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Nutrition and Weight Control Special Report

The Wisdom of Cutting Back on Salt

Americans love salt. On average, we consume twice the recommended daily maximum intake of 2,300 mg of sodium each day, which is 10 to 20 times what is needed for normal body function. Our bodies do need some salt -- but not that much. Here’s what you should know.

Excessive sodium intake causes constriction of small arteries throughout the body and retention of water, which increases the volume of blood to your arteries and veins. These effects can raise blood pressure and cause hypertension, the most important risk factor for a stroke.

Hypertension (high blood pressure) is also a major risk factor for coronary heart disease, the number one cause of death in the United States. Cardiovascular disease, the umbrella term for all diseases of the heart and blood vessels including heart disease, hypertension, and stroke, claims the lives of about 850,000 people yearly. Hypertension also contributes to heart failure, kidney disease, and vision loss. About 73 million American adults have hypertension, defined as a systolic blood pressure of 140 mm Hg or higher and/or a diastolic blood pressure of 90 mm Hg or more.

Most individuals can benefit from reducing the sodium content of their diet. If you already have high blood pressure and are taking medication for it, cutting down on salt can help by making drug therapy more effective. Consuming less salt can reduce medication doses and even replace medication as a way to reduce high blood pressure.

If your blood pressure is normal, it's still worth cutting down on salt, because it may help reduce your chances of developing high blood pressure later in life. Even if you're 55 years old and have normal blood pressure, you still have a 90% chance of developing hypertension sometime later in life. And even if your blood pressure is normal, excess salt can increase the size and thickness of your heart and the stiffness of your arteries, setting the stage for heart disease.

When it comes to cutting sodium, putting away the salt shaker is hardly enough. The vast majority of salt we eat -- 80% -- comes from foods that are processed or served in restaurants. Only 10% of salt is added during home cooking or at the table. The remaining 10% of salt occurs naturally in foods. Try taking these steps to cut out some of the salt in your diet:

 

  • Eat fresh fruits and vegetables more often. They are naturally low in sodium and are good sources of potassium, which helps blunt the blood pressure-raising effects of sodium.

     

  • Choose processed and packaged foods with less sodium. Read labels carefully, since sodium is not always where you think. For example, salad dressings, cheese, breakfast cereals, store-bought cookies and breads, mayonnaise, and canned beans and vegetables can contain substantial amounts of sodium.

     

  • When cooking at home, skip the salt and instead flavor your food with pepper, herbs, spices, vinegar, wine, garlic, onion, lemon, and low-salt versions of mustard, ketchup, and Worcestershire sauce. Choose low- or reduced sodium broths and soups.

     

  • When dining out, ask the server to have your meal prepared without added salt. And of course, limit your consumption of fast foods and ethnic foods that are high in salt.

And finally: OJ Every Day -- Dietary potassium can help reduce the damage caused by sodium by dilating arteries and reducing blood pressure. The National Academy of Sciences advises adults to consume at least 4.7 g of potassium per day to lower the risk of hypertension, kidney stones, and bone loss. However, most people get about half that amount.

To add more potassium to your diet, reach for these potassium rich foods: sweet potatoes, white potatoes, beet greens, soybeans, bananas, spinach, tomato juice, tomato sauce, orange juice, dried peaches, stewed prunes, dried apricots, honeydew melon, cantaloupe, lentils, and kidney beans. An 8-oz glass of orange juice has 450 mg of potassium; a medium banana has about 422 mg.

 

 

 

 

 

What Is a Gene?

Look closely at the chromosomes and you'd see that each is made of bundles of looping coils. If you unraveled these coils, you'd have a six-foot long double strand of deoxyribonucleic acid-DNA.

DNA is a twisted double strand of nucleotides-adenine (A), cytosine (C), guanine (G), and thymine (T).

DNA is a twisted double strand of nucleotides-adenine (A), cytosine (C), guanine (G), and thymine (T).

Believe it or not, if you took all of the DNA in all of your cells and laid it out end to end, it would stretch to the moon and back about 130,000 times.

A DNA molecule is a twisted ladder-like stack of building blocks called nucleotides. There are four types of DNA nucleotides-adenine, cytosine, guanine, and thymine-or A, C, G, and T, for short.

If you could peer into any one of your body's 50 trillion cells, you'd find a fantastically complex and busy world. At the center of this world you'd find a nucleus containing 46 molecules called chromosomes-23 from your mother and 23 from your father. These chromosomes are basically an instruction set for the construction and maintenance of... you.

These two long stacks of building blocks fit together like two sides of zipper, but there's a rule involved: adenine only pairs with thymine, and cytosine only pairs with guanine. So each rung in the DNA ladder is a pair of nucleotides, and each pair is either an A stuck to a T or a C stuck to a G.

You've got six billion of these pairs of nucleotides in each of your cells, and amongst these six billion nucleotide pairs are roughly 30,000 genes. A gene is a distinct stretch of DNA that determines something about who you are. (More on that later.) Genes vary in size, from just a few thousand pairs of nucleotides (or "base pairs") to over two million base pairs.

How Do Genes Work?

