the human brain

موضوع: the human brain الجمعة مارس 26, 2010 1:33 pm

Revealing the Workings, the Wonder, of the Human Brain

Do each of the following, in succession. (This is not a test ):

1. Visualize a place you'd like to be. Maybe it's lounging on a sunny summer day at the beach. Maybe it's in your living room, watching a favorite movie. Create the image of that place in your mind, and hold it for a minute or two.

2. Listen to the sounds in the room around you. Really listen. What do you hear? Low voices in conversation? Muffled laughter in the hall? Phones and computers ringing and beeping? See how many sounds you can differentiate.

3. Silently tap your fingers on the desk, one tap, one finger at a time, in succession. Then reverse the order of tapping. Then tap each finger twice, in succession; then in reverse. Then three times.

4. Starting at 100, count backward by 7s.

5. Remember some event from your past. The first time you rode a bike all by yourself; your grandmother baking your favorite cookies; the first time you kissed someone other than a relative. Put yourself back in that place, and recall everything you can about it: Who was there with you? What were you wearing? What emotions were you feeling?

6. Now pinch yourself. Pick a tender spot on the inside of your elbow, and pinch the skin just
hard enough to feel pain.

In performing these six tasks, you've just activated a good portion of your brain. Even something as simple" as tapping your fingers in succession requires a phenomenal act of coordination among millions of nerve cells throughout the brain, all acting together in perfect timing to produce the signals that command your fingers to move.

If you had been lying inside a PET or fMRI scanner-tools of modern neuroscience that enable scientists to take images of the living brain as it works-the scans would show distinct areas of your brain "lighting up" as you did each task. Tapping your fingers in succession would activate groups of neurons in at least four distinct areas of the brain: the prefrontal cortex,where your brain makes the conscious decision to do the task; the premotor cortex, where you formulate the instructions for doing the task; the motor cortex, a sort of relay station that sends those instructions on to the arm and hand muscles that move the fingers; and the cerebellum, which supervises the whole process and adjusts your actions as necessary in response to external cues, such as where your hand is in relation to the desk. All of this takes place in a mere fraction of a second. Not such a "simple" task after all, from the brain's perspective.

Task number one, visual imagery, lights up the visual cortex in the back of the brain, as well as pathways leading to it from the eyes, along the optic nerve. Differentiating individual sounds around you activates the auditory cortex and associated areas. Tapping your fingers stimulates your motor cortex, which is involved in movement and muscle coordination. Counting backward by 7s is a complex cognitive task, and it calls upon the brain's center for higher thoughts in the prefrontal cortex.

Recalling a memory from your past will likely activate the hippocampus, an inner-brain structure involved in memory, as well as other areas of the brain that correspond to the type of memory. For example, remembering the first time you rode a bike, a motor task, will light up the motor area of the brain; recalling the smell of Grandma's cookies would activate the olfactory center.

Lastly, when you pinched yourself, pain receptors in the nerves of the skin sent signals back to the brain to alert it to the location and intensity of the pain and to initiate corrective action if necessary (i.e., stop pinching!). If the pain was intense, the brain might release endorphins, natural hormones that block the transmission of pain signals. Narcotic drugs such as morphine imitate the action of these natural endorphins to fight pain.

The Most Complex Achievement of Nature

You've just taken a brief tour of your brain. It has taken scientists hundreds of years to figure out the bits of information you've just learned in a few minutes. If that seems like a long time for a little bit of information, consider the complexity of the problem. The human brain is, as neuroscientist Joseph LeDoux, Ph.D., says in The Emotional Brain, "the most sophisticated machine imaginable, or unimaginable." It is composed of more than 100 billion nerve cells, each of which forms as many as 10,000 connections with other neurons. A typical brain weighs about three pounds, just two percent of the total body weight of a 150-pound person. But the brain uses between 20 percent and 25 percent of the body's oxygen and a substantial amount of the calories we consume in the form of the blood sugar glucose. The brain is also a nonstop factory of neurotransmitters that are critical to every thought and feeling we experience. About half of the 30,000 or so genes in the human genome are committed to building and operating the central nervous system (the brain and spinal cord).

The brain, weighing approximately three pounds, makes us distinctively human.
It encases 100 billion or more nerve cells and can send signals to thousands of
other cells at a rate of about 200 miles an hour.

In the process of deciphering the genetic code that is written in our DNA, learning the "blueprint" of our bodies is also likely to pave the way for a better understanding of the brain-based disorders and diseases that plague mankind and open new avenues for treating these disorders. This is an undertaking with enormous implications, because more than 60 million Americans are afflicted with a brain disorder-conditions that range from learning disabilities to depression to traumatic brain injury. That's nearly one in five of us. Look around you. If there are 25 students in your class, statistically 5 of you will be personally affected. Every one of us will personally know or care for someone who is affected by a brain disease or disorder.

