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Your Brain: The Missing Manual by Matthew MacDonald

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Chapter 1. A Lap Around the Brain

For most of this book, you’ll focus on what your brain does, and pay less attention to its plumbing. It’s not that the brain lacks interesting hardware. But you can easily spend a lifetime studying your brain’s biological workings without having the faintest idea why your company laid you off, your spouse ran off with another lover, and your dreams are filled with gorillas in tuxedos serving you shrimp cocktails.

To get practical information that can help with life’s day-to-day challenges, you need to concentrate on your brain’s software—in other words, the thoughts, emotions, and higher-level processes that are endlessly at work in your squishy gray matter. In this book, you’ll explore these phenomena closely. But, before you get started, there are a few underlying details to get out of the way. You need a crash-course in brain basics.

In this chapter, you’ll take a quick tour to see what your brain looks like and how it’s structured. You’ll take a close look at neurons—the tiny wires that convey electrical signals in your brain—and find out how your brain plugs into the rest of your body. Along the way, you’ll dispel a few myths about the brain, peer into its evolutionary history, and learn a few of the secrets of mental health.

A First Look at Your Brain

It’s time to meet your brain.

Lurking in the space between your ears is a very soft, reddish, jelly-like organ. (If you were expecting your brain to be firm and deep grey, like a wrinkled walnut, you are no doubt thinking of a preserved brain. The living brain is much squishier, and it’s covered in deep red arteries.)

Figure 1-1. 

The average human brain weighs in at about three pounds. By comparison, an elephant’s brain tips the scale at 11 pounds while a cat’s brain—brace yourself, cat lovers—is a mere ounce. Bigger animals tend to have bigger brains, and some scientists suggest that a high brain-to-body weight ratio distinguishes the smart species from the dullards. In other words, the larger the brain is as a percentage of body weight, the smarter the creature. This calculation puts a few of our favorite animals at the top of the list (like dolphins and chimpanzees), but it needs a bit of fudgery to deal with really small animals (like birds and mice), which would otherwise appear to be raging geniuses.


You can check out the brain weight of your favorite animal at http://faculty.washington.edu/chudler/facts.html.

Of course, size isn’t everything. Although all mammals have some strikingly similar brain hardware (and, to a lesser extent, so do all creatures that have any sort of brain), there are key anatomical differences. To really understand your brain, you need to dig deeper.

Figure 1-2. 

The Brain: An Archeological Site

Much as archaeologists examining an ancient site often find the ruins of multiple cities, each built on top of the previous one, neuroscientists peering into the brain find newer biological hardware built over the old stuff. In this section, you’ll get the chance to peel back the layers.

The human brain is, like all the products of evolution, a work-in-progress. Although we won’t see the human brain change in our lifetimes, millions of years of evolution have left their fingerprints all over it. Here’s what’s been happening:

  • The human brain has grown, becoming physically larger. In fact, there’s a strong case that humans suffer far more pain giving birth than almost any other animal because of our comparatively huge heads, which we need to carry around our outsized brains.

  • Existing brain hardware has been adapted for different uses. The human brain is remarkably flexible. In deaf children, it can assign brain parts normally used for hearing to other tasks, like understanding sign language. In blind children, the brain can recruit the speech processing regions to interpret the tactile sensation of Braille letters. Over millions of years, similar but more profound shifts can occur. For example, many researchers believe that human speech hijacked some serious brain space in our early ancestors, and crowded out other skills.

  • New features have been bolted on top of old ones. It’s much easier for evolution to change what’s already there than create a whole new brain from scratch. That means there’s some deep, dark animal ancestry in your brain. If evolution were a building contractor, you’d find it leaving a few frightening things in the basement.

In the following section, you’ll slice open your brain (metaphorically speaking) and get a closer look.


No one knows why big-brained humans won the evolutionary arms race. Although it’s tempting to conclude that smarter humans could build better tools (and therefore catch more nutritious animals), the brain has a significant evolutionary disadvantage—it’s a hugely expensive energy hog. One of the more likely explanations for our success is that bigger human brains helped us attract mates and negotiate sticky group dynamics. In other words, we’re all the descendants of a few sexy nerds.

The Outer Wrapper

The outer layer of your brain is the cerebral cortex. It powers conscious perception, abstract reasoning, speech, and creativity. It also accounts for over two thirds of your brain’s weight.

Figure 1-3. 

