What happens when we stretch?

What happens when we stretch?

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From the wikipedia page on stretching:

Stretching is a form of physical exercise in which a specific muscle or tendon (or muscle group) is deliberately flexed or stretched in order to improve the muscle's felt elasticity and achieve comfortable muscle tone.

How is this improvement of elasticity achieved, at the physiological level?

Searching around on this topic brings a lot of articles about the types of stretches (e.g. again, wikipedia) and what are their benefits and disadvantages. But how do they differ at the biological level?

Short answer: Due to habituation. You do something enough number of times, then that event ceases to be something new, and your body adapts to it.

In this case, you train your body to accept more risks allowing greater lengthening, before it begins to internally signal the muscles that an injury is incoming.

Read below for the slightly longer version:

The stretching of a muscle fiber begins with the sarcomere, the basic unit of contraction in the muscle fiber. As the sarcomere contracts, the area of overlap between the thick and thin myofilaments increases. As it stretches, this area of overlap decreases, allowing the muscle fiber to elongate. Once the muscle fiber is at its maximum resting length (all the sarcomeres are fully stretched), additional stretching places force on the surrounding connective tissue.

There are two kinds of muscle fibers: intrafusal muscle fibers and extrafusal muscle fibers. Extrafusil fibers are the ones that contain myofibrils. Intrafusal fibers are also called muscle spindles and lie parallel to the extrafusal fibers and are the primary proprioceptors in the muscle.

When the muscle is stretched, so is the muscle spindle. The muscle spindle records the change in length (and how fast) and sends signals to the spine which convey this information. This triggers the stretch reflex (also called the myotatic reflex) which attempts to resist the change in muscle length by causing the stretched muscle to contract. The more sudden the change in muscle length, the stronger the muscle contractions will be (plyometric, or "jump", training is based on this fact). This basic function of the muscle spindle helps to maintain muscle tone and to protect the body from injury.

One of the reasons for holding a stretch for a prolonged period of time is that as you hold the muscle in a stretched position, the muscle spindle habituates (becomes accustomed to the new length) and reduces its signaling. Gradually, you can train your stretch receptors to allow greater lengthening of the muscles.

You can read more about it (including the role of the lengthening reaction by the gogli tendon organs) here:

It has also been published in a short section in this paper. I am sure you can follow the reference chain if you need to.

Smith, Craig A. "The warm-up procedure: to stretch or not to stretch. A brief review." Journal of Orthopaedic & Sports Physical Therapy 19.1 (1994): 12-17.


Stretching is a form of physical exercise in which a specific muscle or tendon (or muscle group) is deliberately flexed or stretched in order to improve the muscle's felt elasticity and achieve comfortable muscle tone. [1] The result is a feeling of increased muscle control, flexibility, and range of motion. Stretching is also used therapeutically to alleviate cramps and to improve function in daily activities by increasing range of motion. [2] [3] [4]

In its most basic form, stretching is a natural and instinctive activity it is performed by humans and many other animals. It can be accompanied by yawning. Stretching often occurs instinctively after waking from sleep, after long periods of inactivity, or after exiting confined spaces and areas.

Increasing flexibility through stretching is one of the basic tenets of physical fitness. It is common for athletes to stretch before (for warming up) and after exercise in an attempt to reduce risk of injury and increase performance. [5] : 42

Stretching can be dangerous when performed incorrectly. There are many techniques for stretching in general, but depending on which muscle group is being stretched, some techniques may be ineffective or detrimental, even to the point of causing hypermobility, instability, or permanent damage to the tendons, ligaments, and muscle fiber. [6] The physiological nature of stretching and theories about the effect of various techniques are therefore subject to heavy inquiry.

Though static stretching is a part of some pre and post-workout routines, a review article that was published in January 2020 by the Scandinavian Society of Clinical Physiology and Nuclear Medicine, indicated that pre-exercise static stretching did in-fact reduce an individual's overall muscular strength and maximal performance. Furthermore, these findings present a uniform effect, regardless of an individual's age, sex, or training status. [7] For this reason, an active dynamic warm-up is recommended before exercise in place of static stretching. [8] [9] [10] [ medical citation needed ]

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Now, what if I told you, you could make your cells grow, divide, expand, even die, by simply pushing or pulling on them. You’d say, hey, I thought that was the whole role of DNA, right– signaling when cells should do that? Now, I would say you’ve already seen it happening around you– skin growing, expanding to accommodate a pregnancy, right? More skin– bigger belly. Your doctor says, hey, if you want to avoid losing bone mass, osteoporosis– go lift some weights, encourage new bone growth. So scientists, physicians, have known about this mechanical stimulation for many decades, but only recently has the technology of smart robotics been brought into the picture.