Genes are often called the blueprint for life, because they tell each of your cells what to do and when to do it: be a muscle, make bone, carry nerve signals, and so on. And how do genes orchestrate all this? They make proteins. In fact, each gene is really just a recipe for a making a certain protein.

DNA/amino acid code

Here are several examples of how RNA codes for amino acids. These codons are for tryptophan, the amino acid that contributes to the sleepy feeling you may have after eating turkey; phenylalanine, an amino acid used in the aspartame sweetener in diet soda; tyrosine, an important amino acid in intracellular signaling processes; and cystine.

And why are proteins important? Well, for starters, you are made of proteins. 50% of the dry weight of a cell is protein of one form or another. Meanwhile, proteins also do all of the heavy lifting in your body: digestion, circulation, immunity, communication between cells, motion-all are made possible by one or more of the estimated 100,000 different proteins that your body makes.

But the genes in your DNA don't make protein directly. Instead, special proteins called enzymes read and copy (or "transcribe") the DNA code. The segment of DNA to be transcribed gets "unzipped" by an enzyme, which uses the DNA as a template to build a single-stranded molecule of RNA. Like DNA, RNA is a long strand of nucleotides.

This transcribed RNA is called messenger RNA, or mRNA for short, because it leaves the nucleus and travels out into the cytoplasm of the cell. There, protein factories called ribosomes translate the mRNA code and use it to make the protein specified in the DNA recipe.

If all this sounds confusing, just remember: DNA is used to make RNA, then RNA is used to make proteins-and proteins run the show.

"Junk" DNA

Scientists first studying DNA sequences were surprised to find that less than 2% of human DNA codes for proteins. If 98% of our genetic information (or "genome") isn't coding for protein, what is it for?

At first it wasn't clear, and some termed this non-coding DNA "junk DNA." But as more research is done, we are beginning to learn more about the DNA between the genes-intergenic DNA.

Intergenic DNA seems to play a key role in regulation, that is, controlling which genes are turned "on" or "off" at any given time.

For example, some intergenic sequences code for RNA that directly causes and controls reactions in a cell, a job that scientists originally thought only proteins could do.

Intergenic DNA is also thought to be responsible for "alternative splicing," a kind of mix-and-match process whereby several different proteins can be made from one gene.

In short, it now seems that much of the interest and complexity in the human genome lies in the stuff between the genes... so don't call it junk.

It Runs in the Family: Genes and Human Difference

Biologists use two fancy words to describe the relationship between your genes and your physical traits. The first word is genotype. Your genotype is your genes for a given trait. In most cases, you've got two copies of a gene - one from your mother and one from your father.

Pass the Peas, Please

Gregor Mendel was an Austrian monk who did extensive breeding research on pea plants. In doing so, he overturned our understanding of heredity.

At the time of Mendel's work, in the 1860's, most people believed in the blending theory of heredity, the idea that offspring were born with traits constituting a blend or average of the two parents. By this theory, when a red and a white flower are bred together, or crossed, their offspring should all be blended, that is, pink.

Mendel's results suggested otherwise. For example, when he crossed pure-bred tall pea plants with pure-bred short pea plants, he got all tall pea plants-no blending there. He concluded that every organism possesses two "factors" (we now call them genes) for a given trait, and passes on just one of these factors-at random-to its offspring.

Two VERY different people

As different as people can be, we are more alike than we realize. Compare any two people and you'll find that 99.9% of their DNA is the same.

The second word is phenotype. Phenotype is what you actually turn out to be, the way these genes get expressed. Biologists have a saying involving these two fancy words: "Genotype determines phenotype."

Let's take eyelashes, for example. There are 2 kinds of eyelashes in people - long and short. Maybe you've got a short lashes version of a gene from your father, and a long lashes version of a gene from your mother. That's your genotype. And what length of lashes do you actually have? Long - that's your phenotype.

The eyelash example makes an important point. Some genes are dominant, and others are recessive. When you have two different genes for the same trait, and one is dominant (long lashes) while the other is recessive (short lashes), it's the dominant trait that wins out in the phenotype.

But not all genes follow this dominant/recessive model. For example, the gene for blood type is codominant; if you get a gene for type A blood from one parent and type B blood from the other, neither dominates. Instead, you wind up with type AB blood.

Other human traits are polygenic, which means that they are controlled by several genes that contribute in an additive fashion. Skin color is believed to be polygenic. Scientists also think that polygenic inheritance is responsible for inherited predispositions to certain diseases, such as heart disease, arteriosclerosis, and some cancers.

When Genes Go Bad: Mutations and Disease

DNA is constantly subject to mutations, accidental changes in its code. Mutations can lead to missing or malformed proteins, and that can lead to disease.

Sickle-shaped and healthy blood cells

Sickled blood cells (left) and normal blood cells.

Sickle Cell

These are the sickle-shaped blood cells of someone with sickle cell anemia, a genetic disease common among those of African descent.

Sickle cell anemia is the result of a point mutation, a change in just one nucleotide in the gene for hemoglobin. This mutation causes the hemoglobin in red blood cells to distort to a sickle shape when deoxygenated. The sickle-shaped blood cells clog in the capillaries, cutting off circulation.

