in the cortex. The signals that herald the initiation of these genetic programs are unclear, but hormonal changes and other maturational factors are probably involved. The sex hormones estrogen and testosterone, for example, act by turning genes on and off, so that as hormonal changes occur during puberty, the genetic regulation of cell metabolism also changes.
Possible explanation for why teens have difficulty thinking about the consequences of their actions
And it has also been learned that teens are prone to certain types of flawed logic or to ignoring cues about how questions are framed in their decision-making. Again, such observations suggest that one reason adolescents may have limited cognitive ability to simultaneously process information about antecedents and outcomes, hold it in working memory, and use it to make decisions is likely traceable, in part, to brain circuitry not fully developed and still under construction, particularly in the prefrontal cortex of the frontal lobes.
Damage of the lower middle portion of the PFC in adults
Appears to be associated with an impaired ability to simulate the future consequences of current actions, positive or negative, regardless of the emotional significance ("myopia for the future"). Thus, the person appears to be excessively influenced by immediate reward. It is also possible that these individuals may simulate the outcome but not be able to attach an appropriate emotional meaning or significance to it.
When does the brain develop the most neural connections in its lifetime?
BEFORE puberty. As a child, you have the most flexible brain ever. You are a sponge.
Synaptic pruning in adolescence
During puberty, the brain goes through synaptic pruning, effectively diminishing those connections and letting the brain become leaner and more efficient. The brain forms neural circuitries that allow it to perform several tasks simultaneously and efficiently. These circuits mature and are coated with the myelin sheath, which is an insulator that speeds up communications between neurons.
What kind of synapse is the most lost during synaptic pruning in adolescence to adulthood?
Excitatory synapses. Thus, the brain by early adulthood appears to have undergone a reorganization of synaptic balance such that, at least in certain circuits, there is much greater weight on the inhibitory side and less weight on the excitatory side.
Cingulate
INVOLVED IN EMOTION. Connects to the brainstem and spinal cord, modulating our physical reactions to emotions such as sweaty palms, increased heart rate, and that tense feeling in our stomach.
Complexity of brain folds in adolescence
In addition to doubling in size, the brain's surface folds become much more complicated in adolescence. Evidence suggests that this increasingly complex folding may be related to the elaboration of underlying connections among cells. The complexity of the folding patterns becomes increasingly obvious in parts of the brain cortex—the outer mantle—that process cognitive and emotional information (as distinct from parts of the brain responsible for the control of more basic motor and sensory functions). In other words, those parts of the brain related to such higher-order functions as learning and socialization appear to show the greatest changes in adolescence. - In fact, the evolving pattern of folds and crevices reaches a peak and levels off by the late teens, after which it remains stable throughout adult life. - More folds = more surface area. That means that a wrinkly brain can hold more neurons than a smooth brain. - The wrinkles are so important that scientists even have science-y names for them. The ridges are gyri (singular: gyrus) and the folds are sulci (singular: sulcus)
Brain plasticity
It has often been stated that the first 5 years of life is a time of extraordinary brain changeability, or "plasticity." This is reflected in the dizzying pace at which the brain acquires new skills, from learning to walk to learning to read. These new skills and abilities are presumably grounded in an elaboration and stabilization of a synaptic architecture constructed as part of the process of learning these new things.
The process of getting to parallel processing in the outer cortex in adulthood
It is also apparent that regions of the cortex (i.e. the outer mantle layer of the brain) that handle abstract information and that are critical for learning and memory of such concepts as rules, laws, and codes of social conduct seem to become much more likely to share information in a parallel processing fashion as adulthood approaches. This increased information sharing is reflected in the patterns of connections between and among neurons in different regions of the cortex. For example, the branching of neurons in the prefrontal cortex becomes much more complex during adolescence, likely reflecting a more intricate web of information flow. It is as if the cells change their architecture in order to meet the increasingly difficult cognitive and emotional challenges that they are being asked to master. By the end of the twenties, the profile of cell-to-cell contacts reaches an adult pattern and the number of connections reaches a steady state that persists until old age.
Gamma amino butyric acid
Major inhibitory neurotransmitter.
During childhood and adolescence, the amount of _____ increases throughout the brain.
Myelin. So, during the teen years, not only does the number of connections change, but the connections themselves also become faster.
What is the last brain structure to develop in adolescence? At what age?
New research suggests that the prefrontal cortex is the last to develop in the brain (by age 25). It takes the longest to fully mature. The limbic system, however, is fully developed before age 25.
