Curious Folks Ask Part 8

The tension-type headache is the kind most everyone has experienced. Nevertheless, its mechanism is not well understood. The old idea that it results from involuntary muscle contractions that block blood flow to the head has been ruled out, but it may have something to do with "a pain in the neck." Frequent sufferers of tension-type headaches experience increased tenderness of the neck muscles and tendons. Another, albeit controversial, hypothesis is that a tension-type headache is an earlier, less severe phase of a migraine, and that they share a similar mechanism.

Migraines affect 18 percent of women and 6 percent of men each year. The World Health Organization lists migraines among the top 20 causes of disability worldwide. Migraines are severe, often throbbing headaches that intensify with physical activity, and they may be accompanied by nausea and aversion to strong light, smells, or sounds. In some people the migraine is preceded by an aura-neurological symptoms such as flashing lights, blind spots, or numbness.

A migraine begins with a wave of decreased nerve activity that moves across the brain's cortex (surface layer). The decreased nerve activity, also known as the cortical spreading depression (CSD), initiates many changes in the levels of chemicals that brain cells use to communicate with each other and dilates the blood vessels in the dura mater. The swelling blood vessels stretch the nerves around them, causing them to send signals to the trigeminal nerve, which relays pain messages in the face and head.

Stress, certain foods, sleep disruptions, skipping meals, and hormonal changes can trigger migraines. Exactly how these factors trigger the CSD, and why the triggers are different for different people, is not yet understood.

Heart hurt What causes the nausea a.s.sociated with heart attacks?

A region in the brain stem that receives input from the body and other parts of the brain coordinates vomiting. Being able to detect and reject toxins inadvertently consumed with food has an obvious adaptive advantage for an organism. Therefore, it is not surprising that the digestive tract contains sense organs to detect noxious chemicals and convey that information to the brain.

Sense organs that affect nausea are also found in the chest area, including the heart and lungs. Exposing the heart to certain chemicals, mechanically distending it, or electrically stimulating the right cardiac nerve can initiate reflexes involved in vomiting. Sense organs in the left ventricle of the heart that detect tension appear to trigger the nausea a.s.sociated with heart attacks.

The adaptive benefit of this nausea response is unclear. However, since these same sense organs in the heart may be responsible for the nausea that sometimes accompanies heavy exercise, perhaps their role is to serve as a warning to an organism to prevent fatal overexertion.

Knowing thyself My sister has lupus. One of the tests the doctors did was an antinuclear antibodies (ANA) test, which detected antibodies to her cell nuclear contents. But aren't the nuclear contents contained in two membranes (cell and nuclear), so the white blood cells should have no contact with them? Can the white blood cells tell the self DNA apart from non-self DNA?

DNA and other contents of the cell's nucleus are indeed carefully contained within healthy cells. In contrast, when cells die, the contents of the nucleus are often released. So our white blood cells-which defend us against invading microbes-do come into contact with DNA from our body's cells (self DNA).

White blood cells produce protein weapons called antibodies that bind to and neutralize invaders. White blood cells are skilled warriors and make different antibody weapons for different intruders. Therefore, blood tests for many diseases work by identifying specific antibodies in the blood.

For 40 years, the ANA test has been used to help diagnose lupus. Despite the test's long history, two things remain puzzling. First, if everyone's white blood cells come into contact with self DNA, why doesn't everyone have antibodies against it? It turns out that we do, but the antibodies are present in much smaller amounts, and bind to DNA much more weakly, than the anti-DNA antibodies found in lupus patients.

The second puzzle is whether these antibodies play a role in causing the symptoms of lupus. In systemic lupus, the body's immune system attacks its own tissues, including the joints and internal organs. Antibodies would not harm the body by simply binding to DNA released by dying cells.

Researchers have found that the antibodies tend to collect in the kidneys of lupus patients, where they may penetrate into cells. The exact role of these antibodies in lupus remains under investigation.

White blood cells can tell the difference between self and non-self DNA, or at least between DNA from bacteria versus DNA from mammals. Since the DNA building blocks, or bases, used by bacteria (A, T, G, C) are the same as the ones that make up our genes, this is surprising.

