For all its powers, nothing about your brain is distinctively human. We use exactly the same components—neurons, axons, ganglia, and so on—as a dog or hamster. Whales and elephants have much larger brains than we have, though of course they also have much larger bodies. But even a mouse scaled up to the size of a human would have a brain just as big, and many birds would do even better. It also turns out that the human brain is a little less imposing than we had long assumed. For years, it was written that it has 100 billion nerve cells, or neurons, but a careful assessment by the Brazilian neuroscientist Suzana Herculano-Houzel in 2015 found that the number is more like 86 billion—a pretty substantial demotion.
Neurons are not like other cells, which are typically compact and spherical. Neurons are long and stringy, the better to pass on electrical signals from one to another. The main strand of a neuron is called an axon. At its terminal end, it splits into branch-like extensions called dendrites, as many as 400,000 of them. The tiny space between nerve cell endings is called a synapse. Each neuron connects with thousands of other neurons, giving trillions and trillions of connections—as many connections “in a single cubic centimeter of brain tissue as there are stars in the Milky Way,” to quote the neuroscientist David Eagleman. It is in all that complex synaptic entanglement that our intelligence lies, not in the number of neurons, as was once thought.
What is surely most curious and extraordinary about our brain is how largely unnecessary it is. To survive on Earth, you don’t need to be able to write music or engage in philosophy—you really only need to be able to outthink a quadruped—so why have we invested so much energy and risk in producing mental capacity that we don’t really need? That is just one of the many things about your brain that your brain won’t tell you.
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As the most complex of our organs, the brain not surprisingly has more named features and landmarks than any other part of the body, but essentially it divides into three sections. At the top, literally and figuratively, is the cerebrum, which fills most of the cranial vault and is the part that we normally think of when we think of “the brain.” The cerebrum (from the Latin word for “brain”) is the seat of all our higher functions. It is divided into two hemispheres, each of which is principally concerned with one side of the body, but for reasons unknown the wiring is crossed, so that the right side of the cerebrum controls the left side of the body and vice versa. The two hemispheres are connected by a band of fibers called the corpus callosum (meaning “tough material” or literally “calloused body” in Latin). The brain is wrinkled by deep fissures known as sulci and ridges called gyri, which give it more surface area. The exact pattern of grooves and ridges in brains is distinctive to each individual—as distinctive as your fingerprints—but whether it has anything to do with your intelligence or temperament or anything else that defines you is unknown.
Each hemisphere of the cerebrum is further divided into four lobes: frontal, parietal, occipital, and temporal—each broadly specializing in certain functions. The parietal lobe manages sensory inputs like touch and temperature. The occipital lobe processes visual information, and the temporal lobe principally manages auditory information, though it also helps with processing visual information. It has been known for some years that six patches on the temporal lobe, known as face patches, become excited when we look at another face, though which parts of my face excite which of your patches is still largely uncertain, it seems. The frontal lobe is the seat of the higher functions of the brain—reasoning, forethought, problem solving, emotional control, and so on. It is the part responsible for personality, for who we are. Ironically, as Oliver Sacks once noted, the frontal lobes were the last parts of the brain to be deciphered. “Even in my own medical student days, they were called ‘the silent lobes,’ ” he wrote in 2001. That’s not because they were thought to lack functions but because those functions do not reveal themselves.
Beneath the cerebrum, at the very back of the head about where it meets the nape of the neck, is the cerebellum (Latin for “little brain”). Although the cerebellum occupies just 10 percent of the cranial cavity, it has more than half the brain’s neurons. It has a lot of neurons not because it does a great deal of thinking but because it controls balance and complex movements, and that requires an abundance of wiring.
At the base of the brain, descending from it rather like an elevator shaft connecting the brain to the spine and the body beyond, is the oldest part of the brain, the brain stem. It is the home of our more basic operations: sleeping, breathing, keeping the heart going. The brain stem doesn’t get a lot of attention in the popular consciousness, but it is so central to our existence that “brain-stem death” is the fundamental measure of deadness in humans in the United Kingdom.
Scattered through the brain rather like nuts in a fruitcake are many smaller structures—hypothalamus, amygdala, hippocampus, telencephalon, septum pellucidum, habenular commissure, entorhinal cortex, and a dozen or so others—which are collectively known as the limbic system (from the Latin limbus, meaning “peripheral”). It’s easy to go a lifetime without hearing a word about any of these components unless they go wrong. The basal ganglia, for instance, play an important part in movement, language, and thought, but it is only when they degenerate and lead to Parkinson’s disease that they normally attract attention to themselves.
Despite their obscurity and modest dimensions, the structures of the limbic system have a fundamental role in our happiness by controlling and regulating basic processes like memory, appetite, emotions, drowsiness and alertness, and the processing of sensory information. The concept of the limbic system was invented in 1952 by an American neuroscientist, Paul D. MacLean. Not all of today’s neuroscientists agree that the components form a coherent system. Many think they are just lots of disparate parts connected only by the fact that they are concerned with bodily performance rather than with thinking.
The most important component of the limbic system is a little powerhouse called the hypothalamus, which isn’t really a structure at all but just a bundle of neural cells. The name describes not what it does but where it is: under the thalamus. (The thalamus, meaning “inner chamber,” is a kind of relay station for sensory information and is an important part of the brain—there isn’t any part of the brain that isn’t important, obviously—but is not a component of the limbic system.) The hypothalamus is curiously unimposing. Though only about the size of a peanut and weighing barely a tenth of an ounce, it controls much of the most important chemistry of the body. It regulates sexual function, controls hunger and thirst, monitors blood sugar and salts, decides when you need to sleep. It may even play a part in how slowly or rapidly we age. A large measure of your success or failure as a human being is dependent on this tiny thing in the middle of your head.
The hippocampus is central to the laying down of memories. (The name comes from the Greek for “sea horse” because of its supposed resemblance to that creature.) The amygdala (Greek for “almond”) specializes in handling intense and stressful emotions—fear, anger, anxiety, phobias of all types. People whose amygdalae are destroyed are left literally fearless, and often cannot even recognize fear in others. The amygdala grows particularly lively when we are asleep, and thus may account for why our dreams are so often disturbing. Your nightmares may simply be the amygdalae unburdening themselves.*2