Having two copies of the mutated genes cause sickle cell anemia, but having just one copy does not, and can actually protect against malaria - an example of how mutations are sometimes beneficial.

We all start out our lives with some mutations. These mutations inherited from your parents are called germ-line mutations. However, you can also acquire mutations during your lifetime. Some mutations happen during cell division, when DNA gets duplicated. Still other mutations are caused when DNA gets damaged by environmental factors, including UV radiation, chemicals, and viruses.

Few mutations are bad for you. In fact, some mutations can be beneficial. Over time, genetic mutations create genetic diversity, which keeps populations healthy. Many mutations have no effect at all. These are called silent mutations.

But the mutations we hear about most often are the ones that cause disease. Some well-known inherited genetic disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, phenylketonuria and color-blindness, among many others. All of these disorders are caused by the mutation of a single gene.

Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene.

Diseases caused by just one copy of a defective gene, such as Huntington's disease, are rare. Thanks to natural selection, these dominant genetic diseases tend to get weeded out of populations over time, because afflicted carriers are more likely to die before reproducing.

Scientists estimate that every one of us has between 5 and 10 potentially deadly mutations in our genes-the good news is that because there's usually only one copy of the bad gene, these diseases don't manifest.

Cancer usually results from a series of mutations within a single cell. Often, a faulty, damaged, or missing p53 gene is to blame. The p53 gene makes a protein that stops mutated cells from dividing. Without this protein, cells divide unchecked and become tumors.

Genetic Testing

Genetic Tests

Some of the diseases for which genetic tests are currently available:

Cancer (some forms, predisposition only)
Cystic Fibrosis
Down syndrome
Fragile-X Syndrome
Hemophilia
Huntington's Disease
Lou Gehrig's Disease
Neurofibromatosis
Muscular Dystrophy
Phenylketonuria
Sickle Cell Anemia
Tay-Sachs Disease
 

Have you ever had your genes tested? Probably not. DNA testing is still pretty limited, although it is becoming more and more common, especially for fetuses and newborns.

Many prospective parents, especially those with a history of genetic disease in the family, seek genetic testing and counseling before having children. Genetic counselors can evaluate genetic tests and advise people of the risk of conceiving a child with recessive, inherited diseases like sickle cell anemia, Tay-Sachs disease, or cystic fibrosis.

Biochip

Biochips

Biochips, also called DNA arrays or microarrays, are a new technology that promises to speed and simplify a wide range of genetic tests.

Each small glass slide or "chip" contains rows and rows of DNA probes, which test for the presence of a specific DNA sequence or mutation. If the mutation or sequence is present in the DNA being tested, a specific spot on the biochip will glow under special light. A biochip can test for thousands of mutations at once.

Biochips could be the first step towards genetic ID cards. Imagine something like a credit card, except this card would carry all of your genetic information. Your doctors could use this DNA data to tailor your medical care and choose the right drugs and dosages.

Genetic tests can be performed on fetuses by taking cell samples from the womb. The two techniques available are called amniocentesis and chorionic villi sampling. Down syndrome, a condition caused by having an extra chromosome, is tested for this way. After birth, most newborns are given a blood test for phenylketonuria, a genetic disease that can cause mental retardation if it goes undiagnosed.

Not all genetic testing is focused on children. Testing is also done to match organ donors and recipients, to establish paternity or maternity, and in forensics, for identifying evidence from crime scenes. Testing can also help diagnose adult-onset inherited diseases, such as Huntington's disease.

Genetic tests are now available for a range of cancers. These tests don't test for cancer directly, but instead indicate an increased likelihood of developing a cancer. Likelihood is far from certainty, and cancer may or may not develop, since it must be triggered by additional mutations.

Meanwhile, many cancers develop in persons without so-called "cancer genes." For example, the two gene variants that have been linked with breast cancer, called BRCA1 and BRCA2, are involved in only 5% of breast cancer cases.

The decision to be tested can be loaded. What if a gene test told you that age 40 or so, you would start to lose control of your muscles and live a shorter lifespan? This is the case in Huntington's disease, which has no cure. Perhaps you would rather not know, but the information might also guide your life decisions, such as when or whether to have children.

Making Medicines

Genetically engineered corn

Drug Manufacturing Plants

Along with bacteria and farm animals, plants are being genetically engineered to make human hormones, antibodies, and blood-clotting factors. Bananas are being engineered to deliver vaccines.

To make a transgenic plant, scientists mix foreign DNA with protoplasts, plant cells that have had their tough cell walls removed. Then, they run a small electric current through the mixture. The current makes tiny holes in the cell, allowing the foreign DNA to enter. Once the protoplast develops into a plant, the medicine it has been tricked into making can be extracted from the plant's seeds.

Not long ago, if you were diabetic, the insulin your doctor prescribed would have come from a pig. If you required human growth hormone, it would have come from human cadavers, a source that is costly, not to mention a little creepy.

Now, these and other medicines can be made by specially-modified bacteria, called transgenic bacteria. These single-celled organisms have foreign genes along side their own DNA. They live and reproduce like ordinary bacteria, but they also do a bit of extra duty, and produce human proteins for medicines and vaccines.