Today, leading brain researchers report that the more than 1,000 disorders of the nervous system result in more hospitalizations than any other disease group, including heart disease and cancer. Brain damage by stroke, they report, is one of the three greatest medical sources of death, depression causes the greatest disability for adults under the age of 45, and suicides continue to outnumber homicides by almost two to one. The aging of our population makes Alzheimer's and other neurodegenerative diseases an increasing public health concern. At the beginning of the lifespan, autism and related disorders have now been estimated at 1 in 166 births, roughly a tenfold increase in the past decade.

Evolution of the Brain

The modern human brain is the product of millennia of "evolutionary tinkering," says Dr. LeDoux. To figure out how it works, we need to "pick the brain apart in the hope that we will see what evolution was up to when it put the device together."

About half of the 30,000 or so genes in the human genome are committed to building the central nervous system (the brain and spinal cord).

For centuries, "picking the brain apart" was, literally, how scientists learned how the brain worked. Actually, they usually picked apart the brains of other animals for clues as to how the human brain worked. As it turns out, the human brain is remarkably similar to the brains of other mammals, from rats right up to our closest cousins in the evolutionary tree, the great apes. In evolutionary terms, most of the structures in our brain are, in fact, primitive-that is, they have existed in much the same form for eons. These include the parts of the brain that control functions basic to survival, such as breathing, heart rate, and digestion. Such functions are centralized in the brain stem, located in the base of the brain, where the spinal cord meets the brain.

What Makes Human Brains So Special?

Clearly, humans have so-called specialized functions that rats, or even great apes, do not. So what makes humans so special? The key seems to lie in the prefrontal cortex, the forward-most section of the cerebral cortex, which is the brain's outermost layer of gray matter. This is the brain's command and control center, where higher cognitive functions are centralized, including the abilities for thinking, reasoning, believing, planning, and social consciousness-things that set us apart from other animals. The prefrontal cortex is more highly developed in humans than in any other primate, and it may not even exist in other mammals. (This is an area of continuing scientific exploration.)

In addition to examining the brains of other animals, scientists have made great strides in understanding brain function by observing people who have suffered trauma to the brain. Some of the most important breakthroughs in the biology of memory systems in the brain, for example, came from the study of a young man known as H.M., who underwent a radical surgery in which large sections of his temporal lobes were removed to control epileptic seizures. The surgery worked, but it left H.M. with a severe memory disorder in which he could learn, but not retain new information-he couldn't recall having met someone moments after that person had left the room. By observing H.M.'s behavior, and correlating it to the missing parts of his brain, scientists were able to learn which parts of the brain were responsible for certain behaviors. His case single-handedly shaped the course of memory research for decades.

Scientists study the brains of other mammals to understand structures that
bear remarkable similarity to the human brain, particularly those located in
the brain stem.

While H.M. and legions of other brain-injury survivors spurred important advances in understanding the brain, for much of scientific history the brain was a black box, a mystery so profound it was long considered to be the realm of philosophy or religion, not science. The 15th-century philosopher Ren Descartes promoted the idea that the "mind" was separate from the brain or body-an idea that has stubbornly persisted even in this age of modern medicine, argues Antonio Damasio, M.D., Ph.D., in Descartes' Error. Indeed, medical science has only recently begun to recognize the links between psychological phenomena and physical health, or the power of the mind to influence healing in the body.

The drive to understand the brain's mysteries picked up speed as scientific methods advanced. As the 19th century ended, two scientists-an Italian physician named Camillo Golgi and a Spanish anatomist named Santiago Ramn y Cajal-forever changed our understanding of the brain and shared the 1906 Nobel Prize in Physiology or Medicine for their work. While Golgi's theories about brain function were later disproved, his techniques, in which he stained brain tissue with silver nitrate and other substances to reveal its inner structure, are still used today. Just as important, Golgi "opened scientists' eyes to the true complexity of the human brain," notes Bruce S. McEwen, Ph.D., a leading neuroscientist at Rockefeller University. Golgi, they say, was the first to see the brain as a network of connected cells. Even though he was wrong about how the cells were connected, his work spurred others, including Cajal, to look at the brain differently. Cajal advanced what later became known as "the neuron theory," which proposed that nerve cells were not structurally connected, as Golgi thought, but were separate cells connected in some other, unknown way.

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» the aging brain
» the brain interact
» My Brain Made Me Do It
» the brain anatomy