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
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
- 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.
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.
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.
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.
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
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
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
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
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.
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.
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
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.
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
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.
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.
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
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.
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
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.
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.
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
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"
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
From Wikipedia, the free encyclopedia
This article is about features specific to the human brain.
For basic information about brains, see
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
cranium, it has the same general structure as the brains of
mammals, but is over three times as large as the brain of a
typical mammal with an equivalent body size.
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
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
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
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
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 , such as
multiple sclerosis, and many others. A number of psychiatric
conditions, such as
depression, are widely thought to be caused at least
partially by brain dysfunctions, although the nature of such
brain anomalies is not well understood.
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
size of around 1130
cubic centimetres (cm3) in women and 1260 cm3
in men, although there is substantial individual variation.
Men's brains are on average 100g heavier than a woman's, even
when corrected for body size differences
The brain is very soft, having a consistency similar to
soft gelatin or firm
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.
Drawing of the human brain, showing several
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.
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
In a human, comparable cerebral cortex damage produces a
permanent state of
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
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
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
sensory nerves and tracts by way of relay nuclei in the
thalamus. Primary sensory areas include the visual area of
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.
Major gyri and sulci on the lateral surface of the
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.
Anatomists call each cortical fold a
sulcus, and the smooth area between folds a
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
Brodmann, who split the cortex into 51 different areas and
assigned each a number (anatomists have since subdivided many of
the Brodmann areas ). 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
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
. 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 . 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
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
Routing of neural signals from the two eyes to the
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
The corpus callosum, a nerve bundle connecting the
two cerebral hemispheres, with the
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,
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
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.
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
brain article. Some techniques, however, are used mainly in
humans, and therefore are described here.
By placing electrodes on the scalp it is possible to record
the summed electrical activity of the cortex, in a technique
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.
Apart from measuring the electric field around the skull it
is possible to measure the magnetic field directly in a
technique known as
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.
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
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
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.
of brain damage
A key source of information about the function of brain
regions is the effects of damage to them.
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
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.
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
Broca's area, named after
Paul Broca, who discovered the area while studying patients
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
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
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
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
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
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
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
bacteria. Infection of the
meninges, the membrane that covers the brain, can lead to
Bovine spongiform encephalopathy (also known as "mad cow
disease") is deadly in
cattle and humans and is linked to
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
Many brain disorders are
congenital, occurring during development.
Fragile X syndrome, and
Down syndrome are all linked to
chromosomal errors. Many other syndromes, such as the
circadian rhythm disorders, are suspected to be congenital
as well. Normal
development of the brain can be altered by genetic factors,
nutritional deficiencies, and
infectious diseases during
Certain brain disorders are treated by
neurosurgeons, while others are treated by
Visualization of a
diffusion tensor imaging
of a human
. Depicted are reconstructed
tracts that run through the mid-sagittal
plane. Especially prominent are the U-shaped fibers
that connect the two
(the fibers come out of the
image plane and consequently bend towards the top)
and the fiber tracts that descend toward the
(blue, within the image plane).
Johanson, D. C. (1996). From Lucy to language.
New York: Simon and Schuster,
Murre, JM; Sturdy, DP
(1995). "The connectivity of the brain: multi-level
quantitative analysis". Biological cybernetics
73 (6): 529–45.
Neuroanatomy, Ch. 1
Cosgrove et al., 2007
Lynn, Richard (Aug 1994).
"Sex differences in intelligence and brain size: A
paradox resolved.". Personality and Individual
Differences Vol 17 (2): 257–271.
^ 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.
Principles of Neural Science,
Vanderwolf et al., 1978
Gray Psychology 2002
Toro et al., 2008
Fisch and Spehlmann's EEG primer
Buxton, Introduction to
Functional Magnetic Resonance Imaging
Manlove, George (February
"Deleted Words". UMaine Today Magazine.
- Campbell, Neil A. and Jane B. Reece. (2005).
Biology. Benjamin Cummings.
Peter (2002). Psychology (4th ed.). Worth
Kandel, ER; Schwartz JH,
Jessel TM (2000). Principles of Neural Science.
- Thompson, Richard F. (2000). The Brain: An
Introduction to Neuroscience. Worth Publishers.
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
Nervous system (TA
Central nervous system
Peripheral nervous system
central nervous system navs:
peripheral nervous system navs:
noncongen PNS somatic/autonomic/congen/neoplasia,