Central Nervous System PDF

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Central Nervous System

Ward thought he could resist picking up a cigarette, but the temptation was too strong. He had successfully quit smoking in 2006. But when he walked into a smoke-filled poker room 3 years later at a friend’s house in Brooklyn, New York, the scene before him seemed to run in slow motion. He watched as smokers took deep satisfying drags on their cigarettes and exhaled wispy ringlets of contentment. He resisted that night. But poker night became a weekly event, and finally he gave in: he began smoking again. Ward knew that smoking is risky. He’d had countless arguments with his wife, who urged him to stop smoking to protect his own health and hers as well, which is partly why he quit in the first place. But during poker night, none of that seemed to matter. After prolonged use, drugs of abuse change the structure and function of the brain in ways that can wreak havoc on users’ lives. Severely addicted people may stop eating, stop working, in fact stop all activities because nothing matters except the drug—and they will do anything to get it. Without it, they have intense cravings, obsessive thoughts, and are deeply depressed. Today, in 2017, about 20% of Americans smoke tobacco regularly, according to the National Institute on Drug Abuse (NIDA), down from about 45% in 1955. These numbers testify to tobacco’s addictive potential. The component in tobacco that makes cigarettes addictive is the chemical nicotine. nce in the bloodstream, nicotine travels quickly throughout the body, reaching the brain within 8 seconds after tobacco smoke is inhaled Brain - An organ of the central nervous system that integrates and coordinates virtually all functions of the body.

The brain is the master coordinator of the body, controlling virtually all of the body’s activities, including sensation, movement, thinking, and just about all involuntary actions. The brain integrates information it receives from the internal and external environment and produces appropriate actions. The brain is also the seat of memory. Pleasurable experiences recorded in our memories serve as motivation; we tend to seek out the same pleasurable experiences over and over. Ultimately, the decision to smoke a cigarette, or to carry out any other behavior, is made in the brain When we decide to reach for a cigarette, for example, our brain sends a signal to muscles in our arm. These signals travel along nerves, long fibers made up of specialized cells and supportive tissue that transmit signals through the nervous system. A major collection of nerves is the spinal cord, which extends from the base of the brain down to the lower back and is

contained in and protected by our bony spinal column. Together, the brain and the spinal cord make up the central nervous system (CNS). Nerves that travel from spinal cord to distant body sites, like fingers, toes, and heart muscle, compose the peripheral nervous system (PNS). The peripheral nervous system includes all nervous tissue outside the brain and spinal cord leading to and from our limbs and organs. Peripheral nervous tissue in other organs, including the lungs, heart, and digestive organs, keeps our bodies operating without conscious thought. Addiction is a complex psychological and physiological process that affects many parts of the nervous system, but it begins at the level of the cell Sensory Neurons - Cells that convey information from both inside and outside the body to the CNS. Motor neurons - Neurons that control the contraction of skeletal muscle. But unlike most other cells, neurons can dramatically change the charge difference across the membrane, resulting in an electrical signal. Ion channels in the axon membrane open and close to allow specific ions to cross. Action Potential - An electrical signal within a neuron caused by ions moving across the cell membrane.

An action potential can travel down the length of an axon at a rate of 120 m/s— about 270 mph. Myelin - A fatty substance that insulates the axons of neurons and facilitates rapid conduction of action potential

Myelin is critical to nerve transmission: without it, action potentials weaken, losing strength as they pass along the axon, and can peter out before reaching the axon terminals. Diseases in which myelin degenerates over time, such as multiple sclerosis, can cause progressive paralysis, primarily because motor neurons lacking myelin can’t efficiently conduct electrical signals and ultimately can’t transmit those signals to muscles. The myelin sheath is produced by a type of supportive cell called a glial cell that physically wraps around the axon. The great capacity that humans have for conscious thought is a function of the sheer number of neurons found in the human brain: approximately 100 billion. Scientists typically divide the brain into four major regions that orchestrate different functions. The cerebellum, located in the rearmost portion of the brain, controls movement, coordination, and balance. The brain stem, at the base of the brain, coordinates involuntary functions like reflexes, heart rate, and breathing. The diencephalon, located above the brain stem, regulates homeostatic functions like body temperature, hunger, thirst, and the sex drive. The cerebrum is the largest part of the brain, sitting right on top. Its outer layer, the cerebral cortex, is the seat

of our more advanced brain functions, including perception and thinking, and gives us our distinct personalities and most human characteristics. Within each of the four major brain regions are subregions that are further specialized for certain functions. The diencephalon, for example, contains the hypothalamus, which is important for maintaining homeostasis (see Chapter 26). The cerebrum includes the hippocampus and amygdala—two important components of what is sometimes called the limbic system, the primary seat of our emotions and memories. (“Limbic” comes from the Latin limbus, which means “border”; the structures in this system lie along the border of the cerebrum and diencephalon.) Along with two nearby regions, the nucleus accumbens and the ventral tegumental area (VTA), the limbic system is stimulated during pleasurable activities and is a major region involved in addiction. It is this system that researchers sometimes refer to as the “pleasure center” or “reward system” of the brain. Dopamine - A neurotransmitter that is involved in conveying a sense of pleasure in the brain. This is also why even addicts who successfully stop taking drugs for decades can relapse simply by being in an environment that conjures up memories of the drug. The tinkling of a whiskey glass, meeting an old drinking buddy, being in a bar or a room filled with smokers puffing away can be reminders powerful enough to lure former addicts back to their old ways. For Ward, it wasn’t just the smoking at the weekly poker game that rekindled his desire to smoke, it was a combination of events. For one thing, he says he was “extremely stressed” about work. He had started working as a freelance writer, and he was having trouble juggling his deadlines. In the past, he would often rely on smoking to calm himself. “I’m a nervous worker,” he says. “Some people shake their legs, or pace, or exercise to work off stress. Neurotransmitter - A chemical signaling molecule released by a neuron to transmit a signal to a neighboring cell.