Here’s a case in point. A new robot described in the journal Science Robotics this week, sets out to harness the power of pulling. When fastened along a pig’s esophagus over a period of nine days, the robot gently stretched the tissue by more than 10 millimeters– that’s 10 millimeters in nine days– much of that from cell division, alone. So for babies born with rare birth defects called esophageal atresia, this could make a real difference in their lives. So why exactly did this work and what kinds of medical interventions could we perform with pushing and pulling or prodding robots? Well, that’s what we’re going to be talking about this hour. You want to join us– 844-724-8255. 844-SciTalk. You can also tweet us at @scifri.

Let me introduce my guest– Dana Damian, a lecturer, Director of the Biomedical Robotics Lab at Sheffield University in the UK, one of the creators of the esophagus tugging bot. She joins us by Skype. Welcome to Science Friday, Dr. Damian.

DR. DAMIAN: Hello. Hi, thank you. Thank you for this invitation.

IRA FLATOW: You’re welcome. And David Mooney, Professor of Bioengineering at Harvard in Cambridge, welcome, Dr. Mooney.

DR. MOONEY: Thank you. It’s my pleasure to be here.

IRA FLATOW: Dr. Damian, when I hear, “robot,” I think as something that walks around and maybe talks– that’s not quite what you’re describing, though. Describe it for us, please.

DR. DAMIAN: That’s true. Yes, as a matter of fact, in robotics we are very much used to see robots that manipulate, robots that walk, just as you said. So I think where we’ve been taking a little bit of a different approach, because, in the body, it’s not so much about manipulation– well, actually, it a bit of manipulation– it’s not so much about walking, especially at the scale of a centimeter. What the body really does is, or the action that usually take place in the body is a lot about pressure and fluids running through organs, through the body.

IRA FLATOW: Tell us about what you did in the esophagus– you put two rings in there, in the esophagus?

DR. DAMIAN: Yes, yes. Yes, so we have this robot that attaches to an esophagus using two rings. The robot is equipped with sensors– [? four ?] sensor, a position sensor– and so we’re able to measure how long we are displacing the tissue or how much force we apply to the tissue at any time during the treatment. And then, just as you said, we have we have a [? monitor ?] that can apply gentle forces on the tissue of desire values, intensities or signed [INAUDIBLE]. And so what we’ve done, we’ve applied about 2.5 millimeter tissue displacement per day over nine days, and we were demonstrating in vivo trials with swine animal that we elongate the tissue more than 77%.

IRA FLATOW: Why does this happen? Do we know?

DR. DAMIAN: Well, there’s a lot of things that we don’t know, for sure, and maybe David will have his better explanation than I do, but it does look like mechanical tension is a powerful driving force in the physiology of the tissue and the signal that cells receive through this mechanical stimulation can regulate the [? fate ?] of the cell.

IRA FLATOW: David, Dana has been telling us about this tissue– it’s not stretching it, it’s actually making new cells grow. Correct? Well,

DR. MOONEY: So I think the two things are related. So by applying a stretching, she’s then able to induce a growth of the tissue.

IRA FLATOW: Mm-mm. And what does that tell us about what we thought about how cells reproduce?

DR. MOONEY: Yeah, so I think it’s actually a really exciting observation and finding, and certainly has great potential, clinically, to help a variety of different types of patients, including these children that you had mentioned earlier. On the one hand, it extends what we already know. So as you mentioned in your intro, we’ve long appreciated that physical forces regulate a lot of biology. If you think about it, the cells in our bodies live in a very physical world. We walk around, gravity is always pulling on our tissues, there is blood flow through our heart and the vessels. So it makes sense that the cells would respond to these environmental signals and alter how they grow, how they die, how they specialize, and what kinds of functions that they might have. But what’s really new here, is now the extension of a lot of this basic knowledge into the ability to now apply defined mechanical signals and to try to drive regeneration and growth of new tissues.

IRA FLATOW: And so this could possibly have an effect on people once this is as, Dr. Damian was saying, this has been proven– we were showing it in pigs here, but now it might work in people? I mean we might be able to do that?

IRA FLATOW: Tell us how. Give me some examples.