However, although the building blocks are the same, the way they are strung together is different. Specifically, bacteria have many more sequences that are particularly rich in C and G bases than we do. Also, in us this sequence is more likely to be modified by the addition of four atoms called a methyl group. These features allow white blood cells to distinguish bacterial DNA from our DNA.

Halo of stars I've always wanted to know what causes the sensation of seeing stars. Today, after swimming laps, I was lying in the sun. As I opened my eyes, about to get up, I saw intermittent small white dots buzzing around for about 30 seconds. My best guess is that it has something to do with oxygen. Can you shed some light on this for me?

There are at least three possible reasons for seeing stars. Sylvester the cat saw stars when Granny walloped him over the head for trying to eat Tweety Bird. A blow to the head can cause the vitreous fluid that fills the back two-thirds of the eyeball to rub against the retina. In fact, as we age, the vitreous fluid becomes thicker and can push or pull on the retina even with more modest movements of the head.

The retina does not feel pain; it just responds to stimulation by sending a light signal, according David Granet, a professor of ophthalmology at the University of California San Diego School of Medicine. Certain types of exertion cause the "stars" by stimulating the retina. "Of course, a shower of stars, flashing light, or a curtain on vision are all potential warning signs of retinal detachment and should be of concern," he said.

Injury to the retina should be treated immediately to minimize further tearing and bleeding into the eye. If the damage is not too extensive, retinas can be repaired with a laser on an outpatient basis.

Another reason for seeing stars is small clumps of gel that form in the vitreous fluid. These "floaters" cast a shadow on the retina when they pa.s.s in front of it and are most obvious when you are looking at a plain, light-colored background.

The third reason for seeing stars has to do with levels of oxygen and/or nutrients reaching the brain. According to Joseph Scherger, a professor and physician at the UC San Diego School of Medicine, "The brain, including vision, runs on glucose, oxygen, a balance of electrolytes, and ample circulation/blood pressure. One might have visual changes like 'stars' if any of these are low."

Hot, hot, hot I asked my physician if the temperature of a hot flash can be measured and what it might be, and he said he didn't know. I've tried with my home thermometer, and it always reads normal, but I don't feel normal! Can you measure the temperature of a hot flash? If so, how? Where in the body does the signal for a hot flash originate?

Hot flashes seem to be triggered by an overly sensitive body thermostat. A useful a.n.a.logy is a house thermostat that is adjusted so that a temperature increase of a fraction of a degree switches on the air conditioning. A false alarm, such as a waft of heat from opening the oven door, could activate the air conditioning, but since the house was not overly warm to begin with, the AC would rapidly turn off.

When the body's thermostat, which is located in the hypothalamus of the brain, decides it is too hot, it cranks on the body's air conditioning-sweating and dilation of blood vessels in the skin. The rush of warm blood to the skin creates the feeling of intense heat characteristic of the hot flash. If the ambient temperature is not actually high, our AC quickly shuts down. The blood vessels in the skin constrict, and the blood drains away, leaving the skin pale and cold.

Studies have shown that, on average, women who experience hot flashes have a lower core body temperature and a lower sweating threshold-that is, they begin sweating at a lower body temperature-than women who do not experience hot flashes. However, the temperature difference is small, just a fraction of a degree, and therefore requires a very sensitive thermometer to measure.

The standard explanation for the occurrence of hot flashes during menopause is that they are triggered by declining levels of estrogen. Estrogen has been shown to ameliorate hot flashes by increasing the sweating threshold. It is not understood how declining estrogen levels increase the sensitivity of the nerve cells in the brain that control body temperature.

Although considered the hallmark of the menopausal transition, hot flashes can occur at other times of life and can affect both women and men. Also, not all women experience hot flashes during menopause. Research is ongoing to determine if other hormones are involved, and to identify health and lifestyle factors that might increase a woman's risk of having hot flashes.

7. Uniquely human

Odd eats

Why do humans cook food, and when did they start doing this? Do any other animals modify their food intake?