Although the cerebral cortex looks like a heavily wrinkled cauliflower, it’s more like a crumpled sheet of paper. Its deep grooves and bulges allow the brain to cram in many more neurons than the less wrinkled brains you’ll find in other animals. If you were able to stretch your cerebral cortex out flat on your lap, you’d find that it has about the same surface area as a page of newsprint from the New York Times. However, it’s a bit thicker, and doesn’t offer nearly as good a read.


The previous picture shows the brain split down the middle. This view makes it clear that the cerebral cortex wraps along the top, front, and back of your brain. What the figure doesn’t show is how the cerebral cortex also wraps around the sides of your brain. (To see the outside view of your brain, which is nearly all cerebral cortex, see A First Look at Your Brain.)

The Middle Ground

Under the cerebral cortex, you’ll find a set of older brain structures. These brain regions play a key role in memory (see Chapter 5) and emotional drives (such as the pleasure-seeking and pain-avoiding behavior you’ll learn about in Chapter 6).

Figure 1-4. 

Sometimes, these brain structures are grouped together into a ring-shaped region called the limbic system. However, these days many neuroscientists doubt that they actually make up a distinct system that’s separate from the rest of the brain. Instead, they prefer to examine each structure on its own merits.

The Basement

Buried deep inside your brain are its oldest structures, including the brainstem and the cerebellum.

Figure 1-5. 

The brainstem looks like little more than a glorified lump at the top of the spinal cord. It controls body functions that have little conscious control, like breathing, hunger (see Chapter 2), and body temperature. It also plays the role of a massive conduit by funneling all the signals that travel between your brain and your body.

At the back of the brainstem is a fist-size growth that looks like a miniature brain. This region is the cerebellum, and it coordinates balance and movement. New research also suggests that the cerebellum plays a support role for other, more complex tasks. One theory is that it coordinates different regions of the brain so they can perform their work more efficiently.

The History of the Brain

It’s common to describe the deepest regions of the brain as older, because these areas evolved first, in some distant species that was ancestor to us as well as many other modern creatures. For the same reason, we share these brain parts—in an extensively altered form—with other species. For example, bird and reptile brains appear to have a similar brainstem to ours but a vastly shrunken cerebral cortex. Orangutans, which are much closer relatives, have brains that are strikingly similar in almost all their component parts, but much smaller than ours.

It’s a bit fanciful to imagine (as some early brain theories did) that the different layers of our brains are continuously at war. However, it’s not so hard to picture a delicate balance between instinctive, ritualistic, and reactive behaviors that are rooted in the old brain systems and the morality, social sense, and problem solving that draw on the newer brain parts. In fact, it just might explain the paradox of a species that is equally at home in the symphony hall as it is on the field of war.

Ancient Brain, Modern World

From an evolutionary standpoint, the human brain is a relatively recent development, with its sudden increase in size and pumped-up cerebral cortex happening just a few hundred thousand years ago. However, from the perspective of an individual human like yourself, the human brain is unimaginably old. This poses some sticky challenges, because the brain’s survival strategies just aren’t designed for 21st-century living.

This combination of old brain and new world hints at two of the key themes you’ll explore throughout this book:

  • Your brain often works subconsciously. As renowned neuroscientist Joseph LeDoux puts it, consciousness and language are “new kids on the evolutionary block.” As the human brain evolved with its ever-expanding cerebral cortex, it became able to perceive, describe, and reflect on its own actions—many of which are unconscious and nonverbal. So don’t be surprised when you find that your brain does many things without your consent, and many more without your realization. You may be able to understand what’s taking place in the basement of your brain, but you can’t always control it.

  • Your brain’s logic doesn’t always serve you well. Every dieter knows that the brain’s built-in circuitry can lead to trouble when confronting a larger-than-life billboard for the nearest fast-food chain. The problem here is that the brain has been honed by millions of years of evolution to be the perfect tool—for wandering groups of hunter-gatherers in the African savannah. For our ancestors, a good meal was hard to come by. But in the modern world where rich, nutrient dense foods are plentiful, the brain’s natural response (“Eat Now!”) can cause more harm than good. Similarly, it may be that certain brain disorders (say, obsessive-compulsive disorder) and some less-than-pleasant aspects of a properly functioning brain (like stress and nightmares) are the result of hardwired circuitry in older regions of the brain.