Transgenic bacteria

The sweetener in most diet sodas-phenylalanine-is made by transgenic bacteria. Transgenic bacteria also make important medicines.

Farm animals have also been put to work making drugs for humans. It's called gene pharming. For example, antithrombin III, which prevents blood clotting during surgery, is secreted in the milk of transgenic goats. To create these transgenic goats, scientists used a needle thinner than a human hair to inject the DNA sequence for antithrombin III into goat eggs. Then they transplanted the fertilized transgenic eggs into female goats.

Scientists also hope to use genetic technologies to make "magic bullets," drugs that are designed to target specific antigens (disease-causing substances) while leaving healthy tissue alone. "Magic bullets" would have few side effects, because they would consist of antibodies, the same kind of specific weapons that our own immune systems use to kill invaders like viruses, bacteria, or even cancer.

New Therapies

David Vetter,

David Vetter, also known as "the boy in the bubble," died in 1984 at age 12 with a rare immune disease, called SCID. Today, this disease has be treated-with mixed success-using gene therapy.

Parts from Pigs?

Donated human organs like hearts, livers, and lungs are in short supply. Could pig organs be put into people?

Ordinarily, your body would reject a pig organ faster than you can say "bacon." But researchers hope to be able "humanize" pigs for organ donation by adding human genes to their DNA. As bizarre as this idea may sound, researches have already used this technique to transplant genetically altered pig organs into monkeys.

Many of the worst diseases around are caused by glitches in our genes, and the therapies for these diseases often involve a lifetime of drugs (and their nasty side effects) that help but don't really solve the problem. Wouldn't it be great if doctors could just "fix" this faulty DNA?

That is the goal of gene therapy, the insertion of genes into human cells to treat a disorder. It's harder than it sounds. Getting the genes into the right cells, getting them to stay there, and getting them to "express," that is, do their jobs-all present major challenges. Still, there have been some gene therapy successes, and researchers hope for more in the future. So far, gene therapy on blood cells has been most successful, because blood can be easily removed and returned to the body-not so with hearts and livers.

Stemming Controversy

Human stem cells are useful in gene therapy research and treatments-no debate there. Yet there is debate about one of the common sources for stem cells: unwanted human embryos from fertility clinics.

It is possible to do stem cell research using stem cells from other sources, including cord blood (found in placentas and umbilical cords), bone marrow, tooth pulp, and even in the tissue sucked out during liposuction.

However, these non-embryonic stem cells are less versatile, and many researchers believe that embryonic stem cells hold the most potential for such medical possibilities as "replacement" organs and gene therapy.

For example, gene therapy has helped children a rare immune disease called SCID, caused when a faulty gene fails to make a necessary enzyme. Doctors remove some blood cells and use a virus to "infect" the cells with good copies of the faulty gene. Then these blood cells are returned to the body, where they start making the missing enzyme.

The blood cells used in this and other gene therapy trials were stem cells. Stem cells are undifferentiated cells, cells that can grow into other types of cell in the body. Stem cells are extremely interesting to genetic researchers, because it's possible to use them to grow tissues and organs "ex vivo" (outside of the body), as well as to provide cures by injecting genetically-tailored stem cells directly into the body ("in vivo").

Genetic technologies promise many new forms of treatment. Thanks to information provided by the Human Genome Project, a recently-completed effort to identify the entire sequence of human DNA, researchers are envisioning new cures and treatments for a growing list of diseases, including diabetes, heart disease, cancer, AIDS, and Parkinson's. Often, knowing the genetic cause of a disease can help point researchers towards a cure.

Ethics

Oncomouse

A strain of mice used for cancer research, called Oncomouse, was the first mammal to be patented.

The new possibilities created by genetics have brought with them new questions about what is right.

An example: genetic testing is, for now, optional. But many medical tests that start out as optional become less and less optional as time goes by. Who should decide when genetic tests are done? Should insurance companies or employers have access to the results?

If prenatal genetic tests become more common, will people with certain genetic traits, diseases, or even predispositions suffer increased discrimination? Will "designer babies" become the norm? For some, the opportunities presented by genetic testing and therapies smack of eugenics, the use of selective breeding to create "superior" people.

More broadly, who "owns" the genetic information-or the life forms-that come from research? These questions arise with increasing frequency in industry, where patents are granted for genetically engineered plants and animals, and for genetic sequences.

Dolly

This cloned sheep, Dolly, foretells the prospect of human cloning, one of many reproductive possibilities under debate.

Companies argue that without patents, they are left with no guarantee that they can recoup their investment when they discover key genes. But do patents stifle progress and the free exchange of ideas, so important in science?

Protesting genetically modified foods

Food for Thought

The genetic modification of plants for food is a hot-button issue around the world. Genes transplanted to corn, potato, soybean and other food strains can confer desired traits, such as resistance to pests, which lessens the need for toxic pesticides. Though genetically modified foods are thought to be safe to eat, their impact on the environment may not be fully known.

Others worry that reckless experimenting will have unforeseen, potentially catastrophic consequences. For example, the prospect of transplanting animal organs into people (xenotransplantation) raises concern that new diseases could jump from animals to humans, as SARS and "mad cow" disease have.