Early humans probably first realized the value of cooked food when they tasted tubers-root vegetables like potatoes and ca.s.sava-that had been roasted by a lightning-sparked gra.s.s fire. Not only are cooked tubers more delectable, but heat alters the structure of starches and proteins, making them easier to digest and rendering some poisonous vegetables edible.

How long ago humans were able to control fire is still under dispute. By 250,000 years ago, our ancestors could certainly invite the neighbors over for a barbeque. Across Europe and the Middle East, ancient earthen ovens with burned animal bones date from that time. Some anthropologists argue that humans controlled fire almost 2 million years ago. They point to circular areas of scorched earth almost that old discovered in Africa. These "bonfires" contained a mixture of burned wood types, which suggests that they were deliberately set, rather than the remains of a tree struck by lightning.

Wild and domesticated animals can learn to distinguish a food's nutritional properties based on its appearance, smell, or taste. Animals modify their diets according to how their nutritional needs change as they mature, during pregnancy and lactation, and as a result of disease. Zoopharmacognosy-self-medication by animals-is a particularly intriguing aspect of diet modification.

Ill animals of many different species consume things that normally are not a part of their diets but that have medicinal properties. These include laxatives, antidiarrheals, antibiotics, antiparasitics, and antidotes to toxins they have previously consumed. For example, wild chimpanzees with parasitic infections eat leaves from a shrub commonly known as bitter leaf. The leaves contain several chemicals that can kill parasites that cause malaria and other tropical infections.

Many animals eat soil-a habit known as geophagy. Soil is a source of minerals. Geophagy is also a form of zoopharmacognosy. Soil containing certain types of clay combats diarrhea. By binding to toxic plant compounds, soil makes some plants safer to eat. Soil also can enhance plants' pharmacological activities by binding to interfering compounds.

A study showed that zoopharmacognosy requires learning; it is not purely instinctive. In the study, lambs were given food laced with one of three chemicals that cause stomachaches. Then they were given a choice of three medicines, each of which would cure a stomachache caused by only one of the chemicals. Only lambs that had prior experience being cured by the appropriate medicine could select it when given the choice of all three.

Sink like a stone Humans, as primates, typically learn how to swim when they're old enough to follow instructions from swimmers. Can any other primates swim instinctively? What other mammal species do not swim instinctively?

I remember being surprised as a child to learn that our cat could swim very well, although he much preferred terra firma terra firma. Many land mammals can swim, and they tend to use a similar gait in water as they do on land.

It is difficult to know for sure what mammals cannot swim, since many avoid water if given the choice. However, rats, mice, horses, elephants, camels, bears, antelope, skunks, at least some species of bats, and at least one species of armadillo reportedly can swim.

According to the San Diego Zoo's a.s.sociate Curator of Mammals, Karen Killmar, most monkeys can probably swim. This behavior has not been doc.u.mented in all species but has been seen in many. In contrast, there are no reports of great apes (gorillas, chimpanzees, orangutans) swimming. They have been observed in the wild wading in deep water, but not actually swimming. Most researchers do not believe that these species have swimming as an instinctive behavior.

A word of caution to pet owners: Some dogs do not like to swim, and they may panic in deep water, especially if a steep bank makes it difficult for them to climb out. And, of course, a strong current can fatigue even the best swimmer, canine or otherwise.

Dino breath Not considering the millions of years dividing their times on Earth, am I mistaken in thinking that humans could not have survived in the oxygen content of the air during the dinosaur era?

Dinosaurs roamed the Earth from about 230 million years ago to about 65 million years ago. Estimates of the oxygen levels during the dinosaurs' reign differ greatly, but a 2005 study published in the journal Science Science found that the atmosphere's oxygen concentration has increased over the last 205 million years from 10 percent to 21 percent. found that the atmosphere's oxygen concentration has increased over the last 205 million years from 10 percent to 21 percent.

Another study found evidence that about 240 million years ago, oxygen concentrations dropped precipitously and rapidly from about 35 percent to about 12 percent. Therefore, based on these two studies, dinosaurs survived in an oxygen concentration as low as about half of today's levels.

Today, at high or low elevations, 21 percent of the molecules in the air are oxygen, but fewer air molecules (in a specific volume of air) are present at higher elevations. On the Andean and Tibetan plateaus, about 13,000 feet (4 kilometers) above sea level, each breath you take would contain about as many oxygen molecules as it would if taken at sea level back in the time of the earliest dinosaurs.