As you’ll see in this book, your brain includes built-in circuitry that makes office politics seem like a life-or-death struggle (Chapter 6), tosses important facts out of your memory if they aren’t charged with emotion (Chapter 5), and urges you to eat waistband-defying amounts of high-calorie snacks (Chapter 2). Sometimes, you can learn to compensate for your brain or work around its limitations. Other times, you’ll be forced to accept its eccentricities.


Evolution is a powerful brain-shaping force, but it’s slow. It’s a little bit like Microsoft asked you to create the world’s most fantastic accounting software, you whipped it up, took a vacation for 100,000 years, and then came back with the package. Your program might still do the job, but it wouldn’t be ideal.

The Brain’s Wiring

So far, you’ve looked at the brain’s shape, structure, and history. But you haven’t yet seen it in action.

You probably already know that the brain is an electrical appliance more complex than any circuit board. But the brain also communicates with chemicals, using tiny compounds to transmit information, control mood, and interact with the rest of the body. Once you understand a few facts about your brain’s wiring system, you’ll have an easier time tackling some of the more sophisticated topics in this book.


Your brain holds hundreds of billions of nerve cells. These cells come in two flavors: neurons (which get all the attention) and glial cells (which play an essential but often-overlooked supporting role).

Neurons carry electrical signals through your brain, and through the rest of your body. Estimates range, but the most widely cited calculations suggest that you have 100 billion neurons. (If you need an ego boost, compare that with the 300,000 neurons in the brain of the humble fruit fly.) Amazingly, there are at least 10 times as many glial cells, which provide nourishment, protection, waste disposal, speed enhancement (see Wiring the Brain), and other support services for the spotlight-hogging neurons.

Here’s a look at a single neuron:

Figure 1-6. 

Up close and personal, a neuron looks like some form of futuristic vegetation. It receives messages through tree-like branches called dendrites. It then sends an electrical signal down a long tube-like structure called the axon. Add up the cumulative effect of several billion of these electrical impulses and you get a symphony, a treatise on law, or an episode of Buffy the Vampire Slayer.


This picture of the neuron isn’t proportionally accurate. In a real brain, the body of the cell (the top-left section of the picture) would be much smaller, while the dendrites, axons, and axon terminals (the branches at the end of the axon) would snake out far, far longer.


The real magic happens when an electrical signal reaches the end of a neuron. At this point the neuron releases a bundle of chemicals into a tiny gap called a synapse. These chemicals, known as neurotransmitters, drift through the synapse (essentially “swimming” in the fluid of your brain) until they reach the dendrite of another neuron. That neuron can then react by firing its own electrical signal. In this way, a message can ricochet through the human brain, passing from one neuron to another.

Figure 1-7. 

As you might expect, this description is a huge simplification of the messy reality taking place inside your cranium. Here are some of the reasons why the brain’s wiring system quickly becomes more complicated:

  • The brain uses different types of neurotransmitters, which affect different neurons. Estimates suggest that the brain is a chemical soup, using more than 100 different substances to communicate between neurons.

  • An average neuron connects to several thousand other neurons. That means thousands of neurons can simultaneously influence whether a single target neuron fires its signal. Similarly, one active neuron can pass its signal on to thousands more. It all adds up to a very flexible wiring system.

  • Neurotransmitters don’t just trigger a neuron to fire. They can also inhibit a neuron from firing.

  • Neurotransmitters don’t just carry signals between neurons. They can also act as neuromodulators to perform a host of different tasks. For example, a neuromodulator can alter the way a neuron works, change its sensitivity, trigger the creation of new proteins, and drift out of the tiny synaptic gap to affect entire regions of the brain. Many compounds in the brain act as ordinary message-carrying neurotransmitters in some situations, but behave like more powerful neuromodulators in others.


Neuromodulators may play a role in memory, learning, and mood control. For example, antidepressive drugs like Prozac work by increasing brain levels of serotonin, which can act as a neuromodulator. This change affects the way that billions of brain neurons work, in ways even the sharpest scientists don’t currently understand.

If you could scoop out a small lump of your jelly-like brain matter and examine it under the microscope, you’d find a dense thicket consisting of millions of neurons, with dendrites and axons crossing and interweaving in an impossibly tangled fabric. It’s estimated that the total number of connections between neurons (that is, the total number of synapses in the human brain) is in the eye-popping tens of trillions. It’s for this reason that the human brain is sometimes described as the most complex object we’ve ever discovered in our universe. You should feel flattered.

