Phantoms in the Brain: Probing the Mysteries of the Human Mind Read Online Free PDF

from the convoluted outer layer called the neocortex and peer at it under a microscope, 12

    you'll see that it is composed of neurons or nerve cells—the basic functional units of the nervous system, where information is exchanged. At birth, the typical brain probably contains over one hundred billion neurons, whose number slowly diminishes with age.
    Each neuron has a cell body and tens of thousands of tiny branches called dendrites, which receive information from other neurons. Each neuron also has a primary axon (a projection that can travel long distances in the brain) for sending data out of the cell, and axon terminals for communication with other cells.
    If you look at Figure 1.1, you'll notice that neurons make contacts with other neurons, at points called synapses. Each neuron makes anywhere from a thousand to ten thousand synapses with other neurons. These can be either on or off, excitatory or inhibitory. That is, some synapses turn on the juice to fire things up, whereas others release juices that calm everything down, in an ongoing dance of staggering complexity. A piece of your brain the size of a grain of sand would contain one hundred thousand neurons, two million axons and one billion synapses, all "talking to" each other. Given these figures, it's been calculated that the number of possible brain states—the number of permutations and combinations of activity that are theoretically possible—exceeds the number of elementary particles in the universe. Given this complexity, Figure 1.2 Gross anatomy of the human brain, (a) Shows the left side of the left hemisphere. Notice the four lobes: frontal, parietal, temporal and occipital. The frontal is separated from the parietal by the central or rolandic sulcus (furrow or fissure), and the temporal from the parietal by the lateral or sylvian fissure, (b) Shows the inner surface of the left hemisphere. Notice the conspicuous corpus cal−losum (black) and the thalamus (white) in the middle. The corpus callosum bridges the two hemispheres, (c) Shows the two hemispheres of the brain viewed down the top. (a) Ramachandran; (b) and (c) redrawn from Zeki, 1993.
    how do we begin to understand the functions of the brain? Obviously, understanding the structure of the nervous system is vital to understanding its functions5—and so I will begin with a brief survey of the anatomy of the brain, which, for our purposes here, begins at the top of the spinal cord. This region, called the medulla oblongata, connects the spinal cord to the brain and contains clusters of cells or nuclei that control critical functions like blood pressure, heart rate and breathing. The medulla connects to the pons (a kind of bulge), which sends fibers into the cerebellum, a fist−sized structure at the back of the brain that helps you carry out coordinated movements. Atop these are the two enormous cerebral hemispheres—the famous walnut−shaped halves of the brain. Each half is divided into four lobes—frontal, temporal, parietal and occipital—that you will learn much more about in coming chapters (Figure 1.2).
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    Each hemisphere controls the movements of the muscles (for example, those in your arm and leg) on the opposite side of your body. Your right brain makes your left arm wave and your left brain allows your right leg
    to kick a ball. The two halves of the brain are connected by a band of fibers called the corpus callosum. When this band is cut, the two sides can no longer communicate; the result is a syndrome that offers insight into the role each side plays in cognition. The outer part of each hemisphere is composed of cerebral cortex: a thin, convoluted sheet of cells, six layers thick, that is scrunched into ridges and furrows like a cauliflower and packed densely inside the skull.
    Right in the center of the brain is the thalamus. It is thought to be evolutionarily more primitive than the cerebral cortex and is often described as a "relay station" because all sensory