Humans can survive under these low-oxygen conditions. In fact, some Andean miners live for long periods at nearly 20,000 feet, where there is even less oxygen.

Just because human populations can survive at these lower oxygen concentrations does not mean that oxygen levels had no role in the evolution of mammals. Small mammals coexisted with dinosaurs. However, the Science Science paper reported that a dramatic increase in the size and diversity of mammals occurred between 100 and 65 million years ago, during a period of relatively high and stable oxygen levels. paper reported that a dramatic increase in the size and diversity of mammals occurred between 100 and 65 million years ago, during a period of relatively high and stable oxygen levels.

The paper's authors think that the increase in oxygen levels may have facilitated the evolution of large mammals. Larger mammals have fewer blood vessels per unit of muscle than smaller mammals. As a result, larger mammals need higher levels of oxygen in the environment to achieve maximum rates of metabolism.

Dinosaurs may have had lower rates of metabolism, and lower oxygen needs, than mammals. In addition, a group of dinosaurs that includes the brontosaurus is thought to have had a respiratory system similar to that of modern birds, with a series of air sacs that act like bellows to move air through the lungs. The system allows fresh air to flow through the lungs continuously and could have given dinosaurs a survival advantage in low-oxygen conditions.

Modern man Has the h.o.m.o sapiens h.o.m.o sapiens species stopped evolving due to our scientific progress in overcoming the survival of the fittest, or are we still undergoing small changes that are not easily perceived? species stopped evolving due to our scientific progress in overcoming the survival of the fittest, or are we still undergoing small changes that are not easily perceived?

The h.o.m.o sapiens h.o.m.o sapiens species has existed about 200,000 years, but just 10,000 years ago the transition from hunter-gatherer to agricultural societies created significant evolutionary pressures. Specifically, diet changed, and the spread of infectious diseases increased as population densities increased. species has existed about 200,000 years, but just 10,000 years ago the transition from hunter-gatherer to agricultural societies created significant evolutionary pressures. Specifically, diet changed, and the spread of infectious diseases increased as population densities increased.

By comparing the genomes of various modern individuals, geneticists can determine how quickly our DNA sequences have been changing, and whether the changes are random or result from some sort of evolutionary pressure.

One interesting example is the gene for lactase. Lactase breaks down lactose, the main sugar in milk. A version of the gene that permits adults to digest lactose is prevalent in people of European ancestry and some African populations but is very rare in Southeast Asian and sub-Saharan African populations. The geographic distribution and timing of the gene's increase in prevalence correspond to the rise of dairy farming.

Other genes that have changed in one or more populations as a result of relatively recent (over the past 10,000 years) evolutionary pressures include genes that play a role in metabolism, taste and smell, fertility, and skin pigmentation.

In developing nations, where AIDS, malaria, and other scourges kill millions every year, genes that provide resistance to disease are under selection pressure. For example, in regions of the world where malaria is or was recently found, certain versions of genes for hemoglobin-the oxygen-carrying protein in the blood-have become prevalent. These versions of the gene provide some resistance to malaria but can cause blood diseases such as sickle cell anemia.

Some scientists have argued that in developed countries, evolutionary pressures have been relaxed to the extent that humans are no longer evolving. However, others argue that we are still evolving, because not everyone makes equal contributions to the next generation. In addition, they predict that changing climate and increasing populations will create renewed evolutionary pressures.

How evolution may impact how future humans look is impossible to predict. Ironically, the most significant changes in our looks compared to those early farmers are not genetic at all. Increases in height can be tied to better nutrition, obesity is linked to diet and sedentary lifestyles, and smaller jaws develop when people eat softer-textured food. Although genetic factors play a role in height, metabolism, and bone structure, these changes have taken place too rapidly to be purely genetic.

World tour What caused the migration of early humans out of Africa? Did sufficient climatic change occur to make it better to leave than to risk trying to adapt to some sort of environmental change?

