The Adolescent Brain

The Adolescent Brain

jenny l. williamson
Project Director for How Do I Learn: Neuroscience Advances Inform Learning
School of Nursing, University of Washington

Our brains are wired to learn – to form new connections based on what is happening around us. This growth is especially true of infant and child brains. Think about how easily children learn to speak not only their birth language, but any foreign language introduced early enough. Adolescents, too, are wired to learn quickly and easily – although that learning is not always tempered with good judgment or impulse control. (See Adolescent Brain Development: A Period of Vulnerabilities and Oppportunities) Adolescence is a unique period in the brain’s development. It is characterized by both addition and subtraction. There is the addition of new cells and synapses during infancy, childhood and adolescence. There is also the addition of myelin, a white, fatty “insulation” which increases the speed of the brain’s connections. There is a subtraction or pruning of excess neurons and little used connections. After a quick review of neuron function, we will take a closer look at these two processes.

Neuron Image

CC by Nicolas Rougier

The basic working unit of the brain is cell called a neuron. Our nervous system is made up of millions of these neurons all wired together – sort of like an electrical circuit. Electrical signals pass through individual neurons, but when the signal gets to the end of the output part of a neuron, the axon, that it can’t jump the gap between neurons. That small gap separating neurons is called the synapse. Neurons communicate with one another by releasing packets of chemicals, called neurotransmitters, from the ends of axons. These neurotransmitters diffuse across the synapse, lodging on the “catcher’s mitt” or dendrite of the next neuron, making it either more or less electrically excitable. If they are excitatory neurotransmitters, they cause another electrical signal to pass through the receiving neurons and their axons, which, in turn, release neurotransmitters to the neighboring neurons. This process is repeated over and over again as the nervous system responds to the internal and external environment.

So where does the addition and subtraction occur? During childhood and adolescence, the brain is constantly wrapping axons with a white insulation called myelin to increase the speed of the electrical signal within the neuron. Myelination takes a long time and is not complete until the mid-twenties. The brain adds myelin in the cortex (the outside layer of the brain) in a process that starts at the back of the head and moves forward. The last area to become myelinated is the prefrontal cortex, an area right behind your forehead. The prefrontal cortex is where your brain exercises judgment and impulse control. (See Braking and Accelerating of the Adolescent Brain)

Neuroscientists are fond of saying “the neurons that fire together wire together.” This saying refers to the fact that when certain neuronal connections are used often, there is a pathway formed, and the signals within these pathways become faster. But while the brain makes some connections speedier, it also eliminates neuronal connections that are seldom used, pruning these connections so that newer, stronger connections can take their places. (See Dynamic mapping of human cortical development during childhood through early adulthood.) This makes the brain not only faster, but more efficient. Although some pruning is done all through infancy and childhood, the brain does a significant amount of pruning during adolescence. The rule for pruning seems to be “Use it – or lose it!”

The subcortical region of the brain matures faster than the cortical region, especially the prefrontal cortex which is the last to mature. It is this imbalance that leads to the risk-taking nature of many adolescents – for there are no prefrontal brakes to slow down the adolescent brain which seeks sensation. (See Braking and Accelerating of the Adolescent Brain) Happily, the brain DOES mature with time and experience, for as Earl Wilson quips, “Snow and adolescence are the only problems that disappear if you ignore them long enough.” (See Earl Wilson Quotes)

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Posted in Neuroscience