As for gene therapy, some wonder if it's right to tinker with human DNA. Currently, treatments are focused on somatic cells, that is, cells in the body. Any alterations are not passed on to later generations, because DNA in the germ cells-eggs and sperm-is unaffected. But so-called germ line gene therapy is certainly possible. It could cure diseases before they happened, but might cause other unexpected problems that would persist in later generations.

 

Human brain

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Illustration of the human brain and skull

The human brain is the center of the human nervous system and is a highly complex organ. Enclosed in the cranium, it has the same general structure as the brains of other mammals, but is over three times as large as the brain of a typical mammal with an equivalent body size.[1] Most of the expansion comes from the cerebral cortex, a convoluted layer of neural tissue that covers the surface of the forebrain. Especially expanded are the frontal lobes, which are involved in executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision is also greatly enlarged in human beings.

Brain evolution, from the earliest shrewlike mammals through primates to hominids, is marked by a steady increase in encephalization, or the ratio of brain to body size. The human brain has been estimated to contain 50–100 billion (1011) neurons, of which about 10 billion (1010) are cortical pyramidal cells. These cells pass signals to each other via as many as 1000 trillion (1015) synaptic connections [2].

The brain monitors and regulates the body's actions and reactions. It continuously receives sensory information, and rapidly analyzes this data and then responds, controlling bodily actions and functions. The brainstem controls breathing, heart rate, and other autonomic processes. The neocortex is the center of higher-order thinking, learning, and memory. The cerebellum is responsible for the body's balance, posture, and the coordination of movement.

In spite of the fact that it is protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the delicate nature of the human brain makes it susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a wide variety of chemicals that can act as neurotoxins. Infection of the brain is rare because of the barriers that protect it, but is very serious when it occurs. More common are genetically based diseases[citation needed], such as Parkinson's disease, multiple sclerosis, and many others. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought to be caused at least partially by brain dysfunctions, although the nature of such brain anomalies is not well understood.

Contents

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[edit] Structure

Bisection of the head of an adult man, showing the cerebral cortex and underlying white matter

The adult human brain weighs on average about 3 lb (1.5 kg)[3] with a size of around 1130 cubic centimetres (cm3) in women and 1260 cm3 in men, although there is substantial individual variation.[4] Men's brains are on average 100g heavier than a woman's, even when corrected for body size differences[5] The brain is very soft, having a consistency similar to soft gelatin or firm tofu. Despite being referred to as "grey matter", the live cortex is pinkish-beige in color and slightly off-white in the interior. The photo on the right shows a horizontal slice of the head of an adult man, from the National Library of Medicine's Visible Human Project. In this project, two human cadavers (from a man and a woman) were frozen and then sliced into thin sections, which were individually photographed and digitized. The slice here is taken from a small distance below the top of the brain, and shows the cerebral cortex (the convoluted cellular layer on the outside) and the underlying white matter, which consists of myelinated fiber tracts traveling to and from the cerebral cortex. At the age of 20, a man has around 176,000 km and a woman, about 149,000 km of myelinated axons in their brains.[6]

Drawing of the human brain, showing several important structures

The cerebral hemispheres form the largest part of the human brain and are situated above most other brain structures. They are covered with a cortical layer with a convoluted topography.[7] Underneath the cerebrum lies the brainstem, resembling a stalk on which the cerebrum is attached. At the rear of the brain, beneath the cerebrum and behind the brainstem, is the cerebellum, a structure with a horizontally furrowed surface that makes it look different from any other brain area. The same structures are present in other mammals, although the cerebellum is not so large relative to the rest of the brain. As a rule, the smaller the cerebrum, the less convoluted the cortex. The cortex of a rat or mouse is almost completely smooth. The cortex of a dolphin or whale, on the other hand, is more convoluted than the cortex of a human.

The dominant feature of the human brain is corticalization. The cerebral cortex in humans is so large that it overshadows every other part of the brain. A few subcortical structures show alterations reflecting this trend. The cerebellum, for example, has a medial zone connected mainly to subcortical motor areas, and a lateral zone connected primarily to the cortex. In humans the lateral zone takes up a much larger fraction of the cerebellum than in most other mammalian species. Corticalization is reflected in function as well as structure. In a rat, surgical removal of the entire cerebral cortex leaves an animal that is still capable of walking around and interacting with the environment.[8] In a human, comparable cerebral cortex damage produces a permanent state of coma.

The four lobes of the cerebral cortex
The bones of the human skull

The cerebral cortex is nearly symmetric in outward form, with left and right hemispheres. Anatomists conventionally divide each hemisphere into four "lobes", the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. It is important to realize that this categorization does not actually arise from the structure of the cortex itself: the lobes are named after the bones of the skull that overlie them. There is one exception: the border between the frontal and parietal lobes is shifted backward to the central sulcus, a deep fold that marks the line where the primary somatosensory cortex and primary motor cortex come together.

