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

which I mentioned earlier, known as the critical period. During this stage of development, a baby’s brain creates an astonishing two million synapses every second, allowing the infant to reach mental milestones like color vision, grasping, facial recognition, and parental attachment. It’s as if an infant’s brain is sending out billions of antennae, scanning the world for information. For each synapse to survive, it must find another neuron to send information to; this is why the number of synapses in a baby’s brain peaks in childhood. The gray matter—the brain tissue responsible for processing information—continues to thicken throughout childhood as the brain’s cells form extra connections, those limb-like dendrites. Known as arborization, this thickening is like a tree growing extra branches and roots. Stimulation, experiences, repeated sensations—all contribute to the creation of these new neural pathways. In adolescence, this “overgrowth” is responsible for a teen’s heightened capacity to learn new things quickly—everything from operating the new TV remote to speaking Mandarin Chinese. The profusion of gray matter, though, can also cause a kind of cognitive dissonance in which the brain has trouble picking out the right signals from all the “noise.” As a result, by late adolescence the brain has begun to prune away excess synapses and streamline connections.

      Synapses come in two flavors: ones that excite, or turn on, the next neuron, and ones that inhibit, or turn off, the next neuron.

      FIGURE 7A. Inhibitory Cells Can Stop Signaling: Inhibitory cells release inhibitory neurotransmitters onto spines, which will stop a signal in a neuron and turn the cell “off.”

      FIGURE 7B. Excitatory and Inhibitory Synapses: Excitatory axons release excitatory neurotransmitters, such as glutamate, which bind to excitatory receptors and turn the neuron “on.” Inhibitory axons release inhibitory neurotransmitters, like GABA, which bind to inhibitory receptors and turn the neuron “off.”

      Whether or not the synapse is excitatory or inhibitory depends upon the type of neurotransmitter the axon puts out and also on the custom-made receptor, or lock, which is the part of the synapse poised to “receive” the neurotransmitter. If you imagine the neurotransmitter as a simple geometric shape, say a square or a circle, the specific receptor for that “flavor” of neurotransmitter will have the complementary shape in order to make a perfect fit. Just as “you can’t put a square peg in a round hole,” these neurotransmitter “keys” will fit into only the perfect receptor “locks.” This helps the synapse not confuse messages. In addition to the near-perfect pairing of neurotransmitters to receptors, another way the signal is kept clean is that the astrocyte helper cells immediately clean up any leftover neurotransmitter hanging around after it gets released. This happens in milliseconds, as the timing of these signals between brain cells has to be rapid, sharp as a burst.

      Once the neurotransmitter has bound and locked itself into the receptor on the receiving neuron, this pairing sets off a chain reaction. Inside the dendritic side of the synapse, there are lots of proteins that rush to work when the synapse gets excited or inhibited. The signal needs to get down the dendrite to the cell body of the neuron, where it sends a positive charge for an excitatory signal or a negative charge for an inhibitory signal. Depending on which charge is sent, the receiving neuron will get a message to either stop or start functioning. If the message is positive, the receiving neuron will send the information down its own axon and across another synaptic cleft, and so on. A neuron can have up to ten thousand synapses and can send a thousand electrical impulses every second. In one-tenth of the time it takes to blink your eyes, a single neuron can simultaneously send a signal to hundreds of thousands of other neurons.

      Some of the most common excitatory neurotransmitters are epinephrine, norepinephrine, and glutamate. Inhibitory neurotransmitters, like gamma-aminobutyric acid (GABA) and serotonin, act as antianxiety nutrients, calming the body and telling it to slow down. A lack of serotonin can result in aggression and depression. Dopamine is a special neurotransmitter because it is both excitatory and inhibitory. It is also, along with epinephrine and several others, a hormone. When it acts on the adrenal glands, it is acting hormonally; when it acts in the brain, it is a neurotransmitter. As a brain chemical messenger, dopamine helps motivate, drive, and focus the brain because it is integral to the brain’s reward circuitry. It’s the “I gotta have it” neurochemical that not only reinforces goal-directed activity but also can, in certain circumstances, lead to addiction. The more dopamine that is released in the brain, the more the reward circuits are activated, and the more those circuits are activated, the bigger the craving. It doesn’t matter if the craving is at the dinner table or the card table, in the boardroom or the bedroom. For instance, scientists know that high-calorie foods produce more dopamine in the brain. Why? Because higher calories increase our chance for survival. When we crave ice cream or gambling or sex, we may not actually be craving sweets, money, or orgasms. We’re craving dopamine.

      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