Based on genetic and fossil evidence, it is widely accepted that humans originated in Africa. Many researchers recognize two major phases of dispersal. The first, Out of Africa 1, began almost 2 million years ago with h.o.m.o erectus h.o.m.o erectus, the first truly upright-walking human ancestor. The second, Out of Africa 2, began about 100,000 years ago with h.o.m.o sapiens h.o.m.o sapiens, which evolved in Africa between the two phases of dispersal and eventually replaced archaic humans.

Other researchers consider this view simplistic. They generally agree about the early dispersal, but they propose that multiple later dispersals occurred, some of which may have been from Europe and Asia back to Africa. One reason for the uncertainty is that human fossils are rare compared with stone artifacts, and the cultural traits implied by the tools cannot be reliably tied to biological characteristics of the populations that created them.

What caused humans to disperse is unknown. Some explanations focus on unique features of human culture. Other explanations, acknowledging that other successful species also disperse, focus on environmental changes.

One hypothesis is that the first dispersal was caused by one group of humans outcompeting another. This hypothesis is based on the discovery that two technologically distinct populations of humans existed at the time, and only the population of less advanced toolmakers dispersed from Africa, perhaps because they were at a disadvantage on their shared range. A hypothesis based on advances in toolmaking and compet.i.tion between groups also has been proposed to explain the second dispersal.

Oscillations from wet to dry conditions occurred around the times of the early and late dispersals. The climatic changes were accompanied by the dispersal of other large animals, which may have been followed and exploited by human populations.

Whatever the reason the dispersals began, a factor that probably facilitated them by enhancing humans' survival is the reduction in zoonotic diseases. These are diseases, such as sleeping sickness, that typically rely on animals for transmission but also affect humans. Zoonotic diseases are especially prevalent in parts of Africa, compared to cooler, drier climates away from the tropics.

Tree house Why did early man come down from the trees, when he had no protection on the ground from predators such as lions and tigers?

The dominant view among researchers who study human evolution has been that, beginning about 5 million years ago, ground-dwelling humans who could exploit large herds of game on the African savannah arose from tree-dwelling, vegetarian apes. Some maintain that key human traits, including our upright walk and big brains, evolved because of the challenges of life on the open savannah.

Clearly the greater risk of predation and fierce compet.i.tion for prey are problems with this hypothesis. If it is accurate, the transition from trees to savannah was probably gradual and fueled by climate change. The transition is thought to have taken place at a time when the African continent was becoming more arid, which would have resulted in the fragmentation of forested areas. Hominins-bipedal primates-would have been forced to spend more time on the ground moving between wooded areas and would have needed to exploit resources available in gra.s.slands.

Other researchers make the case that humans could not control large plains until they domesticated riding animals-horses in Asia and camels in more arid regions-and that occurred within the past 10,000 years. Dissatisfaction with the savannah-dwelling hypothesis has recently led to the proposal of two alternatives: the aquarboreal hypothesis and the tectonic hypothesis.

According to the aquarboreal hypothesis, the transition from trees to ground occurred in coastal forests, where hominins could gather wetland plants and sh.e.l.lfish. As forests became more fragmented, hominins later dispersed along coastal areas and rivers. A beachcomber phase that included diving could explain humans' excellent voluntary breath control, subcutaneous fat layer, and lack of fur. Such traits are unique among primates but are found in dolphins, hippos, and walruses.

According to the tectonic hypothesis, hominins evolved and expanded in the African Rift, which extends from north to south along eastern Africa. The rift is a rugged terrain formed by volcanic activity and plate tectonics-movement and deformation of the Earth's crust. Within this complex topography, agile bipeds could gain a tactical advantage over faster-moving quadruped prey and find protection from predators. Humans may have dispersed from Africa into Europe and Asia along a virtually continuous line of tectonically active terrain, as suggested by the locations of the earliest and best-doc.u.mented sites of human occupation outside of Africa.

Loners Many different animals, such as birds, have hundreds of different species. Why aren't there tens or hundreds of different species of humans?