DR. MOONEY: Yeah. Well, first of all, I’ll actually put this in little bit historical context. So we already do some of this already, even though maybe oftentimes people don’t think about it. Many of us have children that we’ve taken to an orthodontist and had some procedures done, in terms of the braces. And when you apply braces, you’re also applying a physical force on the tissue– in this case, the tooth structure– to get not just the teeth to move, but the bone underneath those teeth to actually remodel. In the area of wounds, we currently have some therapies where people who have significant wounds that aren’t healing well on their skin, we apply vacuum in the clinic and basically pull on those tissues to try to enhance healing.

So there certainly is precedent for this that’s out there. And so what I think the exciting thing here is the ability to now take this into the body and use these soft robotic systems that are very different than the hard robots that I think most of us think about from the movies. And now be able to use those in the body because they can apply very gentle, very reproducible forces on the cells and the tissues to induce this type of remodeling and regrowth and regeneration. And not only can we do it outside the body, but we potentially actually, inside the body, we can potentially have these devices outside to apply these types of loads, as well. So you know, we have soft materials, and we can place these in the body surgically. We can wrap them around tissues in the body, and then program them to deliver the kind of forces that we’ve learned are important and can drive regeneration.

IRA FLATOW: Dr. Damian, how hard is it? We hear Dr. Mooney talk about putting a robot in the body– how hard is it to design a robot that’s supposed to function inside the human body?

DR. DAMIAN: It is quite challenging and it truly depends on the medical– on the clinical condition that we are targeting to resolve. We’ve been going through a lot of trials and uncertainties in designing this robotic infant and tell you the truth, when I first saw it actually working, I felt like I’m controlling a rover on Mars just because it is in an inaccessible place. And so one once we place it there inside, we need to make sure that it is going to work 100% of the time. And I think this is a huge– this is a huge challenge. We need to go through many stages of design. So how can we design such a robot that takes into account, not only the targeted tissue, but also the environment around? So we have soft tissue, like the esophagus, like the lungs– but we also have hard tissue– speaking about the robotic infant that we’ve developed– such as ribs. And so we want to have a robot that is both soft, but is also durable.

DR. MOONEY: And well, it took us quite a while to understand all these uncertainties in order to embed the requirement in this design. So we’ve come up with an encapsulation that is soft, it’s wrinkled– in order to take into account that gentle interaction with the soft tissue– but we also had to embed polyester mesh inside it just in order to take into account the stress from the ribs.

DR. DAMIAN: –which could [? tear ?] the encapsulation at any time, and that would be so devastating for both the human or the animal, as well as for the robot, because we have a lot of electronics inside which can oxidize or which can short, just because there’s going to be some fluid running into the [? unit. ?]

IRA FLATOW: Dr. Mooney, people are going to listen–

IRA FLATOW: I’m sorry, people are going to listen to this and say, hey, I have a child who’s suffering from this or that or I have a friend– this is not ready for humans yet, is it?

DR. MOONEY: Not the type of intervention we’re talking about today, but as in many types of advances in medicine, there will be a stepwise transition. So to put these devices in the body, as we just heard, has a lot of challenges, and there will have to be a lot of work done to make sure that can be done safely and reproducibly. But if, for example, we instead are applying these outside the body, it actually lowers the bar and the hurdles to do it substantially. And that’s something that you have the opportunity to move much more quickly, I think, towards human clinical trials. If you think about it, today, many people already get massage therapy, which is, in some ways, similar to what we’re talking about today– where an individual pushes and pulls on tissues to try to alter. Now in massage, we typically don’t know exactly what it is we’re manipulating, and it’s also difficult, if not impossible, to have a defined and repeatable force done over and over again, multiple days. But with a soft robotic that you place outside the body, you can accomplish that. So how I see this playing out is we’ll have devices outside the body, initially, that can induce regeneration of certain tissues– probably not the ones deep inside body organs– and then, as those get developed and go to the clinic, then we’ll be continuing to make advances on these internal soft robotic devices.

IRA FLATOW: How would this interact with stem cell research? What you’re saying, reproducing cells, is a lot of what STEM cell research is aimed at. Can this robotic idea– the pushing and pulling– do away with some of the need to use stem cells in some certain cases?

