Frances Jensen - The Teenage Brain - A neuroscientist’s survival guide to raising adolescents and young adults

Inhibiting a neural response is just as important as activating one when it comes to “executive” brain function. Examples of things that bind to inhibitory synapses are sedatives such as barbiturates, alcohol, and antihistamines. Synapses will be critical in our discussion of the adolescent brain because both the number and the type of synapses in our brains change as we age. They also change in relation to the amount of stimulation our brains experience. One topic that will come up later is the effect of illegal and illicit drugs and alcohol on these synapses, which we will cover in the chapter on addiction.

A popular instrument used by researchers to test inhibition is the Go/No-Go task in which subjects are told to press a button (the “Go” response) when a certain letter or picture appears, and not to press it (the “No-Go” response) when the letter X appears. Several studies have shown that children and adolescents generally have the same accuracy, but the reaction times, the speed at which a subject successfully inhibits a response, dramatically decrease with age in subjects age eight to twenty. In other words, it takes longer for adolescents to figure out when not to do something.

Signals move from one area of the brain to another along fiber tracks, and some of these tracks travel down through the core regions of the brain in order to send signals to and from the spinal cord. Brains are intricately interconnected by these fibers, and research using special brain scans is rapidly evolving to look at these connections. Because axons are designed to have a rapid pulse of electricity run through them to the connection point at the synapse, they act like electrical wires conducting an electrical signal. And just as an electrical wire needs insulation in order for the electricity not to dissipate along its length, so do the axons. Since we don’t have rubber in our brains, our axons are coated with a fatty substance called myelin. (See Figure 6.) The brain requires myelin in order to function normally, to get a signal from one region of the brain to another and also down to the spinal cord. As we said before, myelin is made by oligodendrocytes, and has a white hue due to its fatty content: hence the term “white matter.” By essentially “greasing” the “wires,” myelin allows signals to travel down axons faster, increasing the speed of a neural transmission as much as a hundredfold. Myelin also aids the speed of transmission by helping to cut down the synapses’ recovery time between neural firings, thereby allowing a thirtyfold increase in the frequency with which neurons transmit information. The combination of increased speed and decreased recovery time has been estimated by researchers as roughly equivalent to a three-thousand-fold increase in computer bandwidth. (Myelin also is the target of attack in the disease multiple sclerosis, or MS. Patients with MS have areas of inflammation in their white matter that come and go, and this is why they can lose functions like walking, sometimes only temporarily until the inflammation passes.)

At birth, a baby’s cortex contains little myelin; this explains why the electrical transmissions are so sluggish and an infant’s reaction times so slow. However, the baby’s brainstem is almost as fully myelinated as an adult’s, so it can control automatic functions like breathing, heartbeat, and gastrointestinal function necessary to stay alive. Connections to and from many other areas of the brain occur after birth, beginning with the motor and sensory areas at the bottom and back of the brain. As these areas become wired with myelin, infants are better able to process basic information from their senses—their eyes, ears, mouth, skin, and nose. Within the first year, the neural tracts that support brain regions involved in vision and other primary senses, as well as those involved in gross motor activity, are completed. This is, in part, why it takes about a year for a baby to become coordinated enough to walk. Much of the brain becomes insulated by age two, and high-level areas involved in language and fine motor coordination follow over the next few years when children are particularly primed to learn to talk and improve their fine motor skills. The more complex areas of the brain, especially the frontal lobes, take much, much longer and are not finished until a person is well into his or her twenties.

All of this learning is dependent on excitation, the driving force in our brains. Excitatory signals between neurons build brain connections and are required for brain development. Excitation can come from outside or inside your brain, but regardless, if a particular pathway of cells and their synapses are activated repeatedly, the synapses between them strengthen. Thus, cells that “fire” together“wire” together.

In the developing brain, especially in early childhood, as groups and pathways of neurons and their synapses get activated, the process of excitation “turns on” the molecular machinery in the cell. This actually results in the building of more synapses, a process we term synaptogenesis (birth of synapses). Synapses are increased in infancy through adolescence, peaking in early childhood. Because synaptogenesis is so dependent upon brain cells being activated by one another, a child’s brain has more excitatory than inhibitory neurotransmitters and synapses compared with an adult’s brain, where there is more balance between the two.

Excitation is a key element of learning. The period in early life in which excitation is so prominent is also called the “critical period,” when learning and memory are more robust than in later life. This allows the brain to be very sensitive to excitation and grow. Unfortunately, the abundant excitation in the developing brain carries a price: the risk for overexcitation. This explains why diseases that are a result of overexcitation, like epilepsy, are more common in childhood than adulthood. Seizures are the main symptom in epilepsy, and they are caused by too many brain cells turning on together without enough inhibition to balance them.

FIGURE 8. The Young Brain Has More Excitatory Synapses Than Inhibitory Synapses:The number of synapses increases from infancy through adolescence, peaking in early childhood.

Arborization, or the branching out of neurons, peaks in the first few years of life but continues, as we’ve seen, into adolescence. Gray matter density peaks in girls at age eleven and in boys at age fourteen, and waxes and wanes throughout adolescence.

White matter, or myelin, however, has only one trajectory in adolescence: up. Jay Giedd and colleaguesat the National Institute of Mental Health scanned the brains of nearly one thousand healthy children, ages three to eighteen, and discovered this pattern of wiring. As we saw in Figure 4, researchers at the University of California, Los Angeles, built on those findings and compared the scans of young adults, ages twenty-three to thirty, with those of teenagers, ages twelve to sixteen. They found that myelin continues to be produced well past adolescence and even into a person’s thirties, making the communication between brain areas ever more efficient.

FIGURE 9. Gender Differences in Rate of Cortical Gray Matter Growth:Like the body, the male brain is on average larger than the female brain. Rates of growth in male and female brains also are different. In females, the growth rate of two areas important for cognitive maturity—the frontal lobes and the parietal lobes—peaks in the early teen years, but in males the peak does not occur until the late teens.