In Bones, Stones and Molecules Bones, Stones and Molecules (2004), authors David Cameron and Colin Groves comment that researchers who work with the fossil record consider the present era, which has only one (2004), authors David Cameron and Colin Groves comment that researchers who work with the fossil record consider the present era, which has only one h.o.m.o h.o.m.o species, as a unique time in the history of our lineage. In other words, multiple species of humans probably coexisted at various times in human history. species, as a unique time in the history of our lineage. In other words, multiple species of humans probably coexisted at various times in human history.

Based on the known fossil data, most scientists propose that archaic humans first dispersed from Africa approximately 1.8 million years ago. Populations settled in different regions and evolved independently.

Modern humans (h.o.m.o sapiens) probably emerged in Africa between 250,000 and 150,000 years ago. They later dispersed, and by 40,000 years ago, h.o.m.o sapiens h.o.m.o sapiens occupied most parts of Africa, Asia, Europe, and Australia. At that time, the Neanderthals ( occupied most parts of Africa, Asia, Europe, and Australia. At that time, the Neanderthals (h.o.m.o neanderthalensis) still occupied parts of Europe and Asia. h.o.m.o erectus h.o.m.o erectus likely still existed in Indonesia (although the fossil evidence is a bit sketchy). likely still existed in Indonesia (although the fossil evidence is a bit sketchy).

What happened next has been the subject of much speculation. One hypothesis is that modern humans clashed violently with the indigenous human populations they encountered, eventually eliminating them. Another hypothesis is that some interbreeding occurred, and that, for example, we each have a little Neanderthal in us. Finally, it may be that modern humans were simply more successful at competing for the available resources, and the other human species just died out.

The fossil record is not complete enough to tell us what happened to all early humans. The greatest amount of information is known about the extinction of the Neanderthals, 27,000 years ago. Evidence to suggest that h.o.m.o sapiens h.o.m.o sapiens engaged in ma.s.s genocide of engaged in ma.s.s genocide of h.o.m.o neanderthalensis h.o.m.o neanderthalensis is lacking. Similarly, the available DNA evidence suggests that interbreeding between modern humans and Neanderthals was uncommon-at least, we did not inherit Neanderthal genes. is lacking. Similarly, the available DNA evidence suggests that interbreeding between modern humans and Neanderthals was uncommon-at least, we did not inherit Neanderthal genes.

It is most likely that modern humans drove Neanderthals to extinction by outcompeting them. Modern humans seem to have hunted and gathered over larger areas than the Neanderthals, who tended to remain in the valley systems they had long occupied. Therefore, modern humans were more efficient at exploiting the environment for limited resources. h.o.m.o erectus h.o.m.o erectus also seems to have become extinct at the same time also seems to have become extinct at the same time h.o.m.o sapiens h.o.m.o sapiens appeared in their region, likely also as a result of compet.i.tion for limited resources. appeared in their region, likely also as a result of compet.i.tion for limited resources.

Living link?

On the Science Channel, I saw Oliver, the "questionable chimpanzee." I remember seeing him on the news in the past. I was always intrigued with his upright walk, strong manlike shoulders, and wise eyes. I wondered about his relatives. I found out that Oliver had 47 chromosomes, while chimpanzees have 48 chromosomes. Humans have 46 chromosomes. Does that make Oliver a "link" between a chimp and a human?

Starting in the 1970s, Oliver was promoted as a missing link or "humanzee" because of his unusual physical and behavioral traits, as well as the rumor that he had 47 rather than 48 chromosomes. According to reports, his arms and legs were too long, his ears a funny shape, his head too bald, and his face too small for him to be a chimpanzee. He also walked with a locked-knee bipedal (two-legged) gait.

Primatologists who examined Oliver pointed out that chimps vary widely in their physical characteristics. In addition, most of Oliver's teeth were pulled when he was very young to prevent him from biting people. As a result, the muscles in his lower face and temples, and even the bones in his jaws, remained underdeveloped. His training could account for his bipedal gait.

Despite these explanations, doubts about his karyotype-chromosome number and characteristics-remained. An early genetic test attributed to unidentified "American scholars" depicted 47 chromosomes followed by a question mark and was exploited by Oliver's owners to promote him as a missing link.