DR. MOONEY: Yeah. So it’s a really provocative idea that instead of culturing outside the body and transplanting stem cells, which is how most of us think about regenerative medicine happening today, to instead directly target those cells in the body that already exist. And, for example, as you’re saying, apply a certain stress to induce them to proliferate and then have them specialize and become the tissue type of interest. There is proof of principle for that already. For example, in the area of skeletal muscle, it’s been demonstrated that one can induce regeneration of muscle, which is caused by a stem cell population simply by applying mechanical cues. And what we’re talking about with Damian’s work, at this point, it’s not completely clear if there is a stem cell contribution. But at the end of the day, there likely is at least some stem cells that are participating in the building of the bulk of the tissue where these blood vessels. So likely, stem cells are being targeted there. And we have found in the lab, many groups that stem cells are exquisitely sensitive to these types of physical cues.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from PRI, Public Radio International, talking with Dr. Dana Damian and Dr. David Mooney talking about mechanically manipulating cells to get them to move. And what about in cancer cells? If we push and pull cancer cells the right way– might we disable them?

DR. MOONEY: That’s actually, again, a really striking concept. If you think about it, how we oftentimes detect cancer– if you think about breast cancer– when a woman does a self-exam, she’s actually looking for a region of the breast tissue that’s stiffer. So we intrinsically know that there’s a different mechanical environment for cancer, and people have appreciated this for a long period of time. So that naturally led to the question of whether cancer– whether this stiffness is a result of the cancer, or perhaps it actually causes the cancer? And over the last decade or so, there’s been a tremendous amount of work that’s shown that the mechanical environment of cancerous cells plays a really dramatic role in their further development of malignancy– their ability to move and migrate and colonize other parts of the body. And so that actually is an area where there’s a lot of research now to try to alter the mechanical environment of cancer cells to try to, in essence, either prevent them from being able to metastasize or perhaps even return them back to a normal state.

IRA FLATOW: Let me see if I can get–

DR. MOONEY: Oh, no, please, go ahead.

IRA FLATOW: Sounds exciting. I just want to see if I can get a call in from a listener before we have to go. Quinton in San Antonio. Hi, Quentin.

QUINTON: I’d like to ask– you said the experiment lasted for nine– eat and function normally or if it required IV fluids.

IRA FLATOW: Yeah, he sort of dropped out a little bit. So you’re asking was the pig able to eat and function normally with the robot on?

DR. DAMIAN: Yes, I heard that. Yes. The short answer is, yes. And the long answer, to explain it to you better– we’ve mounted that robot on a healthy esophagus, so [? non-interrupted ?] esophagus. And so we would have a normal tube, basically, and while the animal can keep on moving and eating and drinking water, we’re doing– or the robot is doing– the job that it’s supposed to do.

IRA FLATOW: And can you give us other kinds of illnesses that you might, or other different kinds of tissue you might aim, Dr. Damian?

DR. MOONEY: Sure, sure. So we have targeted, currently, the esophagus because it’s a rather simple soft tissue. It has mostly wall and the transportation of the food from the mouth to the stomach. And it has some muscular layers there in order to transport the food. But we’re also looking now at the short bowel. There is this devastating condition called, “short bowel syndrome,” where children are born with a shorter bowel. So that means they have an impaired digestion. And so we have– from our preliminary trials, it looks like we can also elongate this tissue.

IRA FLATOW: And for adults who have bowel shortening surgery? Might this?

DR. DAMIAN: Yes, it looks like this could also apply, yet this is something that we still have to demonstrate.

IRA FLATOW: Wow. This is quite interesting. You know, as I said before, this is not a new finding. We know– we’ve known for decades about this happening, yet it’s now coming to the fore right now as new research is published about how successful you researchers have been. I want to thank both of you– Dana Damian, Lecturer, Director of the Biomedical Robotics Lab, Sheffield University in the UK and one of the creators of the esophagus tugging bot, and Dr. David Mooney Professor of Bioengineering at Harvard in Cambridge. Thank you both for taking time to be with us today.

Scars and Keloids

Most cuts or wounds, with the exception of ones that only scratch the surface (the epidermis), lead to scar formation. A scar is collagen-rich skin formed after the process of wound healing that differs from normal skin. Scarring occurs in cases in which there is repair of skin damage, but the skin fails to regenerate the original skin structure. Fibroblasts generate scar tissue in the form of collagen, and the bulk of repair is due to the basket-weave pattern generated by collagen fibers and does not result in regeneration of the typical cellular structure of skin. Instead, the tissue is fibrous in nature and does not allow for the regeneration of accessory structures, such as hair follicles, sweat glands, or sebaceous glands.

Sometimes, there is an overproduction of scar tissue, because the process of collagen formation does not stop when the wound is healed this results in the formation of a raised or hypertrophic scar called a keloid. In contrast, scars that result from acne and chickenpox have a sunken appearance and are called atrophic scars.

