Welcome to Your Brain Read Online Free PDF Page B

“They’re Made Out of Meat,” Terry Bisson describes alien beings with electronic
    brains who discover a planet, Earth, on which the most sophisticated organisms do their thinking with
    living tissue. The aliens refer to brains as “thinking meat.” (Gross, we know.) The idea that your
    brain can generate dreams, memory, breathing, and every mental process in your life may seem hard
    to believe—but it’s true.
    This is particularly impressive in view of the brain’s size. Considering its many functions, the
    brain is packed into a very small space. Billions of neurons and additional supporting cells
    communicate with one another using an astronomical number of synaptic connections—and the entire
    operation fits into an object weighing about three pounds, the size of a small cantaloupe.
    Like a cantaloupe—and the rest of your body—your brain is made of cells. Brain cells come in
    two types: neurons, which talk to one another and to the rest of the body, and glial cells, which
    provide essential support to keep the whole show going. Your brain is made up of about one hundred
    billion neurons—which have a long, skinny, complicated shape—and many more glial cells. From a
    distance, the brains of different animals do not look alike. (Compare the shrew and whale brains in
    the picture.) They all work according to the same principles, however.
    Signals within a neuron are carried by electricity. Each neuron has a net excess density of
    negative charge on the inside of the membrane that surrounds it relative to the outside, due to an
    uneven distribution of positive and negative ions like potassium and chloride. This unequal
    distribution of charge creates a voltage difference across the membrane, like a much smaller version
    of the voltage difference that allows a nine-volt battery to give a shock to your tongue. (Actively
    moving ions across the membrane to maintain this charge distribution requires more energy than
    anything else that the brain does.)
    To send electrical signals from one part of the neuron to another, the neuron opens channels that
    allow the ions to move across the membrane, creating a current that carries an electrical signal down
    the membrane. Neurons receive inputs through branched, treelike structures called dendrites, which
    put together information from a bunch of different sources. The neuron then sends an electrical signal
    down a long, wirelike structure, called an axon, which triggers a chemical signal to another neuron,
    and so on. Axons can carry signals over long distances; your longest axons run from your spine to the
    tips of your toes. In contrast, the longest known axons in whales are sixty feet (about twenty meters) in
    length. The longest axons belonging to the shrew, whose brain is pictured on the penny, are a mere
    two inches (about five centimeters). In all cases, electrical signals spread using similar molecules
    and according to the same biological principles.
    Did you know? Your brain uses less power than your refrigerator light
    Neurons and synapses are so efficient that the brain uses only twelve watts of power—
    yet it can do a lot more than the little light in the back of your refrigerator. Over the course
    of a day, your brain uses the amount of energy contained in two large bananas. Curiously,
    even though the brain is very efficient compared to mechanical systems, in biological terms,
    it’s an energy hog. The brain is only 3 percent of the body’s weight, but it consumes one-
    sixth (17 percent) of the body’s total energy. Unfortunately, that doesn’t mean that you
    should snack more to keep your energy up when you’re studying. Most of the brain’s energy
    costs go into maintenance, keeping you ready to think by maintaining the electric field
    across each neuron’s membrane that allows it to communicate with other neurons. The
    added cost of thinking hard is barely noticeable. Look at it this way: you’re always paying
    to support your brain, so you