The karyotype mystery was not solved definitively until 1998, when Oliver was moved to a sanctuary. The results of genetic tests published that year in the American Journal of Physical Anthropology American Journal of Physical Anthropology showed that Oliver had 48 chromosomes and that his DNA sequence was highly similar to that of the Central African variety of chimpanzee. The researchers narrowed Oliver's probable birthplace to Gabon by comparing Oliver's DNA sequence to DNA sequences from other chimpanzees of known origin. showed that Oliver had 48 chromosomes and that his DNA sequence was highly similar to that of the Central African variety of chimpanzee. The researchers narrowed Oliver's probable birthplace to Gabon by comparing Oliver's DNA sequence to DNA sequences from other chimpanzees of known origin.

Even if Oliver had 47 chromosomes, this would not make him a missing link. During the production of egg and sperm cells, a process called meiosis separates chromosome pairs and reduces the number of chromosomes by half so that an offspring gets one chromosome of each pair from each parent. Errors can occur during meiosis and result in offspring with more or fewer than the normal chromosome number. For example, people with Down syndrome, caused by an extra 21st chromosome, and those with Klinefelter's syndrome, caused by an extra X chromosome, have 47 chromosomes.

Tears for fears Why is it that when you are about to cry, a lump forms in your throat?

Our bodies instinctively interpret negative emotions such as anger, sorrow, and fear as stress. In the face of a stressful situation, our nervous systems switch from "rest-and-digest" mode to "fight-or-flight" mode. This response is a relic from our pre-civilization days, when stress usually was a result of life-threatening situations.

The switch between modes is a function of the autonomic nervous system (ANS). Because its actions are mostly out of our conscious control, the ANS is also referred to as the involuntary nervous system. It is composed of the sympathetic nervous system, which activates the fight-or-flight response, and the parasympathetic nervous system, which controls opposite but complementary actions to promote recuperation and restart regular body maintenance activities.

To prepare the body to confront or run from danger, the sympathetic nervous system stimulates the adrenal glands to produce adrenalin, dilates the pupils, increases heart rate and blood pressure, and diverts blood from the intestines to make it available to the muscles in the limbs. The cessation of digestion can result in the nausea that often accompanies sorrow.

The sympathetic nervous system also increases air intake into the lungs. To allow more air to enter the lungs, the throat must open. The opening is relatively small during normal breathing. In response to the stress that initiates crying, the glottis-the gap between the vocal cords and the a.s.sociated muscles in the throat-expands as wide as possible.

In contrast, during the act of swallowing, the airway needs to be closed to keep out food and liquids. As food is pushed down the esophagus, the upper portion of the airway is lifted by the muscles at the back of the throat, the glottis constricts, and the epiglottis-the flap of cartilage lying just beneath the base of the tongue-is closed over the glottis.

The sensation of a lump in the throat is a result of the glottis muscles being told to open and close at the same time. In other words, the glottis is caught in a tug-of-war. The feeling is usually relatively short-lived, but some people under stress experience it for weeks or months. The sensation is called globus syndrome or globus hystericus if medical tests rule out injury or disease as a cause.

Blind dreams Do people who are born blind dream? If so, what do they see in their dreams?

For most people, dreaming is an intensely visual experience. Visual imagery is nearly always present in the dreams of sighted people, and their dreams are usually in color. Auditory sensations occur in more than half of their dreams, but only a small percentage involve taste, smell, and touch.

Blind people who lost their vision after about age 5 continue to have visual imagery in their dreams. They see new friends, places, and things in their dreams, not simply memories they've retained since before they lost their sight. The fact that dreams do not reflect people's current visual impairment reveals that the dreaming brain does not simply reproduce perceptions (albeit in a new narrative), but actually constructs things that have never been experienced while awake.

In contrast to people who lost their sight later in life, people completely blind since birth (congenitally blind) or shortly thereafter lack the rapid eye movements usually a.s.sociated with dreaming. However, they do dream, and they tend to describe their dreams in the same visual language as a sighted person. When asked to elaborate, it becomes clear that the "pictures" in their heads have been created through prior experience with other senses. In dream reports made by congenitally blind people, more than half of the sensory references are to touch, smell, and taste. The rest of the sensory references are auditory.