Scarring of skin after wound healing is a natural process and does not need to be treated further. Application of mineral oil and lotions may reduce the formation of scar tissue. However, modern cosmetic procedures, such as dermabrasion, laser treatments, and filler injections have been invented as remedies for severe scarring. All of these procedures try to reorganize the structure of the epidermis and underlying collagen tissue to make it look more natural.

Practice Question

Why do scars look different from surrounding skin?

What happens to my body when I exercise?

There’s a lot going on inside your body when you go out for that morning jog.

Exercise diverts blood from your liver and digestive system to your skeletal muscles. Hormones tell the body to convert fat into glucose, reduce the pain you feel and improve your mood. Muscles generate lactic acid as a by-product of intensive exercise and, as this builds up, the pH of the blood around the muscles drops. This drop in pH eventually prevents the muscles contracting further. At this point, you need to rest to allow the lactic acid to be metabolised.

The brain makes neurotransmitters, like serotonin, dopamine and GABA. This is part of the reason why the brain consumes more energy during exercise.

Adrenaline levels rise, which stimulates the heart to beat faster. Capillaries in the muscles open wider, increasing blood flow there by up to 20 times.

The muscles of the ribcage assist the diaphragm to pull in up to 15 times more oxygen than at rest. Breathing gets faster but also deeper.

Your two million sweat glands can produce 1.4 litres of sweat per hour. Waste heat is carried away by the latent heat of evaporation as it dries.

As you exercise, the large muscles in your arms and legs squeeze the veins running through them, pumping blood back to your heart.

High-impact and weightlifting exercises stimulate bone formation and reduce the rate of calcium loss as we get older.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.

Stress and your mind

Stress has marked effects on our emotional well-being. It is normal to experience high and low moods in our daily lives, but when we are stressed we may feel more tired, have mood swings or feel more irritable than usual. Stress causes hyperarousal, which means we may have difficulty falling or staying asleep and experience restless nights. This impairs concentration, attention, learning and memory, all of which are particularly important around exam time. Researchers have linked poor sleep to chronic health problems, depression and even obesity .

Losing sleep affects your ability to learn. from

The way that we cope with stress has an additional, indirect effect on our health. Under pressure, people may adopt more harmful habits such as smoking, drinking too much alcohol or taking drugs to relieve stress. But these behaviours are inappropriate ways to adapt and only lead to more health problems and risks to our personal safety and well-being.

So learn to manage your stress, before it manages you. It’s all about keeping it in check. Some stress in life is normal – and a little stress can help us to feel alert, motivated, focused, energetic and even excited. Take positive actions to channel this energy effectively and you may find yourself performing better, achieving more and feeling good.

What Happens When We Stuff Ourselves At Holiday Time?

Raymond Biesinger

On a Sunday that’s usually a week or two after “Western” Easter, my parents set up an electric spit to roast a whole lamb in their suburban Massachusetts backyard. We welcome guests to our Greek Orthodox Easter celebration with a kiss on both cheeks we nibble on tiropitakia, little cheese pies made with phyllo dough, and kokoretsi, organ meats wrapped in intestines and cooked on the spit next to the lamb. When the lamb is ready, we begin the meal by cracking open dyed hard-boiled eggs. Over the next few hours, we eat way too much lamb, moussaka, dolmades, and tzatziki, finishing off with cookies, cakes, and chocolate bunnies bought at deep discount (cheap Easter sweets are one of the perks of celebrating according to the Julian calendar). Happy, sleepy, and extremely full, we adjourn well before sunset to rest and digest.

This ritual will be recognizable to millions of holiday overindulgers. Most will be too sleepy to wonder what’s happening inside their cells and nerves, or which enzymes and hormones control the biochemistry of postprandial sleepiness. But as a biologist, I can’t help myself.

At maximum capacity, the stomach can hold a gallon of food

That you can eat until you feel like you might burst without actually bursting tells us a lot about the physics and physiology of the stomach and the neuroscience of appetite. At maximum capacity, the stomach can hold a gallon of food, about sixty-five times its empty volume. As the stomach stretches to accommodate additional food, it inflates like a balloon, pushing against the other organs in the abdomen and making it increasingly uncomfortable to keep eating. Eventually the stomach will start pushing on the diaphragm, making it difficult to take a deep breath.

Well before you reach that maximum volume, the body begins to take action. The stomach is lined with bundles of nerves that can sense the level of stretching and work with gastrointestinal and peripheral hormones to signal fullness to the brain. Should you press onward past the first feelings of fullness, the nerve signals get more insistent.