Researchers who study the functions of the cortex divide it into three functional categories of regions, or areas. One consists of the primary sensory areas, which receive signals from the sensory nerves and tracts by way of relay nuclei in the thalamus. Primary sensory areas include the visual area of the occipital lobe, the auditory area in the temporal lobe, and the somatosensory area in the parietal lobe. A second category is the primary motor area, which sends axons down to motor neurons in the brainstem and spinal chord. This area occupies the rear portion of the frontal lobe, directly in front of the somatosensory area. The third category consists of the remaining parts of the cortex, which are called the association areas. These areas receive input from the sensory areas and lower parts of the brain and are involved in the complex process that we call perception, thought, and decision making. The amount of association cortex, relative to the other two categories, increase dramatically as one goes from simpler mammals, such as the rat and the cat, to more complex ones, such as the chimpanzee and the human. [9]

Major gyri and sulci on the lateral surface of the cortex

The cerebral cortex is essentially a sheet of neural tissue, folded in a way that allows a large surface area to fit within the confines of the skull. Each cerebral hemisphere, in fact, has a total surface area of about 1.3 square feet.[10] Anatomists call each cortical fold a sulcus, and the smooth area between folds a gyrus. Most human brains show a similar pattern of folding, but there are enough variations in the shape and placement of folds to make every brain unique. Nevertheless, the pattern is consistent enough for each major fold to have a name, for example, the "superior frontal gyrus", "postcentral sulcus", or "trans-occipital sulcus". Deep folding features in brain such as the inter-hemispheric and lateral fissure, and the insular cortex are present in almost all normal subjects.

Brodmann's classification of areas of the cortex

Different parts of the cerebral cortex are involved in different cognitive and behavioral functions. The differences show up in a number of ways: the effects of localized brain damage, regional activity patterns exposed when the brain is examined using functional imaging techniques, connectivity with subcortical areas, and regional differences in the cellular architecture of the cortex. Anatomists describe most of the cortex—the part they call isocortex—as having six layers, but not all layers are apparent in all areas, and even when a layer is present, its thickness and cellular organization may vary. Several anatomists have constructed maps of cortical areas on the basis of variations in the appearance of the layers as seen with a microscope. One of the most widely used schemes came from Brodmann, who split the cortex into 51 different areas and assigned each a number (anatomists have since subdivided many of the Brodmann areas[citation needed]). For example, Brodmann area 1 is the primary somatosensory cortex, Brodmann area 17 is the primary visual cortex, and Brodmann area 25 is the anterior cingulate cortex.

[edit] Topography

Topography of the primary motor cortex, showing which body part is controlled by each zone

Many of the brain areas Brodmann defined have their own complex internal structures. In a number of cases, brain areas are organized into "topographic maps", where adjoining bits of the cortex correspond to adjoining parts of the body, or of some more abstract entity. A simple example of this type of correspondence is the primary motor cortex, a strip of tissue running along the anterior edge of the central sulcus, shown in the image to the right. Motor areas innervating each part of the body arise from a distinct zone, with neighboring body parts represented by neighboring zones. Electrical stimulation of the cortex at any point causes a muscle-contraction in the represented body part. This "somatotopic" representation is not evenly distributed, however. The head, for example, is represented by a region about three times as large as the zone for the entire back and trunk. The size of a zone correlates to the precision of motor control and sensory discrimination possible[citation needed]. The areas for the lips, fingers, and tongue are particularly large, considering the proportional size of their represented body parts.

In visual areas, the maps are retinotopic—that is, they reflect the topography of the retina, the layer of light-activated neurons lining the back of the eye. In this case too the representation is uneven: the fovea—the area at the center of the visual field—is greatly overrepresented compared to the periphery. The visual circuitry in the human cerebral cortex contains several dozen distinct retinotopic maps, each devoted to analyzing the visual input stream in a particular way[citation needed]. The primary visual cortex (Brodmann area 17), which is the main recipient of direct input from the visual part of the thalamus, contains many neurons that are most easily activated by edges with a particular orientation moving across a particular point in the visual field. Visual areas farther downstream extract features such as color, motion, and shape.

In auditory areas, the primary map is tonotopic. Sounds are parsed according to frequency (i.e., high pitch vs. low pitch) by subcortical auditory areas, and this parsing is reflected by the primary auditory zone of the cortex. As with the visual system, there are a number of tonotopic cortical maps, each devoted to analyzing sound in a particular way.

Within a topographic map there can sometimes be finer levels of spatial structure. In the primary visual cortex, for example, where the main organization is retinotopic and the main responses are to moving edges, cells that respond to different edge-orientations are spatially segregated from one another[citation needed].

[edit] Lateralization

Routing of neural signals from the two eyes to the brain

Each hemisphere of the brain interacts primarily with one half of the body, but for reasons that are unclear, the connections are crossed: the left side of the brain interacts with the right side of the body, and vice versa. Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross the midline at brainstem levels. Visual input follows a more complex rule: the optic nerves from the two eyes come together at a point called the optic chiasm, and half of the fibers from each nerve split off to join the other. The result is that connections from the left half of the retina, in both eyes, go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain. Because each half of the retina receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field—an arrangement that presumably is helpful for visuomotor coordination.

