I'm currently listening to Stress and Your Body, a set of Great Courses lectures by Professor Robert Sapolsky. Sapolsky is a professor of biological sciences, neurology, and neurosurgery at Stanford University. His lab focuses on how stress affects the nervous system. He also has extensive field work, studying a particular population of wild baboons in East Africa - where he examines how social rank, personality, and sociality affect vulnerability to stress-related disease. He is a fantastic lecturer, and if you get the chance to watch a YouTube video of him lecturing, go for it.
The main text for Stress and Your Body is Sapolsky's own Why Zebras don't get Ulcers, though there are a few scientific papers and books to supplement this course.
Lecture 1: Why Don't Zebras Get Ulcers? Why do we?
- When a zebra gets stressed by a lioness, it activates all sorts of pathways that help it survive the fight-or-flight response. Systems that use a lot of energy, like digestive and immune, slow way down to conserve energy for the run. Blood pressure rises to get oxygen to the limbs. When the zebra successfully escapes, these pathways turn off.
- When we get stressed, we often worry about things in the future - impending disasters that may or may not come. Tests. Presentations. Work. Since there's always something to worry about, your body is always suppressing those systems that are turned off during the fight-or-flight response.
- For the zebra, the fight-or-flight response is helpful, but to the constant worrier, it causes extreme wear-and-tear on your body.
- Reading: Why Zebras Don't Get Ulcers Chapter 1
Lecture 2: The Nuts and Bolts of the Stress Response
- The sympathetic nervous system is for stress / fight-or-flight; whereas the parasympathetic system is for non-stress situations. These systems control the automatic functions of your body like digestions, immune system, heat beat, constriction of blood vessels, etc.
- To really simplify things, you can think of the brain as triune; including the hypothalamus, limbic system, and cortex. The hypothalamus does all the regulatory stuff (like the parasympathetic and sympathetic control). The limbic system controls emotion. The cortex is all about abstract reasoning and long-term memory.
- Upon certain stimulation (like stress or fight-or-flight), your brain stimulates the release of hormones (chemical signals) that control your body's response.
- Reading: Why Zebras Don't Get Ulcers Chapter 2
Lecture 3: Stress and Your Heart
- When the sympathetic nervous system (fight-or-flight setting) activates during stress, your blood pressure goes up. This is great for short periods when the blood needs to rush to your limbs so that you can run. But when it's chronically up (due to chronic stress), damage is done to your blood vessels and heart - much as damage is done to pipes and pumps when they are run too hard for too long. Tears occur. Plaque builds up along the tears. Vessels get blocked by plaque. Heart attack.
- Reading: Why Zebras Don't Get Ulcers Chapter 3
Lecture 4: Stress, Metabolism, and Liquidating Your Assets
- When you eat, you break down (digest) those foods into their base components - glucose, amino acids, and fats. These can get stored in your fat cells, be made into proteins for your muscles, or be used as ready energy.
- When you become stressed, you break down the stored fat or protein to make energy for your muscles so that you can activate the fight-or-flight response. This is great when you're running from a lion. But if you're chronically experiencing stressors, you break down fat, store it again, break it down, store it, in cycles. Each time you break down fat you use energy, and each time you store fat you use energy. Thus, chronic stress leads to a net loss of energy, resulting in fatigue.
- Insulin is released by your pancreas when it senses raised levels of glucose in the blood. Insulin signals the body to store fat and absorb glucose.
- When you are stressed, your body releases signals called glucocorticoids which tell your body to stop responding to insulin. This is because the last thing you want to do when you are running from a lion is waste energy storing fat. What you want to be doing is breaking fat down. But when your glucocorticoids are chronically raised, your body is always insensitive to glucose - thus raising the levels of fat in the blood and leading to heart problems. Also, because your body is resisting the effects of insulin, the pancreas pumps out more insulin, leading to diabetes.
- Reading: Why Zebras Don't Get Ulcers Chapter 4. It also suggested Diabetes for Dummies, but I didn't bother.
Lecture 5: Stress, Overeating, and Your Digestive Tract
- About 2/3 of people eat more than usual when stressed, but some eat less than usual. This is because during stress a couple of types of hormones are released.
- Initially upon stress, your body suppresses digestion and appetite in order to save energy for the fight-or-flight response; however, after the stressor is over, your body replenishes your energy stores by making you crave carbs.
- If you look at the image above, you'll see that when you are stressed, your brain releases CRH. This eventually stimulates the release of glucocorticoids like cortisol. CRH suppresses appetite. Glucocorticoids increase appetite.
- CRH has a short half-life in the body. Flucocorticoids have a much longer half-life.
- If someone is under constant stress (such as a chronic illness) their stress may reduce their appetite because CRH is being released non-stop.
- However, many people who claim to be under "non-stop stress" are often cycling between stress and non-stress several times a day. These people are on the stress-rebound long enough each time that the CRH (which suppresses appetite) is mostly gone while the glucocorticoids (which increase appetite) lingers on non-stop. This leads to over-eating.
Lecture 6: Stress and Growth - Echoes From the Womb
- Stressful environments of neonates can have an impact on body function (growth, metabolism, anxiety, etc) then and later on in adult life.