Key MRI study in teens' response to emotions expressed on faces
One key MRI study found that when identifying emotions expressed on faces, teens more often activated their amygdala—the brain area that experiences fear, threat and danger— whereas adults more often activated their prefrontal cortex—the area of the brain linked more to reason and judgment—and performed better on the task. Behaviorally, the adult's responses were more intellectual, the teens' more from the gut. These findings and others suggest that although the plasticity and changeability of the adolescent brain are extremely well suited to meet the demands of teen life, guidance from parents and other adult institutions are essential while decision-making circuitry is being formed.
Dopamine inputs to the PFC during adolescence
One set of cells that shows this pattern in the prefrontal cortex is the set that carries the messenger chemical dopamine. This chemical has been found to be critical for focusing attention on environmental stimuli when it is necessary to choose between conflicting options, especially when the goal may not be obvious and choices based on memory, not impulse, are required. Dopamine inputs to the prefrontal cortex grow dramatically during adolescence, probably representing one of the neuronal mechanisms that increase the capacity for more mature judgment and impulse control. Indeed, beginning in adolescence, the dopamine reward signal becomes especially important in the frontal lobe as ideas, per se, become increasingly reinforced and valued. Since learning is based on reward, the adolescent begins to have the ability to follow an idea in pursuit of a goal, rather than to simply act on instinct.
What is the PFC responsible for?
PFC is responsible for skills such as setting priorities, plans and ideas, forming strategies, controlling impulses, allocating attention, understanding social conduct, rules, and laws.
Lower middle portion of the prefrontal cortex
Responsible for judgement regarding decisions and their implications in cause, effect, and consequences.
What's an observation that seeks to explain teens' lack of inhibition?
Such observations suggest that one reason adolescents may have difficulty inhibiting inappropriate impulses is that the circuitry needed for such control is not fully mature in early adolescence, thereby making such tasks relatively difficult. (remember that the prefrontal cortex is the last thing to fully mature in the adolescent's brain, by age 25).
Synaptic pruning in adolescence (more in-depth)
The abundance in gray matter from childhood is pruned in a back- to-front sequence through the teen years. MRI studies also show an opposite front-to-back wave of increases in white matter (myelin) through childhood and adolescence. In essence, brain functions are sculpted to reveal and allow increasing maturity in thought and action. Both of these pruning forces are likely influenced by genetic or environmental influences. We still don't know what the forces actually are, though. - In the earliest stages of brain development, primarily before birth, there are many more brain cells and connections formed than can possibly survive. A process of competitive elimination, or "pruning," follows this vast overproduction. Those cells and connections that are used survive; those that aren't used wither.
When brain circuits become fully myelinated explanation
The age at which brain circuits become covered with myelin corresponds, more or less, to the time at which they become functionally mature and achieve their adult role. - Areas of the brain that process complex abstract information—e.g., learning and memory in the service of goal directed behavior—develop these coats relatively late. - Parts of the brain involved in such sensory and motor functions as moving the arms and the eyes are fully myelinated by the first few years of life. The cabling of the prefrontal cortex and related regions, however, is not fully myelinated until well into the third decade of life.
Gray matter in childhood brain
The cortical gray matter thickens throughout childhood as the brain cells grow an exuberance of connections to other brain cells.
Connection between cingulate and hippocampus
The hippocampus helps retrieve memories, to put situations in historical context, and to remember past details about a situation that might be important. A myelin-covered bridge of axons called the superior medullary lamina connects these two areas. The laying down of myelin in this circuit is one of the most active processes in all the brain, with the myelin content doubling during the teen years. Thus, like the PFC, functions that enhance teens' ability to connect "gut feelings" with intellectual elements are under major construction.
Hippocampus and amygdala in circuitry processes during adolescence
The manner in which this circuitry processes environmental stimuli changes in adolescence. The changes reflect, in part, the fact that the prefrontal cortex controls the responses of these lower centers. Changes intrinsic to these structures might also independently contribute to differential responses or influence prefrontal cortex development and adult function. As the prefrontal cortex matures, a stimulus that might earlier have initiated an automatic behavioral routine or a simple emotional arousal comes to be treated with a more reasoned or deliberate response. The prefrontal cortex's control of more automatic response patterns is another manifestation of the reorganization of brain activity that emerges during adolescence.
Arborization
The receiving zone of the cell grows synapses primarily on extensions called dendrites, which look somewhat like the roots of a tree. This process is called arborization, from the Latin word arbor for tree, and reflects the increased "bushiness" of individual cells as their connections grow like extra branches, twigs, and roots. The dendrites grow out from the cell body, home to its nucleus (or headquarters) and hub of its DNA—its genetic blueprint. This root structure increases in size and complexity as the synaptic population grows.
