有關腦的很多誤解! Misconceptions about brain!!

Brain matter

 

Throughout
history, the human brain has been remarkably good at dismissing itself.
Everyone from ancient Egyptians to Aristotle has downplayed the role of the
mysterious stuff between our ears. Famed anatomist Galen gave the brain credit
as commander of movement and speech, but even he brushed aside the white and
gray matter, figuring the fluid-filled ventricles inside the brain did most of
the work.

 

Human brains
are big...

The average
adult brain weighs just under 3 pounds (between 1.3 and 1.4 kilograms). Some
neurosurgeons describe the texture of a living brain
as that of toothpaste, but according to neurosurgeon Katrina Firlik, a better
analogy can be found in the local health-food store.

"[The
brain] doesn't spread like toothpaste. It doesn't adhere to your fingers the
way toothpaste does," Firlik writes in her memoir, "Another Day in
the Frontal Lobe: A Brain Surgeon Exposes Life on the Inside" (Random
House, 2006). "Tofu — the soft variety, if you know tofu — may be a more
accurate comparison."

If you aren't
charmed by that description, consider this: About 80 percent of the contents of
your cranium is brain, while equal amounts of blood and cerebrospinal fluid,
the clear liquid that buffers neural tissue, make up the rest. If you were to
blend up all of that brain, blood and fluid, it would come to about 1.7 liters,
or not quite enough to fill a 2-liter soda bottle.

Credit:
Dreamstime.

...But they're
getting smaller

Don't get too
cocky about your soda-bottle-sized brain. Humans 5,000 years ago had brains
that were even larger.

"We do
know from archaeological data that pretty much everywhere we can measure —
Europe, China, South Africa, Australia — that brains have shrunk about 9 cubic inches (150
cubic centimeters), from an average of about 82 in³ (1,350 cm³).
That's roughly 10 percent," University of Wisconsin at Madison
paleoanthropologist John Hawks told LiveScience in 2009.

Researchers
don't know why brains might be shrinking, but some theorize that they're
evolving to be more efficient. Others think our skulls are getting smaller
because our diets include more easily chewable foods and so large, strong jaws
are no longer required.

Whatever the
reason, brain size doesn't directly correlate with
intellect, so there's no evidence that ancient man was brainier than humans of
today.

Our brains burn
through energy

The modern
brain is an energy hog. The organ accounts for about 2 percent of body weight,
but it uses about 20 percent of the oxygen in our blood and 25 percent of the
glucose (sugars) circulating in our bloodstream, according to the American
College of Neuropsychopharmacology.

These energy
requirements have spurred a debate among anthropologists about what fueled the evolution of big brains in the first place. Many
researchers credit meat, citing evidence of hunting in our early ancestors. But
meat would have been an unreliable food source, say other scientists. A 2007
study published in the Proceedings of the National Academy of Science found
that modern-day chimps know how to dig for calorie-rich tubers on the savanna.
Perhaps our ancestors did the same, boosting their brainpower with veggies.

As for what
motivated the brain to balloon in size, there are three major hypotheses:
climate change, the demands of ecology, and social competition.

Wrinkles make
us smart

What's the
secret to our species' smarts? The answer may be wrinkles. The surface of the
human brain is convoluted by deep fissures, smaller grooves called sulci, and
ridges called gyri. This surface is called the cerebral cortex and is home to
about 100 billion neurons, or nerve cells.

The folded,
meandering surface allows the brain to pack in more surface area — and thus,
more processing power — into the limited confines of the skull. Our primate relatives show varying degrees of
convolution in their brains, as do other intelligent creatures like elephants.
In fact, research done by Emory University neuroscientist Lori Marino has found
that dolphins have even more pronounced brain wrinkles than humans.

Most of our
brain cells aren't neurons

The old saw
that we use just 10 percent of our brainpower isn't true, but we now know that
neurons make up just 10 percent of our brain cells.