Synapse - The site of transmission of a signal between a neuron and another cell; includes the axon terminal of the signaling neuron, the space between the cells, and receptors on the receiving cell. Synaptic cleft - The physical space between a neuron and the cell with which it is communicating. For example, the neurotransmitter serotonin, which is active in the CNS and the gastrointestinal tract, regulates anxiety, appetite, and sleep. Another important neurotransmitter is acetylcholine, which is involved in learning, memory, and muscle contraction. Many drugs—including both drugs of abuse and therapeutic medicines—interact with neurotransmitters. The class of antidepressant drugs known as selective serotonin reuptake inhibitors (SSRIs), which includes Prozac and Zoloft, helps to treat depression by preventing the

reuptake of serotonin from the synaptic cleft, thus prolonging the activity of this moodelevating neurotransmitter. Nicotine binds to receptors on neurons and triggers the release of multiple neurotransmitters in the brain, which accounts for the range of sensations that smokers experience: a reduction in anxiety, for instance, as well as an increased ability to concentrate. And as we saw, nicotine dramatically increases the release of dopamine from certain neurons, such as those in the VTA. dopamine’s main job is to convey information related to elation and pain. The joy we get from a meal, sex, a winning poker hand, or indeed anything that gives us pleasure is conveyed in part by dopamine But when a person smokes, snorts cocaine, or takes heroin, for example, dopamine levels in the synaptic cleft increase dramatically. With so much dopamine available, practically all of the brain’s dopamine receptors become activated simultaneously. The immediate effect is euphoria. But there is a downside. Because so much dopamine is produced, the brain becomes overwhelmed and tries to dampen the drug’s effect by switching off some of its dopamine receptors. When the drug wears off, fewer receptors are functioning— bringing down mood. As they come down from a high, addicts will likely feel even more unhappy and depressed as the dopamine response system is dampened. Eventually, many addicts need to take drugs simply to feel normal. Without drugs, they suffer the physical symptoms of withdrawal, which may include depression, anxiety, and intense cravings for a dopamine fix. The specific symptoms and their intensity vary depending on the drug In a variety of tests, drug addicts seem to make poorer decisions, with detrimental consequences. drugs such as cocaine can change the shape of neurons in specific parts of the brain and consequently may impair their ability to transmit signals Brain-imaging studies have also shown that addicts consistently have lower than normal levels of blood flow in the frontal regions of the cerebrum during withdrawal from cocaine, and higher than normal levels while they are on the drug. This region of the cerebrum is involved in decision making. In a variety of tests, drug addicts seem to make poorer decisions, with detrimental consequences. 13 children and young people age 4 to 21 over 10 years showed that that some parts of the brain, such as the prefrontal cortex, are not fully developed until the mid-twenties or so. Taking drugs at an early age may hinder normal development of this region. The researchers found that the number of a nicotine receptor called alpha-5 influences how susceptible mice are to nicotine addiction. When given the opportunity to self-administer nicotine, normal mice will stop after reaching a certain dose. Mice with no alpha-5 receptors,

however, won’t stop until they’ve taken a much higher dose. Humans also have alpha-5 receptors in varying numbers, and scientists hypothesize that people with fewer receptors are less sensitive to nicotine and may become more easily addicted. Tobacco, for example, is responsible in some way for one out of every five deaths in the United States, according to NIDA. Smokers have a higher incidence of both heart and lung disease. And smoking causes cancer. Tobacco use is the leading preventable cause of death and disease in the United States, killing some 440,000 people a year. Nora Volkow, head of NIDA, has shown that methamphetamine (crystal meth) users have poorer short-term memory and score much lower on tests of motor skills, such as quickly walking a straight line. Volkow and other researchers have shown that after a period of abstinence from drugs, the dopamine system can repair itself—but it generally takes longer than a year Two popular antismoking drugs, Chantix and Zyban, for example, work by competing with nicotine for binding sites on neurons. By partially activating dopamine release, the drugs help reduce nicotine cravings in someone trying to quit smoking. They also make smoking less pleasurable because nicotine from cigarettes cannot bind to nicotine receptors while the drugs are present. However, there have also been reports of serious side effects of these drugs, such as suicidal thoughts, so use of these medications should be considered carefully Before Ward quit smoking in 2006, he was up to a pack and a half a day. Smoking had increasingly become a point of tension between him and his non-smoking wife. She desperately wanted him to quit, both for the sake of his health and because of the risks to herself of heart disease and cancer from second -hand smoke. A medical examination showed that Ward already had risk factors for heart disease, including high blood pressure. His doctor told him to stop smoking. “She didn’t ask me to stop,” Ward says. “She simply said, ‘You are going to stop smoking in two weeks.’” She prescribed a nicotine patch—a skin patch that delivers nicotine to the bloodstream and eases nicotine withdrawal symptoms—and other medication, which helped him quit. But after that fateful poker night in 2009, Ward caved into his cravings: he went back to smoking. This time, instead of his old pack-and-a-half-a-day habit, he managed to cut down to only a few cigarettes a week, all of them smoked on his weekly poker night. Two years after Ward started smoking again, he moved. The weekly poker nights were gone, and gone, too, were the social cues that had tempted him in the first place. There are more antismoking social influences in his life now than ever. Most of his friends do not smoke, he exercises more, and perhaps most important of all, he wants to model healthy behavior for his 4-year-old daughter: “I don’t want her to see me smoking.”

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