The corpus callosum, a nerve bundle connecting the two cerebral hemispheres, with the lateral ventricles directly below

The two cerebral hemispheres are connected by a very large nerve bundle called the corpus callosum, which crosses the midline above the level of the thalamus. There are also two much smaller connections, the anterior commisure and hippocampal commisure, as well as many subcortical connections that cross the midline. The corpus callosum is the main avenue of communication between the two hemispheres, though. It connects each point on the cortex to the mirror-image point in the opposite hemisphere, and also connects to functionally related points in different cortical areas.

In most respects, the left and right sides of the brain are symmetrical in terms of function. For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several very important exceptions, involving language and spatial cognition. In most people, the left hemisphere is "dominant" for language: a stroke that damages a key language area in the left hemisphere can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.

A substantial part of our current understanding of the interactions between the two hemispheres has come from the study of "split-brain patients"—people who underwent surgical transection of the corpus callosum in an attempt to reduce the severity of epileptic seizures. These patients do not show unusual behavior that is immediately obvious, but in some cases can behave almost like two different people in the same body, with the right hand taking an action and then the left hand undoing it. Most such patients, when briefly shown a picture on the right side of the point of visual fixation, are able to describe it verbally, but when the picture is shown on the left, are unable to describe it, but may be able to give an indication with the left hand of the nature of the object shown.

It should be noted that the differences between left and right hemispheres are greatly overblown in much of the popular literature on this topic. The existence of differences has been solidly established, but many popular books go far beyond the evidence in attributing features of personality or intelligence to the left or right hemisphere dominance.[citation needed]

[edit] Sources of information

Information about the structure and function of the human brain comes from a variety of sources. Most information about the cellular components of the brain and how they work comes from studies of animal subjects, using techniques described in the brain article. Some techniques, however, are used mainly in humans, and therefore are described here.

Computed tomography of human brain, from base of the skull to top, taken with intravenous contrast medium

[edit] EEG

By placing electrodes on the scalp it is possible to record the summed electrical activity of the cortex, in a technique known as electroencephalography (EEG).[11] EEG measures mass changes in population synaptic activity from the cerebral cortex, but can only detect changes over large areas of the brain, with very little sensitivity for sub-cortical activity. EEG recordings can detect events lasting only a few thousandths of a second. EEG recordings have good temporal resolution, but poor spatial resolution.

[edit] MEG

Apart from measuring the electric field around the skull it is possible to measure the magnetic field directly in a technique known as magnetoencephalography (MEG).[12] This technique has the same temporal resolution as EEG but much better spatial resolution, although not as good as fMRI. The greatest disadvantage of MEG is that, because the magnetic fields generated by neural activity are very weak, the method is only capable of picking up signals from near the surface of the cortex, and even then, only neurons located in the depths of cortical folds (sulci) have dendrites oriented in a way that gives rise to detectable magnetic fields outside the skull.

[edit] Structural and functional imaging

A scan of the brain using fMRI

There are several methods for detecting brain activity changes by three-dimensional imaging of local changes in blood flow. The older methods are SPECT and PET, which depend on injection of radioactive tracers into the bloodstream. The newest method, functional magnetic resonance imaging (fMRI), has considerably better spatial resolution and involves no radioactivity.[13] Using the most powerful magnets currently available, fMRI can localize brain activity changes to regions as small as one cubic millimeter. The downside is that the temporal resolution is poor: when brain activity increases, the blood flow response is delayed by 1–5 seconds and lasts for at least 10 seconds. Thus, fMRI is a very useful tool for learning which brain regions are involved in a given behavior, but gives little information about the temporal dynamics of their responses. A major advantage for fMRI is that, because it is non-invasive, it can readily be used on human subjects.

[edit] Effects of brain damage

A key source of information about the function of brain regions is the effects of damage to them.[14] In humans, strokes have long provided a "natural laboratory" for studying the effects of brain damage. Most strokes result from a blood clot lodging in the brain and blocking the local blood supply, causing damage or destruction of nearby brain tissue: the range of possible blockages is very wide, leading to a great diversity of stroke symptoms. Analysis of strokes is limited by the fact that damage often crosses into multiple regions of the brain, not along clear-cut borders, making it difficult to draw firm conclusions.

[edit] Language

Location of two brain areas that play a critical role in language, Broca's area and Wernicke's area

In human beings, it is the left hemisphere that usually contains the specialized language areas. While this holds true for 97% of right-handed people, about 19% of left-handed people have their language areas in the right hemisphere and as many as 68% of them have some language abilities in both the left and the right hemisphere. The two hemispheres are thought to contribute to the processing and understanding of language: the left hemisphere processes the linguistic meaning of prosody (or, the rhythm, stress, and intonation of connected speech), while the right hemisphere processes the emotions conveyed by prosody.[15] Studies of children have shown that if a child has damage to the left hemisphere, the child may develop language in the right hemisphere instead. The younger the child, the better the recovery. So, although the "natural" tendency is for language to develop on the left, human brains are capable of adapting to difficult circumstances, if the damage occurs early enough.

The first language area within the left hemisphere to be discovered is Broca's area, named after Paul Broca, who discovered the area while studying patients with aphasia, a language disorder. Broca's area doesn't just handle getting language out in a motor sense, though. It seems to be more generally involved in the ability to process grammar itself, at least the more complex aspects of grammar. For example, it handles distinguishing a sentence in passive form from a simpler subject-verb-object sentence — the difference between "The boy was hit by the girl" and "The girl hit the boy."

