Consider the human brain: Somehow, its several billion invisible cells serve to produce emotions, thoughts, and dreams. The deeply wrinkled surface of the brain's identical halves contains the substance that makes a mind, able to soar on the notes of a bird or the colors of a sunrise, suffer the deepest anguish from pain or loss, or simply be bored. Millions of tiny changes in molecular currents within the brain, each instant, create all of these emotions, and can lead a person to seek the source of all things in the universe.
While humans, the great apes, and perhaps the whales, dolphins, and elephants, have the most complicated brains, all mammalian brains share similar structures. As animals evolved more complex behaviors, their brains tended to add outer layers to primitive core structures. Thus, a rat has a basic mammalian brain structure that monkeys and humans share, but monkeys and humans have more complicated coverings, containing added circuitry.
The Ancient Brain: The Limbic System
If you split open the two halves of the human brain, as you would split the halves of a walnut, the inner face would show a small part of what neuroscientists call the limbic system. Most of this limbic system lies hidden deep within each half, buried below the covering folds of the cortex, the outermost layer of the brain. The outer, cortex covering of the brain is the source of reasoning, neuroscientists have found, while the inner, limbic system is the source of reacting.
Reptiles have the beginnings of a limbic system, and all animals further along on the evolutionary tree share this reptilian heritage, but have built on it. Parts of the limbic system seem to respond in emotionally charged situations. Memories of an emotional nature are stored here as well.
Most knowledge of the limbic system was gained through experiments on animals. For example, in cats, scientists placed wires finer than a hair into the limbic structure called the amygdala (named for the Latin word for almond, because of its shape). By passing a small electric current through that wire, the very picture of rage could be created: a cat's fur would instantly rise up, its back would arch, and it would hiss and show its claws. Other cats, with the amygdala removed surgically, would stay placid no matter what.
Such experiments on animals gave hints on how the human brain works. Careful studies of people who had disease or accidents that affected specific parts of their brains confirmed what scientists had found in animals. For instance, people with epilepsy, a disorder involving uncontrolled electrical activity within the brain, had unusual rage reactions if the electrical storm took place within the temporal lobe. The temporal lobe is the part of the brain beneath the skull's temples that surrounds the amygdala.
One man who had epilepsy of the temporal lobe would pick up a kitchen knife and for no reason threaten his wife during one of his epileptic attacks. He had surgery to remove part of the temporal lobe, which successfully interrupted the abnormal brain activity. The man then no longer fell into these uncontrolled rages.
Many structures besides the amygdala make up the limbic system. These different regions of the limbic system are all clusters of the cell bodies of many millions of nerve cells. These nerve cells are also called neurons. Fibers leading from one set of clustered neurons make connections to other sets of neurons in the limbic system, and also to other parts of the brain. The covering of the human brain, known as the cerebral cortex, is complexly wrinkled and convoluted (having many folded, crumpled curves). This allows more surface area to pack in more cortical neurons. The neurons of the cortex carry out the rational thoughts that, in people, are able to govern emotions. Planning and thoughts based on situations recalled from the past can help suppress, or prevent, immediate emotional reactions generated in the limbic system.
A Basic Blueprint of the Brain
The two halves of the brain each contain regions that control the movement of the limbs and receive sensations from the skin, with each half of the brain responsible for the opposite side of the body. The brain also makes sense of reports from the sense organs in the head. Even special sensors in joints tell the brain how the body is arranged in space—that is, they register the body's posture. Unconsciously, the brain monitors the degree of tension in each muscle, and adjusts to allow coordinated movement. (A good deal of coordination is carried out by the cerebellum, the structure underneath the main part of the mammalian brain.)
The somatosensory area, which receives sensations, and the motor cortex, which sends out commands for moving the body's parts, spread down the middle sides of the cortex, midway between the front and back of the brain. In the back of the brain is an area called the optic cortex, which records and analyzes sights from the eyes. In between the optic cortex, and the somatosensory and motor cortex, are areas that govern hearing. Language comprehension is generated by a region near the hearing center. Producing speech is enabled by a place in the cortex near the motor area for the face.
More complex processes are carried out in other cortical regions, primarily in the temporal lobe (mentioned earlier) and in the frontal lobes of the cerebral cortex. These regions at the front end of the brain are involved in planning and judgment.
All of these areas in the cortex interact with the limbic system below the brain's surface. Deeper still in the core of the brain, joining the halves, are clusters of neurons providing the controls over breathing, heart rate, and other basic bodily functions. The structure called the thalamus, which is located between the brain's core and the limbic system, registers pain and relays painful messages to the limbic system and cortex.