The Nervous System

Often, we think of the human brain as a single device—a sort of biological computer made out of water, fat, and DNA. But the brain is actually a multi-pronged organ whose influence extends far beyond the head. In fact, the long tentacles of dendrites and axons stretch right out of the brain and into nearly every corner of the human body, uniting every muscle and organ into a body-wide network called the nervous system.

So far, you’ve learned how neurons can pass information between themselves. But the neurons on the outskirts of the nervous system get their input from something else. Depending on the type of neuron, they may fire signals in response to changes in heat, pressure (used for the sense of touch and sound), chemicals (for taste and smell), or light (for vision). These signals are then ferried up through the spinal cord to the brain. For example, a touch on your toe runs through just two giant neurons to reach your brain.

Similarly, an outgoing chain of neurons lets your brain send messages to the far corners of your body. When your brain needs to exert its control over a body part—either consciously or unconsciously—it simply triggers the right combination of neurons. The last neuron in the chain triggers the release of a chemical that kicks off the desired body process in another cell.

For example, if you stub your toe while line dancing, nearby neurons detect the deformation of your skin. These neurons pass the message up to the brain, which interprets this electrical activity as head-slapping agony. Your brain then triggers the neurons that will jerk the foot away. The last neurons in this sequence release a neurotransmitter to some nearby muscle tissue, compelling your muscles to contract and move your leg.

Of course, the low-level story is far more detailed. Even the simplest response involves many different neurons. For example, as you jerk your leg away using one group of muscles, you brain needs to relax another muscle group to prevent injury. Furthermore, the nervous system reacts to many different types of neurons in the same area of the body. This is one of the reasons that humans are “blessed” with so many types of pain. The dull ache of damaged tissue is picked up by a neuron that reacts to chemical changes, the flash of pain from a burn is triggered by neurons that react to extremely high heat, the sting of a cut is caused by neurons that react to the incision, and so on.


The neurons that transmit different sensations take different pathways in the spinal column and sometimes travel with different speeds. Throbbing pain is the slowest, which is why you have a brief moment to contemplate the pain you’re about to feel after smacking your toe on a door jamb.

The Endocrine System

As you’ve learned, your brain pulls all the strings. It controls a vast range of body processes simply by signaling the right neurons. However, neurons don’t stretch everywhere, and they aren’t nuanced enough to take every interaction into account. For that reason, your brain has another system that allows it to control the body—the endocrine system.

The endocrine system consists of a group of small organs known as glands. These glands work their magic by secreting various chemicals (called hormones) into your bloodstream. These hormones trigger reactions in other body parts. For example, the thyroid gland controls the speed of your metabolism. The adrenal gland controls the “fight or flight” response—it fires you up into a state of acute stress when an SUV steals the mall’s last parking spot on Christmas Eve.

Figure 1-8. 

Much as the brain rules the nervous system, it’s also the master of the endocrine system. Its key control mechanism is the pituitary gland.

The Pituitary Gland

To communicate with the glands in your body, the brain needs to release hormones into your blood. Its job is complicated by a defensive wall called the blood-brain barrier, which separates your brain from your bloodstream. The blood-brain barrier prevents most toxins, bacteria, viruses, and hormones from passing into the brain. The only substances that can pass through are extremely small ones, or ones that are soluble in fat. Fortunately, oxygen, alcohol, and caffeine make the cut. Other compounds need to be ferried across by specialized transporters. (One example is glucose, the sugar molecule that supplies energy to your brain.)

Much as the blood-brain barrier locks substances out of your brain, it also prevents substances from passing from your brain into your blood. To get around this limitation, the brain uses a built-in hormone dispenser called the pituitary gland. This pea-sized gland hangs out of the bottom of the brain, allowing it to slip hormones into your bloodstream whenever your brain gives the signal.


The pituitary gland is often called the master gland, because it releases the hormones that tell the other glands (like the thyroid and adrenal glands) what to do. In this way, your brain can use the pituitary gland to exact precise control over the state of your body.

Although you may have vaguely heard about the pituitary gland before, it’s already had a profound effect on your life. The brain uses the pituitary gland to release life-changing hormones at key points in your life. These hormones trigger growth and sexual development (as you’ll see in Chapter 9), the contractions of birth, and milk production. Clearly, your brain is in charge of a lot more than you might have expected.

