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

was that while his seizures were by and large gone, so, too, was his ability to turn short-term memories into long-term memories. Essentially, H.M. could remember his past—everything before the time of the operation—but for the rest of his life he had no short-term memory and could not remember what happened to him, what he said or did or thought or felt or whom he met, in the decades following the surgery. H.M.’s loss, as often happens in the history of science, was neuroscience’s gain. For the first time researchers could point to a specific brain region (the temporal lobe) and brain structure (the hippocampus) as the seat of human memory.

      Next door to the hippocampus, in another part of the limbic system under the temporal lobe, is another key brain structure, the amygdala, which is involved in sexual and emotional behavior. It is very susceptible to hormones, such as sex hormones and adrenaline. It is sort of the seat of anger, and when stimulated in animal experiments, it has been shown to produce rage-like behavior. The limbic system can be thought of as a kind of crossroads of the brain, where emotions and experiences are integrated.

      A slightly unbridled and overexuberant immature amygdala is thought to contribute to adolescent explosiveness; this explains in part the hysteria that greets parents when they say no to whatever it is their adolescent thinks is a perfectly reasonable request. Cross that immature amygdala with a teen’s loosely connected frontal lobe, and you have a recipe for potential disaster. For example, the sixteen-year-old patient of a colleague of mine was so incensed when his parents said driving was a “privilege” (for which he did not yet qualify), and not a “right,” that he stole the car keys and drove away from the house. He didn’t get very far, though. He forgot the garage door was closed and plowed right through it. One of my colleagues also told me that, because he himself had three grown daughters, rather than sons, he had few “terrible teen tales” to tell. Then he reconsidered: “Oh, yes, there was the weekend we were away and the ‘couple of friends’ became a party that got out of hand, including the raid on our wine cellar, a minor fender bender with our stolen liquor in the trunk, and maybe a navel ring (which I never knew about until years later after it disappeared). But all’s well that ends well.”

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       Under the Microscope

      If you pick out any random region of brain and look at it under a microscope, you’ll find it jam-packed with cells. In fact, there is almost no space between the billions of cells in the brain. Evolution made sure of that, putting to use every cubic micron wisely. A cell is the body’s smallest unitary building block, and each has its own command center, called a nucleus, a large oval body near the center of the cell. There are more than two hundred different types of cells making up every organ, tissue, muscle, etc. A unique cell type in the brain is the neuron. This is a cell we will talk about frequently in this book. Thoughts, feelings, movements, and moods are nothing more than neurons communicating by sending electrical messages to one another.

      I remember my first time looking at brain cells under a microscope. In the mid- to late 1970s the only way to study changes in neurons, for instance the changes that occur during learning, was by looking through a microscope at individual cells over a given period of time. Today, we have amazing tools—brain imaging scans and specialized microscopes—that allow us to look into the brain and see cells and synapses change in real time. If you are learning something right now, as you read this, your neurons will change in about fifteen minutes, creating more synapses and receptors. Changes start within milliseconds of learning something new, and can take place over a period of minutes and hours. When I look at brain cells under a microscope, I think of the billions of neurons that are interconnected and how we’re still trying to figure out the wiring. What we know now is that no two human brains are wired exactly the same, and experience shapes us all differently. It’s the final frontier, our own internal frontier, and we’re just now beginning to see all the patterns.

      There are 100 billion neurons in the human brain and you could place about 30,000 of them on the head of a pin, but placed end to end the neurons in just one person’s cortex would stretch for 100,000 miles—enough to circle the globe four times. At birth, we have more neurons than at any other time in our life. In fact, our brains are at their densest before birth, between the third and sixth months of gestation. Dramatic pruning of much of that gray matter occurs in the last trimester and first year of life. Still, by the time a baby is born, he or she has a brain brimming with neurons. Why? An infant’s overabundance of neural cells is needed to respond to the barrage of stimuli that comes with entering into the world. In response to all those new sights, sounds, smells, and sensations, neurons branch out in the baby’s brain, creating a thick forest of neural connections. So why aren’t all babies tiny Mozarts and Einsteins? Because when we are born, only a very small percentage of that overflow of neurons is wired together. The information is going in, being absorbed by the neurons, but it doesn’t know where to go next. Like someone plunked down in the middle of a strange and bustling metropolis, the infant brain is surrounded with possibilities and yet has no map, no compass, to navigate this strange new world. “All infants are born in a state of psychedelic splendor similar to an acid trip” is how Daniel Levitin, a neuroscientist at McGill University in Montreal, Canada, colorfully describes it.

      A neuron responds to a stimulus with a burst of activity, called an action potential, which is actually an electrical signal that passes, in relay fashion, from the point of contact with the stimulus down the receiving limb of the neuron, called the dendrite, through the cell.

      When we see the color red, smell a rose, move a muscle, or remember someone’s name, action potentials are happening.

      FIGURE 6. Anatomy of Neuron, Axon, Neurotransmitter, Synapse, Dendrite, and Myelin: Signals between cells flow in one direction, from an axon to a dendritic spine, through a synapse. Axons with myelin coating transmit signals faster than those without. At the synapse, a transfer of neurotransmitter molecules binds to the synaptic receptor on the spine.

      If you think of each neuron’s cell body as a point in a relay, there must be an incoming and an outgoing signal. Once the outgoing signal reaches the axonal bouton, or end point, it sets off a reaction causing the bouton to release packages of chemical messengers, called neurotransmitters. The point of contact between two neurons is called a synapse, and is actually a space no more than two-millionths of an inch wide. The synapse is truly where the action takes place in the brain. The signal heads down the neuron through the axon to the synapse and is then released as a chemical message. Like liquid keys, these neurotransmitters cross the synapse and lock onto the neuron on the other side, and in this way carry information from one cell to another. Once opened, the receptor causes a chain reaction of signals going down the receiving cell, triggering a pulse, or an action potential, which travels from a dendrite and through the cell body and out the axon toward another cell.

      In order for neurons to survive, they need helper cells called glia. There are several types of glia: astrocytes, microglia, and oligodendrocytes. To put it simply, the astrocytes defend the neuron by helping to nourish it and cleaning up the unwanted chemicals around it. This helps keep the brain’s neurons at optimal functioning level. The microglia are tiny cells that move around the neuron and really activate when there is an infection or inflammation—they move through brain tissue to the site of action to fight these injuries, like an army-in-waiting. But because the brain is efficiently designed, microglia also have an everyday purpose, a kind of housekeeping duty, so that even when they are not activated, they are still helping maintain the health and well-being of the synapses. Oligodendrocytes are the cells that make the myelin that goes around the axon of neurons. These cells are tightly packed in the white matter, wrapping whitish-colored myelin around axons to insulate them, much like rubber around an electrical cord, allowing faster speeds of signal transmission down the axon.

      While you are born with the vast majority of your neurons, most of the synapses in the cortex are not fully formed. In lower areas, like the brainstem, synapses