The second language area to be discovered is called Wernicke's area, after Carl Wernicke, a German neurologist who discovered the area while studying patients who had similar symptoms to Broca's area patients but damage to a different part of their brain. Wernicke's aphasia is the term for the disorder occurring upon damage to a patient's Wernicke's area.

Wernicke's aphasia does not only affect speech comprehension. People with Wernicke's aphasia also have difficulty recalling the names of objects, often responding with words that sound similar, or the names of related things, as if they are having a hard time recalling word associations[citation needed].

[edit] Pathology

A human brain showing frontotemporal lobar degeneration causing frontotemporal dementia

Clinically, death is defined as an absence of brain activity as measured by EEG. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory, personality, and movement. Head trauma caused, for example, by vehicle or industrial accidents, is a leading cause of death in youth and middle age. In many cases, more damage is caused by resultant edema than by the impact itself. Stroke, caused by the blockage or rupturing of blood vessels in the brain, is another major cause of death from brain damage.

Other problems in the brain can be more accurately classified as diseases than as injuries. Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and Huntington's disease are caused by the gradual death of individual neurons, leading to diminution in movement control, memory, and cognition.

Mental disorders, such as clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic function. These disorders may be treated by psychotherapy, psychiatric medication or social intervention and personal recovery work; the underlying issues and associated prognosis vary significantly between individuals.

Some infectious diseases affecting the brain are caused by viruses and bacteria. Infection of the meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as "mad cow disease") is deadly in cattle and humans and is linked to prions. Kuru is a similar prion-borne degenerative brain disease affecting humans. Both are linked to the ingestion of neural tissue, and may explain the tendency in human and some non-human species to avoid cannibalism. Viral or bacterial causes have been reported in multiple sclerosis and Parkinson's disease, and are established causes of encephalopathy, and encephalomyelitis.

Many brain disorders are congenital, occurring during development. Tay-Sachs disease, Fragile X syndrome, and Down syndrome are all linked to genetic and chromosomal errors. Many other syndromes, such as the intrinsic circadian rhythm disorders, are suspected to be congenital as well. Normal development of the brain can be altered by genetic factors, drug use, nutritional deficiencies, and infectious diseases during pregnancy.

Certain brain disorders are treated by neurosurgeons, while others are treated by neurologists and psychiatrists.

Visualization of a diffusion tensor imaging (DTI) measurement of a human brain. Depicted are reconstructed axon tracts that run through the mid-sagittal plane. Especially prominent are the U-shaped fibers that connect the two hemispheres through the corpus callosum (the fibers come out of the image plane and consequently bend towards the top) and the fiber tracts that descend toward the spine (blue, within the image plane).

[edit] See also

[edit] Notes

  1. ^ Johanson, D. C. (1996). From Lucy to language. New York: Simon and Schuster, p. 80.
  2. ^ Murre, JM; Sturdy, DP (1995). "The connectivity of the brain: multi-level quantitative analysis". Biological cybernetics 73 (6): 529–45. doi:10.1007/BF00199545. PMID 8527499.  edit
  3. ^ Carpenter's Human Neuroanatomy, Ch. 1
  4. ^ Cosgrove et al., 2007
  5. ^ Lynn, Richard (Aug 1994). "Sex differences in intelligence and brain size: A paradox resolved.". Personality and Individual Differences Vol 17 (2): 257–271. doi:10.1016/0191-8869(94)90030-2. http://psycnet.apa.org/?fa=main.doiLanding&uid=1995-13422-001. 
  6. ^ Marner L, Nyengaard JR, Tang Y, Pakkenberg B. (2003). Marked loss of myelinated nerve fibers in the human brain with age. J Comp Neurol. 462(2):144-52. PubMed
  7. ^ Principles of Neural Science, p 324
  8. ^ Vanderwolf et al., 1978
  9. ^ Gray Psychology 2002
  10. ^ Toro et al., 2008
  11. ^ Fisch and Spehlmann's EEG primer
  12. ^ Preissl, Magnetoencephalography
  13. ^ Buxton, Introduction to Functional Magnetic Resonance Imaging
  14. ^ Andrews, Neuropsychology
  15. ^ Manlove, George (February 2005). "Deleted Words". UMaine Today Magazine. http://umainetoday.umaine.edu/issues/v5i1/stroke.html. Retrieved 2007-02-09. 

[edit] References

  • Gray, Peter (2002). Psychology (4th ed.). Worth Publishers. ISBN 0716751623. 
  • Kandel, ER; Schwartz JH, Jessel TM (2000). Principles of Neural Science. McGraw-Hill Professional. ISBN 9780838577011. 
  • Thompson, Richard F. (2000). The Brain: An Introduction to Neuroscience. Worth Publishers. ISBN 0-7167-3226-2
  • Vanderwolf, CH; Kolb; Cooley (Feb 1978). "Behavior of the rat after removal of the neocortex and hippocampal formation". Journal of comparative and physiological psychology 92 (1): 156–75. ISSN 0021-9940. PMID 564358.  edit

[edit] External links