The Electrical Nature of the Brain
The brain's cells, the neurons, work by tiny electric currents that in turn release molecules to relay a message to the next neuron. An example of these connections and relays is what would take place if you stub your toe. The pressure and resulting damage to the toe stimulates pain fibers, which are the endings of nerve cells. These fibers are a thousand times finer than a human hair. The pain fibers are entwined within the cells just below the surface of the skin. An electrical impulse is generated in these microscopic fibers, and travels to the main body of the pain-sensing neurons, which lie within the spinal cord.
The pain message then proceeds up along the even longer fiber coming out of these spinal, pain neurons, traveling toward the brain. The pain neurons thus have one fiber reaching to the skin that receives the pain impulse, called the dendrite, and another that runs through the spinal cord and delivers pain to the brain, the axon.
At the thalamus, the pain impulse reaches the pain fiber's axon terminal, and here, the electrical message becomes chemical. At this special ending of the neuron, the current causes tiny amounts of chemicals called neurotransmitters to be released. Neurotransmitters continue the message to the next neurons. In this manner, pain from the big toe is relayed all the way to the cortex.
Different Brain Areas, Different Neurons
The pain neuron is simple, with its two main fibers. Other neurons in the brain have elaborately branching dendrites, resembling the bare branches of a tree. (Dendrite is a Latin word meaning "branches.") These two types of neurons are shown in the accompanying figure. Each of these many dendrites are covered all over with thousands of spines—and each spine is clasped by the ending of the axon of another neuron. This complicated, branching structure of the brain explains its amazing ability to carry out complex mental processes. Try to calculate how many connections are possible among a few billion neurons, each equipped with several thousand spines studding its hundreds of dendrites.
Brain Structures in Addiction
The brain must have ways to regulate the vast array of impulses and the flow of chemicals that create its activity. In fact, scientists have only begun to explain how the brain is controlled. Drug addiction is a response to an introduced substance that at first disrupts the precisely controlled actions of the brain. In response, the brain attempts to bring itself back into balance despite the drug.
Reward System. Animals learn to find food, comfortable places to sleep and hide, and mates based on feelings of pleasure that are generated within the structures of the limbic system. Even animals with more complex behaviors, such as people, still depend on the primitive reward system for meeting basic needs as well as the ability to enjoy the more complex rewards of human culture. Scientists have traced a limbic circuit governing pleasure, and it is this system that is responsible for the addictive nature of abused drugs.
A key limbic structure involved in generating rewarding feelings is termed the nucleus accumbens. It receives messages by means of a messenger molecule called dopamine, and so does part of the brain's cortex, called the prefrontal cortex (toward the front end of the brain). The prefrontal cortex is a region that recruits memories and higher thought processes needed for rational planning. The prefrontal cortex appears in humans but is primitive or absent in most other species.
Studies of addiction in rats can be used to study the basic chemistry behind human addiction. These studies might also help scientists to develop ways to make quitting easier. With rats that are made addicted to drugs, researchers can try out existing therapeutic drugs or discover new ones. Then, clinical trials are carried out in people.
Researching the Brain
By the close of the 1990s, brain researchers had new tools to look inside the living brain and watch it work. Neuroscience has also uncovered a range of growth factors that are critical in building and maintaining a brain. These new tools and discoveries have been employed in the field of addiction research.
Data from research on animals provide evidence that opiate abuse causes toxic (poisonous) actions on a number of brain cells in a key limbic region responsible for memory. In adult rats, the number of newly formed cells in this region, called the hippocampus, was cut back by long-term exposure to morphine. For existing brain cells in animals, morphine was found to decrease the length and number of the spines on neurons that receive incoming transmitter messages.
Drug abuse can also literally shrivel neurons. Repeated injections of morphine directly into a brain region known to be part of the limbic pleasure circuit in rats made certain neurons in this region 25 percent smaller. The injections cause the neurons to make lots of their messenger molecule, dopamine, and they apparently adapt by shrinking to shut down production.
Addictive drugs are chemicals that are able to hijack the brain's own chemistry. By mimicking molecules that the brain uses to send messages, the drugs activate regions of the brain that guide the body's movements and emotions. Research is showing that long-term use of these drugs not only changes brain chemistry, but that abusing drugs may also permanently change the structures of the brain on which they act.
WANT TO BE A BRAIN SURGEON?
A really fun way to learn how the brain is organized is to be a virtual neurosurgeon on the Internet. Check out this URL: <http://www.pbs.org/wgbh/aso/tryit/brain/> to try your hand at playing doctor.
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