- Put a newborn in a pod and give it all the nutrients it needs but don't give it physical caresses, the growth of the newborn is temporarily stunted.
- Take a baby mouse away from it's mother and handle it gently for 3 minutes and the mouse's growth and release of glucocorticoids decreases. This turns out to be due to increased maternal grooming once the baby has been returned to it's mother.
- But the environment doesn't start outside the womb, stress of fetuses can also lead to the same problems.
- In the Dutch Hunger Winter of the Nazis starved a population of Dutch people, including pregnant women. The (now) adult children of these women have higher prevalence of metabolic disease.
- What's more, the grandchildren of Dutch Hunger Winter pregnant women have higher rates of metabolic disease because the adult children of Dutch Hunger Winter pregnant survivors have become incredibly efficient at taking nutrients out of their bloodstream. That means that their fetuses get less expose to mother's nutrients, giving them metabolic disease as well.
Lecture 7: Stress, Growth, and Child Development
- Stress Dwarfism is a disease in which a child is so stressed he or she stops growing entirely.
Lecture 8: Stress and Female Reproduction
- The female reproductive system is mostly under the control of four hormones: luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, and progesterone.
- The hypothalamus releases LH and the pituitary gland releases FSH. These two hormones mature eggs and produce estrogen. These hormones peak at about 14 days into a woman's menstrual cycle, as seen in the image above.
- After the egg is matured, the luteal phase begins. The egg is released, and progesterone peaks while LH, FSH, and estrogen levels drop. At this phase, the lining of the uterus builds up in case the egg is fertilized.
- During stress, the hypothalamus lowers the rate of LH secretion. Reduced levels of LH stimulate a negative feedback loop which lowers release of FSH. Production of estrogen likewise decreases. Lower estrogen levels reduce the likelihood of egg maturation and release.
- On top of that, stress hormones like glucocorticoids inhibit the release of progesterone. Thus, the lining on the walls doesn't build up, decreasing the likelihood of implantation of the lucky fertilized egg which has survived all the other adversities.
Lecture 9: Stress and Male Reproduction
- Lueteinizing hormone (LH) and follicle stimulating hormone (FSH) don't only affect estrogen in women, they also stimulate release of testosterone in men. As in women during stress, FSH and LH levels drop - and so does testosterone. Strangely, though, stress-related drops in testosterone have almost no effect on men.
- Stress affects men in another way, though - erectile dysfunction. If you remember from lecture 2, the sympathetic nervous system is turned off during stress, and the parasympathetic system is turned off. The sympathetic nervous system (fight-or-flight) constricts blood vessels so that the blood can get pumping and the zebra can run from the lion. The parasympathetic system does the opposite - it dilates blood vessels.
- An erection in humans is due to dilated and heavily-filled blood vessels in the penis (activation of the parasympathetic nervous system). Ejaculation occurs when the parasympathetic system deactivates again - thus activating the sympathetic nervous system. When a man is under a lot of stress his sympathetic system activates, and he often can't get an erection, or if he does, he ejaculates too soon.
Lecture 10: Stress and the Immune System
- The immune system is incredibly complicated. As you can see from the picture above, there are many cell types in the immune system. These cells interact with each other in ways that we can't comprehend. Because it's so complicated, and it's not necessary, I won't go into detail about what we do know.
- Stress affects the immune system in several ways. One way is on the short-term immune system. You see increased rates of autoimmune diseases, like Crohn's disease, multiple sclerosis, and rheumatoid arthritis. These are diseases in which the immune system thinks that part of your body is a foreign invader. The immune system attacks whatever body system it thinks is foreign, leading to problems with that system.
- Although it activates the short term immune system, it manages to simultaneously deactivate the long-term immune system, leading to more illnesses like the common cold.
Lecture 11: Stress and Cancer
- Severe immune suppression like that seen in AIDS can inhibit the body's defense against cancer, but stress doesn't affect the immune system as strongly as AIDS. Thus, the link that most people believe exists between cancer and stress may be a myth - though believing the stress is affecting your cancer can make you more miserable because you expect to be more miserable.
- Laboratory experiments on mice do show an increased risk of tumors taking hold when the mouse is stressed; however, Sapolsky cautions against applying these studies to humans. First of all, the cancer is injected into these animals. It isn't naturally developed like it is in the human.
- There are some retrospective studies in humans showing that people who have been more stressed have increased likelihood of developing cancer; however, Sapolsky cautions against applying these studies to real life since they are retrospective. In other words, they are studies in which people are asked about their stress levels after they have already developed cancer. Because so many people think stress increases the likelihood of cancer, they are more likely to say they were stressed before developing cancer. The studies are biased.
- The only way to do a good study to find the relationship between cancer and stress is to follow hundreds of thousands of people for decades and ask them over and over what kind of stressors they are exposed to. Then they determine if stress is linked to cancer development; however, this study would be prohibitively expensive.
Lecture 12: Stress and Pain
- Stress can blunt pain on the short term, but it can also make you hyper-sensitive to pain on the long term.
Lecture 13: Stress, Learning, and Memory
- In the short term, stress can help your memory because it brings more oxygen to the brain with higher blood pressure. You're more likely to remember incidents surrounding an isolated stressful incident, for instance.
- In the long term, stress blunts your brain's ability to form strong memories.