Between “full” and reflexive vomiting—the body’s final defensive strategy for overfullness—there is a lot of room for holiday overeating. It’s easy to ignore those early signals, convincing ourselves that we’ve still got room to try a few things we couldn’t fit on our plates the first time, and still more room for dessert. And, in fact, the abundance of choice presented by holiday feasts actually enhances our penchant for overeating.

The variety-induced overeating typical of holidays is known as the “smörgåsbord effect,” and was first identified in 1956 by the French physiologist Jacques Le Magnen. To study the effects of food flavors on appetite, Le Magnen made tiny feasts for rats. When he fed the rats unlimited amounts of a single type of food, they would eat until they felt full, and then stop. But when he gave the rats a smörgåsbord with four different flavors of rat chow, the rats would eat about three times as much as normal, filling up again on each new flavor.

Humans are like rats in that way: when we’re eating one food, we get a little more bored and a little more full with each bite—the “hedonic rating” (basically the empirical enjoyability) of the meal goes down with every mouthful. If you’ve ever waddled out of a fancy restaurant, overstuffed after eating a tasting menu where many dishes were parceled out in tiny portions over a couple hours, you’ve experienced the reverse: without that sensory boredom kicking in, you can eat more and more enthusiastically throughout the meal.

This phenomenon was rediscovered in experiments on humans in the 1980s. Researchers served varied four-course dinners in their labs and asked the diners to rate their satisfaction at different points throughout the meal. They found that people would eat up to 44 percent more than when offered only a single dish, and that satisfaction and appetite were renewed by each new flavor.

At a fundamental level, our hunger instincts are controlled by the levels of fats and sugar in our bloodstream, and we eat in order to maintain these nutrients at a stable level. When our blood sugar begins to go down, we start to feel hungry, and hormones tell our brain that it’s time to eat again. But while we’re eating, both sensory pleasure and stomach stretching happen quickly. How we eat—and especially how we eat during the holidays—is influenced by forces beyond just our metabolism and our stomach capacity, namely our willpower and our senses.

Holiday Stuffing

Inevitably, we end up ignoring our bodies’ early warnings and overeat during the holidays. But how does this all translate to the inevitable postmeal yawns and shuttering of eyelids?

One oft-cited explanation is the sugar high and subsequent insulin crash phenomena. Consider the Halloween feast: kids gorge themselves on pillowcases full of refined sugar. The sugar rapidly enters the bloodstream through the lining of the stomach. Cells in the pancreas absorb the sugar from the blood and start converting it into energy. The subsequent change in the level of energy activates a cascade of biochemical switches. At the last step in the cascade, specialized vesicles inside the pancreatic cells open to release insulin. The hormone insulin controls how the body’s cells and tissues process sugar.

All this happens in a matter of minutes. As the fresh dose of insulin flows through the bloodstream, it tells the muscles and fat cells to absorb the sugar and to start converting it into energy. Hence, sugared-up kids bouncing off the walls. But the sugar high has never been proven in double-blind studies: kids get hyper when they get treats whether they have real sugar or not. The psychology and rituals of food—like the excitement of trick-or-treating—have a bigger effect than sugar itself.

The sugar crash is likewise disputed. Insulin makes our tissues absorb sugar quickly, but that shouldn’t cause blood sugar to dip below normal levels. Let’s consider another theory. For the post-Thanksgiving food coma, blame often falls on everybody’s favorite celebratory poultry: turkey. High levels of the amino acid tryptophan in turkey are converted into melatonin, the hormone that regulates sleep-wake cycles in the brain. Turkey does have a lot of tryptophan, but so does chicken, fish, cheese, and eggs—tryptophan levels aren’t enough to explain how sleepy you feel after overeating at Thanksgiving.

What makes holiday feasts sleep-inducing—Thanksgiving, in particular—is the combination of all of the above. First, you just eat a lot during the holidays. The same nerve bundles in your stomach lining that signal your brain to slow down your gorging also tell the brain to divert more of your body’s energy to digestion. Second, you eat a lot of carbs in the form of mashed potatoes, stuffing, and dinner rolls. The simple sugars trigger the release of insulin into the bloodstream. Insulin’s main job is to tell cells to absorb that sugar, but it also activates the absorption of some—but not all—amino acids, and raises the relative concentration of tryptophan. Tryptophan gets left behind to enter the brain. Cells in the brain convert tryptophan first into serotonin, a neurotransmitter that makes you feel happy, and then into melatonin, which makes you sleepy.