The other 90
percent, which account for about half the brain's weight, are called glia,
which means "glue" in Greek. Neuroscientists used to think glia were
simply the sticky stuff that holds neurons together. But recent research has
shown glia to be much more. A 2005 paper in the journal Current Opinions in
Neurobiology laid out the roles of these unsung cells, which range from mopping
up excess neurotransmitters to providing immune protection to actually promoting
and modulating synapse growth and function. (Synapses are the connections between neurons.) It turns out the
silent majority isn't so silent after all.

The brain is an
exclusive club

Like bouncers
at a night club, an assembly of cells in the brain's blood system, called the
blood-brain barrier, lets only a few molecules into the nervous system's inner
sanctum – the brain. The capillaries that feed the brain are lined with tightly
bound cells, which keep out large molecules. Special proteins in the barrier
transport necessary nutrients and substances into the brain. Only an elite few
make it through.

The blood-brain
barrier protects the brain, but it can also keep out lifesaving medications.
Physicians trying to treat brain tumors can use drugs to open the junctions
between cells, but that leaves the brain temporarily vulnerable to infection.
One new way to sneak meds past the barrier might be nanotechnology. A 2009
study published in the journal Cancer Research showed that specially-engineered
nanoparticles can cross the barrier and attach to tumor tissue. In the future,
combining nanoparticles with chemotherapy drugs could be one way to target
tumors.

The brain
starts as a tube

The foundation
for the brain is set early. Three weeks after conception, a sheet of embryonic
cells called the neural plate folds and fuses into the neural tube. This tissue
will become the central nervous system.

The neural tube
grows and differentiates throughout the first trimester. (When cells
differentiate they specialize into various tissues needed to create body
parts.) It isn't until the second trimester that glia and neurons begin to
form. The brain doesn't wrinkle up until even later. At 24 weeks, magnetic
resonance imaging shows just a few nascent grooves in the otherwise smooth
surface of the fetal brain, according to a 2000 study in the journal Radiology.
As the third trimester begins in week 26, the grooves deepen and the brain
begins to look more like that of a newborn.

Teen brains
aren't fully formed

Parents of
stubborn teenagers rejoice, or at least relax: That adolescent attitude stems, in part, from the
vagaries of brain development.

The gray matter
of the brain peaks just before puberty and is pruned back down throughout
adolescence, with some of the most dramatic development happening in the
frontal lobes, the seat of judgment and decision-making.

A 2005 study
published in the journal Child Development found that the parts of the brain
responsible for multitasking don't fully mature until we're 16 or 17 years old.
And research presented at the BA Festival of Science in 2006 revealed that
teens also have a neural excuse for self-centeredness. When considering an
action that would affect others, teens were less likely than adults to use the
medial prefrontal cortex, an area associated with empathy and guilt. Teens
learn empathy by practicing socializing, the researchers said. So much for
grounding them until they're 20.

Brains never
stop changing

Scientific
wisdom once held that once you hit adulthood, your brain lost all ability to
form new neural connections. This ability, called plasticity, was thought to be
confined to infancy and childhood.

Wrong. A 2007
study on a stroke patient found that her brain had adapted to the damage to nerves
carrying visual information by pulling similar information from other nerves.
This followed several studies showing that adult mice could form new neurons.
Later studies found more evidence of human neurons making new connections into
adulthood; meanwhile, research on meditation showed that intense mental
training can change both the structure and function of the brain.

Women aren't
from Venus after all

Popular culture
tells us that women and men's brains are just different. It's true that
male and female hormones affect brain development differently, and imaging
studies have found brain differences in the ways women and men feel pain, make
social decisions and cope with stress. The extent to which these differences
are genetic versus shaped by experience — the old nature-versus-nurture debate
— is unknown.

But for the
most part, male and female brains (and brainpower) are similar. A 2005 American
Psychologist analysis of research on gender differences found that in 78
percent of gender differences reported in other studies, the effect of gender
on the behavior was in the small or close-to-zero range. And recent studies
have debunked myths about the genders' divergent abilities. A study published
in the January 2010 Psychological Bulletin looked at almost half a million boys
and girls from 69 countries and found no overall gap in math ability. Focusing on our differences
may make for catchy book titles, but in neuroscience, nothing is ever that
simple.