Has a patient ever stumped you with a question you just weren’t ready for? “Why are yawns contagious?” … “Why do teeth turn yellow?” … “Why do some people have curly hair while others have straight hair?” … “What is that dangling thing at the back of your throat, and what does it do?” … “If humans get heart attacks, why don’t animals?” … Or the classic: “Why can’t you tickle yourself?” So that you’ll be ready the next time a curious patient asks one of these perplexing questions, here are the answers: ‘Why are yawns contagious?’ No one really knows for sure why you yawn after the person next to you yawns—or even when you simply see a photo of someone yawning. But it is indeed called “contagious yawning,” and there are a number of theories about why it happens. First, it’s not contagious in the same way that a virus is contagious. There appears to be no material or biophysical connection between one person “catching” a yawn from another. Rather, a popular hypothesis holds that yawns are contagious because of a kind of deeply ingrained empathy between humans. This empathy may take the form of spontaneously mirroring or matching another: I smile, then you smile. I yawn, so you yawn. Researchers also suspect that these reflexive social cues trace back to our animal instincts and herd mentality. However, the actual evidence for the empathy hypothesis is inconsistent and inconclusive. What we do know is that contagious yawning does occur, but not the same way in everyone. For instance, babies and toddlers may smile when their mothers smile, but don’t usually yawn when their mothers yawn. Older folks, too, are believed to be less susceptible to contagious yawns. Among adults, some 40% to 60% are likely to catch a yawn from another. So, if you’re one of the people who often yawns when another person yawns, then you could be a more sensitive, empathetic individual. Or maybe not. ‘Why do teeth turn yellow?’ Everyone wants a nice set of pearly whites. That’s how our teeth looked when we were babies. So, what happened as we got older? There are two fundamental causes of tooth discoloration: intrinsic factors and extrinsic factors. Intrinsic factors include metabolic diseases and systemic conditions—such as congenital or developmental defects—that cause yellow, brown, or even black-spotted teeth. Tetracycline antibiotics can turn teeth yellow or gray in developing babies and children, and browner with age. (On a side note, because tetracycline fluoresces under ultraviolet light, so do affected teeth, which glow bright yellow under a UV lamp.) Age is another intrinsic—and inevitable—cause of discolored teeth. As we age, a secondary layer of dentin (which is darker than primary dentin) grows beneath the enamel. Meanwhile, the enamel naturally thins as we get older, resulting in darker-looking teeth. Extrinsic factors include food, drink, and other causes. Coffee and tea, for example, stain teeth, as do red wine and cola. Smoking or chewing tobacco turn teeth yellow and brown along the gum line. Trauma to the tooth or enamel can also allow increase staining. ‘Why do some people have curly hair while others have straight hair?’ The first reason why some people have curly hair while some have straight hair has to do with genetics. Scientists used to think that curliness was dominant and straight hair was recessive, but we now know that several genes (many of them still unknown) play a role in shaping our hair. The second reason follows from the first. Genetics help determine the shape of a person’s hair follicle (the pore from which a single hair grows). It turns out that follicles aren’t the same shape for everyone. People with straight hair have hair follicles that run straight down into the dermis, while people with curly hair have hair follicles that are curved. In addition, the “bore” of the follicle also affects the curliness. A perfectly round follicle produces a straight hair, while an elliptical follicle grows a curly hair. Because the shape of your hair follicles doesn’t change after infancy, this largely explains why you usually have straight or curly hair for life. The third reason is keratin, or rather the distribution of it. Keratin, the largest component of hair, is a tough, fibrous protein that gives the hair its structure. In people with straight hair, keratin is distributed symmetrically in the hair shaft. But in curly hair, keratin is distributed asymmetrically. The eyelash, which curves but doesn’t coil, provides an interesting example in that a certain type of keratin is more heavily distributed from the convex side of the follicle. ‘What is that dangling thing at the top of your throat, and what does it do?’ As you surely recall from your anatomy classes, that drop-shaped tissue dangling from the back of the soft palate is the uvula. But do you recall what it does? Some doctors believed the uvula was a vestigial remnant from lower animals, which kept insects from going down the animals’ throats while running. (In fact, humans are the only animals with a full uvula.) Others suggested it helped our ancestors to drink water while bent over a primeval stream, for instance. Some believed it served no purpose at all. Even within the past century, the uvula was often removed as part of tonsillectomy because it was thought to contribute to infection. Traditional uvulectomy is still practiced in parts of Africa and the Middle East for various beliefs: to prevent disease and throat infections, to aid in breastfeeding, to improve speech, to cure “weakness,” and other reasons. In developed countries, surgical uvulectomy is sometimes performed to relieve symptoms of obstructive sleep apnea. But, still, what exactly does the uvula do? This little appendage may have at least three important jobs, which were only determined within the past 30 years or so. The uvula’s main job, simply put, is to keep your milk and cookies from spurting out of your nose when you’re eating and drinking. When you swallow, the soft palate and uvula move upward and backward to seal off the nasopharynx, helping to keep foods and liquids from surging into the nasal cavity. Along with other structures in the throat, the uvula helps stimulate the gag reflex. This prevents you from eating something too large for your throat and then choking. The uvula is also able to produce a large volume of saliva, which appears to keep the throat from getting too dry. “The uvula effectively bastes the mucosa of the pharynx and helps keep it moist and well lubricated,” wrote otolaryngology researchers in a literature review. ‘If humans get heart attacks, why don’t animals?’ Yes, humans are the only animal to have heart attacks. It’s certainly easy to imagine that if a gorilla ate a lot of bacon cheeseburgers and sat around watching a lot of TV, it too might develop atherosclerosis and suffer a heart attack. But it won’t—that gorilla won’t be healthy, but it won’t get a heart attack, either. (Keep in mind that even a healthy human on a vegetarian diet can still have a heart attack.) Just this year, researchers reported that our human ancestors lost a single gene a few million years ago, which may have led to a greater risk of cardiovascular disease in the human species. That missing gene may also result in an even higher risk for humans who have a hankering for bacon cheeseburgers (or any red meat, actually). Humans lack the CMAH gene, which went missing due to a mutation that occurred 2 to 3 million years ago (possibly due to a malarial parasite), the researchers explained. To test this theory, they deleted the CMAHgene in mice and then fed them a high-fat diet, which resulted in a nearly twofold increase in the severity of atherosclerosis compared with control mice. When the researchers added the equivalent of a large amount of red meat to their already high-fat diet, the experimental mice had an additional 2.4-fold increase in atherosclerosis. So, until we get back that CMAH gene, avoiding bacon cheeseburgers will reduce—but never totally eliminate—your risk for a heart attack. ‘Why can’t you tickle yourself?’ “It is well known that you cannot tickle yourself,” wrote neurologists in an article literally titled Why can’t you tickle yourself? (Of course, you can tickle yourself. You just won’t feel the urge to laugh.) These researchers showed that the cerebellum predicts sensations when you cause them but not when someone else does. “When you try to tickle yourself, the cerebellum predicts the sensation, and this prediction is used to cancel the response of other brain areas to the tickle,” explained lead researcher Sarah-Jayne Blakemore, PhD, professor, Cognitive Neuroscience, University College London, London, UK. Two brain regions are involved in processing the sensation of tickling. “The somatosensory cortex processes touch and the anterior cingulate cortex processes pleasant information. We found that both these regions are less active during self-tickling than they are during tickling performed by someone else, which helps to explain why it doesn't feel tickly and pleasant when you tickle yourself,” she noted. Source