After all the turkey and stuffing, you may manage to find room for a few bites of pumpkin pie, your appetite reinvigorated by the sight and smell of a new stimulus. The extra dose of simple sugars releases another spike of insulin, and perhaps your brain makes a little bit more melatonin. Sleep is thus irresistible. You pass out on the couch.

In the morning, your stomach is empty or somewhere close to it, your insulin levels are low, and you’re ready to do it all over again.

Christina Agapakis is a biologist, designer, and science writer. She makes art with microbes, soil and food.

What Happens To Your Body When You Miscarry

You're probably aware of the general details of a miscarriage. It's a pregnancy that ends in the loss of a baby before the 20th week. But the actual process of a miscarriage (as well as its physical, mental, and emotional side effects) is still shrouded in shame and silence. Miscarriage continues to be a taboo topic, and thus a hidden trauma. It's rarely spoken about publicly, and as a result, the ins and outs of the miscarrying body aren't widely known. But it's a mistake to think that concealment will help with miscarriage recovery or prevention. If you or someone close to you ever has (or ever does) miscarry, being able to identify what happens to your body may allow you some understanding amidst the trauma and pain of the experience.

You might already know this, but different miscarriages vary widely in their symptoms and effects. This really shouldn't be surprising — after all, no female body is the same, and neither is any pregnancy. That said, there's such a lack of awareness about miscarriage that you can't really blame anyone who assumes they're all mostly similar. Leaving aside such rare conditions as ectopic pregnancies (wherein fetuses develop outside the womb), "average" miscarriages can be wildly different from woman to woman — and not just because of how far along they are in their pregnancy when it occurs.

Given how common miscarriages are (22 to 75 percent of all pregnancies end in miscarriage in the first six weeks), we should know far more about them than we do. If you're interested in learning more about what happens to the female body during miscarriage, read on. But be warned — this will be graphic and potentially upsetting.

1. Early Symptoms Include Bleeding And Back Pain

The first sign of a miscarriage in most situations is "spotting," or light bleeding. However, that's not a guarantee up to one in four women experience bleeding at some point in their pregnancy. If it's light and clears up within a few days, it's not likely a sign of a miscarriage, but you should still see a doctor.

In a miscarriage, blood flow increases instead of eventually going away, and it's sometimes accompanied by the beginnings of cramps in your lower back or abdominal area. Other symptoms include a sudden appearance of whitish-pink mucus in the vagina and the sudden absence of pregnancy symptoms, such as morning sickness or sore breasts.

2. The Cervix Will Soften So That The Baby Can Exit

Cervical softening is a natural part of birth, but the process of expelling a non-viable fetus needs its involvement, too. During miscarriage, cervixes often "soften" to let the material from the womb pass through — but sometimes they don't. When this happens, medical professionals have to step in. Incidentally, this is why it's recommended that people don't have sex until about two weeks after a miscarriage. Until the cervix closes up again, penetrative sex raises your risk of an internal infection.

3. The Body May Have Contractions

In some women, the process of miscarriage actually stimulates the body to start producing contractions similar to those that would help out with birth. In this case, though, the body's trying to rid itself of the products of the pregnancy, including the fetus, placenta, and other tissue. These tend to feel like cramps from hell, and may result in the expulsion of blood clots or bits of tissue. Additionally, they can happen either suddenly or over a period of days.

4. A Woman May Pass The Gestational Sac Or Fetal Tissue

This is one of the hardest parts about miscarriage to discuss. But it's an unfortunate truth: Women who miscarry toward the end of the first trimester, (when the fetus is more fully developed) may find that they actually pass the gestational sac in which the embryo has been growing. Others pass gray-colored "fetal tissue" or parts of the placenta.

This is obviously a very traumatic experience, and it's OK to be extremely upset about it. It's a different story for everybody, though. Some women have reported that they never saw anything recognizable — just lots of tissue and blood.

5. Sometimes, The Body Won't Remove All The Tissue By Itself

Miscarriages aren't all about waiting for the bleeding to stop. There are actually three different ways to "manage" miscarriage: expectant (the mother waits for the miscarriage to finish naturally), medical (she's given medicine to help the process), and surgical (any leftover tissue is taken out by doctors). It depends on whether the body struggles to remove all of the remnants of a pregnancy by itself. The medical option either uses oral drugs or vaginal pessaries. The surgical options include a minor outpatient procedure called dilation and curettage, or D&C, which uses a vacuum to "clean" out things that might be hurting the woman.