Incidentally, the part of the brain that controls the pituitary gland is the hypothalamus. You’ll meet the hypothalamus several times in this book, starting in Chapter 2 (The Secret Gears of Appetite).

Mental Fitness

You’ve now completed your first tour of the brain. Although you don’t yet know all the reasons for the peculiar behavior of the planet’s dominant species, you now have some of the tools that you can use to start asking the right questions. This makes it a good time to take a step back and change focus from low-level biology to more general guidelines. In this final section, you’ll consider how you can keep your mental machine running in tip-top shape through the decades.

First, it’s important to realize that the solution isn’t to grow a bigger brain. After birth, it’s rare for new neurons to appear in the brain. In fact, the story of the brain’s development (which is told in Chapter 10) is largely the story of neurons and synapses dying off in waves as your body lumbers into old age. But don’t panic yet. There’s good reason to think that the loss of a few million neurons over the years is no big deal. In fact, it just might be part of the brain’s natural housekeeping.

Rather than count the number of neurons in your head, it’s more important to take note of the connections between them. As you’ve already learned, neurons are constantly being rewired. In healthy brains, the ratio of synapses to neurons grows as the number of neurons declines. In other words, leaner brains can become more efficient to compensate for their loss of neurons.

So what can you do to keep your brain in its best working form? There may be no way to dodge bad genes, bad luck, injury, and disease, but studies of brain aging consistently identify a few characteristics in old-aged but nimble-brained people. Here are a few practical guidelines if you hope to become a quick-witted fast-talking 90-year-old cribbage shark:

  • You are what you do. The brain is constantly rewiring the connections between your neurons, strengthening the ones you use and weakening the ones that you don’t. In other words, when you spend a day munching Cheetos, watching American Idol reruns, and lamenting the tragedy of your life, you aren’t just whiling away the time. You’re also training your brain to be a better Cheetos-eater, TV watcher, and chronic worrier. Fall into this pattern for a few years, and your brain just won’t look the same.

  • Use it or lose it. The brain may not be a muscle, but there’s good evidence that the human body doesn’t waste effort maintaining mental hardware that you never use. Surprisingly, it seems that it’s never too late to ramp up your thinking. Many studies suggest that suddenly giving your brain more to do, even late in life, can overcome recent brain decline and foster broad, long-term improvements.


    There’s no magic brain-honing activity. But broad, integrative tasks like studying a new language, learning to play a musical instrument, changing jobs, writing a book, picking up a new hobby, and planning the perfect crime are all good prospects.

  • Embrace something different. The brain craves novelty. The best way to keep your brain stimulated is to activate as much of it as often as you can. There’s a fun side to this advice (“Indulge your curiosity!” “Engage strangers in long conversations!”), and a more challenging side (“Turn off the TV and learn differential calculus!”). The bottom line is that most of the time, the human body craves dull and easy stability. However, the brain thrives with constant challenges, tricky concepts, extreme concentration and, well, work.


    Don’t think you can hone your brain with all-night Sudoku marathons. After the first 100 boards, your brain will have adapted itself to the patterns and strategies of the game, and will be able to polish off a board with far less neural work. On the one hand, this is a welcome development—after all, smart people use less activity for things they’re good at. However, if your goal is to keep your brain strong, repeating the same type of challenge over and over again is no different than training with baby weights. For the maximum benefit, do something difficult and do something different.

  • Exercise the body to help the mind. Studies suggest the keenest old brains have owners who exercise regularly. The best bet seems to be modest aerobic exercise, such as a daily jog or brisk walk. It’s unclear why this helps, although it could well be that exercise stimulates other body processes that benefit the brain.

  • It wouldn’t hurt to strum a tune. The popular media is filled with tantalizing studies suggesting that a bit of music listening or music making can boost test scores and cultivate a baby genius. The truth is that the human brain is unlikely to respond to a magic music pill. However, exposing your brain to as many different influences as possible is always a surefire way to promote its development. Learning music as a discipline—in other words, as something to read, play, or improvise—is likely to draw on regions of the brain that are left dormant through the rest of your day-to-day life. (That said, if you’re already an accomplished musician, your brain has long-ago transformed the challenging problems of making music into deeply ingrained neural patterns that take little effort. As a result, you’ll get more brain stimulation by taking up accounting.)

  • Give your brain good food and rest. Don’t forget food and sleep. You’ll learn all you need to know about brain-healthy eating and sleeping in the next two chapters.

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