6. Pregnancy Hormones Will Take A While To Readjust

This is particularly unfortunate, but hCG, the hormone associated with pregnancy in women (which is what's detected in urine-based pregnancy tests) may take a while to return to its normal (basically undetectable) levels in the body after a miscarriage. How long this takes depends on when in the pregnancy the miscarriage occurred. If it happens between the eighth and tenth week, hCG will take longer to disappear.

These hormones may also stick around if you still have fetal or placental tissue inside you. What's worse is that until the hCG levels go back to normal, you may still have positive pregnancy tests.

7. The First Period After A Miscarriage Will Be Different

An evocative personal essay over at xoJane points out something that isn't often mentioned as an aftereffect of miscarriage: Your periods will likely be all over the place. Some will be heavier, as they attempt to eject the thickened lining that the body had prepared for pregnancy, and others will simply be all over the place due to the radical hormonal changes that have altered your body's menstrual clock. Either way, "snapping back," mentally or physically, doesn't easily happen.

8. A Chemical Pregnancy Has Different Loss Symptoms

Some exceptionally early miscarriages (those before the sixth week) are called "chemical pregnancies" because the body starts sending out hormonal signals of pregnancy before the fertilized egg actually gets fully implanted in the uterus. If the implantation doesn't succeed, the egg won't develop — so even if you have a positive pregnancy test, your first ultrasound may show no sign of a pregnancy. Chemical miscarriages can sometimes go completely undetected, and happen in 50 to 60 percent of first pregnancies.

9. Some Miscarriages Show No Symptoms At All

This is the crucial thing to remember. Not all miscarriages are the same, and some of the most traumatic miscarriages are the ones without any signs. These are called "missed" or "silent miscarriages," and they're the ones that happen without any signals whatsoever — some even occur without bleeding. They may result from problems with the embryo or the gestational sac, an infection, or some other reason. The one thing they have in common is that they generally only become clear when you go in for a scan.

If you've ever miscarried, just remember that you're not alone, and you're not a failure or a freak. If you're pregnant or ever become pregnant, visit a doctor immediately if you start to see any of the above signs. It may be nothing, but it will definitely be worth your time (and your peace of mind) to get it checked out.

What happens if you eat your own sperm?

When sperm is ingested by swallowing semen, the sperm will be broken down and absorbed into the bloodstream as if consuming water, milk, or gelatin. If it's semen (the liquid that carries the sperm from the penis) that a person is worried about, ingesting one's own semen is safe if that person is free of sexually transmitted infections (STIs).

On the other hand, swallowing one's own semen is unsafe if a person has certain STIs. In this case, the risk depends on what STI a person has, its method of transmission, and the area(s) of infection. With swallowing semen, the primary concern is with infections that localize in the genitals, mouth, and/or throat. If the infection can be transferred through semen, and it can infect different locations independently, then there is a chance that the infection can spread to the mouth or throat. This type of infection includes gonorrhea and chlamydia.

Certain STIs, such as human papillomavirus (HPV, the virus that can cause genital warts), herpes, and syphilis, spread through direct, skin-to-skin or oral-genital contact. Some men can, and do, go down on themselves. If they have HPV, herpes, and/or syphilis, the infection(s) can spread from their penis to their lips, mouth, or throat.

Swallowing one's own semen does not pose health risks with respect to systemic infections (e.g., HIV). These infections are in the bloodstream and their symptoms are present throughout the body.

So if you're STI-free, feel free to indulge yourself! Just be careful you don't spoil your dinner.

Show/hide words to know

Organism: a living thing that can be small like bacteria or large like an elephant.

When you wake up in the morning, what do you notice first? A smell? A sight? A sound? Image by Bablekan.

You wake up, but stay in bed for a moment, sensing the world around you. You feel your muscles pull as you stretch under the soft covers and slowly open your eyes. You might see light coming in through the blinds, hear a car honk outside, smell someone cooking breakfast, or taste dry toothpaste on your lips from brushing your teeth the night before.

As a living organism, it’s important for you to be able to sense and respond to the environment around you. Humans and many other animals have five main senses that help them understand the world around them. How do each of these senses work, and what happens when they don’t work properly?

Starting here, you can take a tour of the senses that many animals, including humans, experience:

Explore how each of our five senses work. Image by Allan-Hermann Pool.

Which sense do you rely on the most? And how you might start to rely on other senses more if one of your senses were to stop working?

Additional images via Wikimedia Commons. Hand and flower by Øyvind Holmstad. Fennec foxes by Anass ERRIHANI.