字幕表 動画を再生する 英語字幕をプリント Hey everyone! Just wanted to explain the change of scenery. Usually, we film these videos in a studio, but we're gonna be filming from my house from now on. Hair check, hair check. I'm gonna stress about one thing today is my hair. Alright! Aging is inescapable, for now anyway. And aging itself encompasses a lot of physiology. Some aspects of aging are hard to picture and are the topic of ongoing research, but one that we know quite a bit about is muscle. But muscle is complex. Not only is it always adapting to how we use it, but it changes multiple times over the course of our lives. Today, we'll learn about how our muscles change from the time we're just a fetus, to our last mortal moments. Before we get too deep in the weeds here, we need to clarify what we mean by muscle. Some of you may remember back from episode one that muscle is one of our four distinct types of tissue along with nervous, epithelial, and connective tissue. Each type of muscle has a different function, and there are three types. We've got two types of muscle that we control involuntarily: the special cardiac muscle in our hearts and smooth muscle around our blood vessels and certain organs. We don't consciously contract our heart muscles. Thank goodness, that would be exhausting. The focus of this episode though is skeletal muscle, the type of muscle that lets you move your body. Skeletal muscle is the most massive group of tissue in your body making up for a serious chunk of your body weight, so where does it come from? This tissue is one of the first ones formed while you're still an embryo, so that's where our journey begins. In your early days as an embryo, your cells divide into three distinct layers — the endoderm, mesoderm, and ectoderm. Literally the inner, middle, and outer layers. These layers become the different tissues of your body, and it's the mesoderm that we're interested in. It becomes the tissues of your cardiovascular system, reproductive system, connective tissue, and skeletal muscle. During development, you have cells in the mesoderm layer called myoblasts — these things are cells that are dedicated to becoming muscle fibers. Myoblasts grow and replicate until they encounter a certain chemical that gets them to start the process of turning into proper muscle. After they encounter that chemical, if they bump into another myoblast, they'll start linking together into chains of myoblast cells. Now, one thing that makes skeletal muscle fibers unique compared to cardiac or smooth muscle is how it has multiple nuclei in each fiber. That's because during this next stage as an embryo, myoblasts fuse together into structures called myotubes then they ditch their individual cell membranes as they fuse into one fiber. Each myoblast had a nucleus, so the myotubes, as well as mature muscle fibers, have multiple nuclei too. Some of those myoblasts don't differentiate though, and they hang around as satellite cells, a type of cell that sometimes gets called muscle stem cells. These things are super useful. Our mature muscle fibers don't divide like other cells do, but satellite cells can divide and grow if our muscles are injured. Although, that statement comes with a lot of it depends attached to it. There's quite a bit that can influence how those satellite cells work. At this point in the journey, you are a human that's alive in the world, crawling around with about six hundred muscles, eating mashed bananas or Cheerios or whatever they feed babies these days. I don't know, I don't hang out with any. From the time you're a newborn to your younger years, you keep roughly the same number of muscle fibers, but each one gets bigger because of those satellite cells. This muscle cell growth is called hypertrophy and it explains the vast majority of muscle growth that happens in your body. If you've lifted weights before, you might've heard that term thrown around a bit in reference to those hashtag gains. And it's true, your muscles can hypertrophy as a result of a weight lifting routine. But hypertrophy also means an increase in any cell size. So your fat cells can hypertrophy too, that kind of thing. Plenty of research has investigated the possibility of muscle hyperplasia, or growing new muscle fibers, but at this point in our knowledge of muscle, it seems like our muscles grow mostly because our existing fibers get bigger. Weightlifting aside, at this point in your lifecycle, your muscles are growing. After a few years, puberty happens and your muscles grow again. And your muscles keep growing in size and strength until about your twenties and maybe thirties. It seems kind of silly to say this, but every body is different. Your body is still awesome regardless of when your muscle mass peaks. I also included that maybe in that last line because nothing special happens to your body at the stroke of midnight when you turn thirty. Although that was when I got my first grey hair. As life goes on, and you creep into your thirties, this is about when aging starts to have its effect on your muscles. Sometime after your thirties, you'll lose about one to two percent muscle mass every year. By age 70, you're looking at twenty five to thirty percent muscle loss from your peak. This aging-associated muscle loss is called sarcopenia, which involves a few long term processes that all happen at the same time. It's kind of a weird, in-between term that sometimes refers to a disease but also just being a thing that happens as a result of age-related changes. Unfortunately, because there's so much happening at once, it's hard to figure out what causes muscle loss, and therefore how we can fix it. It involves both changes in function and physical changes that both influence each other. Some of it is influenced by your nervous system. As you age, the nerves that control your skeletal muscles reorganize themselves. We start to lose type 2 muscle fibers, which are fast twitch fibers that let us generate power, and start to rely on the slow twitch, type 1 fibers. Part of that involves losing connections between nerves and type 2 muscle fibers. With less muscle working together for any given movement, and switching to weaker fibers in general, your strength decreases and eventually, so does the size of your muscles. Muscle aging comes with all kinds of changes in the composition of muscle. We also see an increase in the fat and connective tissue between and around muscle fibers. Plus, we see a decrease in our muscle's ability to repair itself. Part of this is because we don't make as much new protein to go into those muscles. We also see fewer of those important satellite cells, again, cells that become skeletal muscles, so we can't take advantage of their repair and regeneration abilities like we used to. And then we start losing mitochondria. If you remember back to our mitochondria episode, certain cell types have more mitochondria than others, and skeletal muscle has a lot for its size. They need to make quite a bit of energy to do so much work, so in order for muscle to work properly, they depend on functioning mitochondria. And when muscle mitochondria don't work as well, neither do your muscles. We're still not totally sure why mitochondria stop working as well, but we think it has to do with slight tweaks in gene regulation, or how your body turns certain genes on or off. There are a few other reasons for muscle loss as well like increased general inflammation, hormonal changes, and we just tend to be less active as we get older. So it won't come as a surprise that the most effective treatments for preventing muscle loss due to normal aging include a combination of nutrition and exercise. I know I just spent the last two minutes talking about how your muscle wastes away as you get older, but I'll leave you with a bit of hope. Exercise is one of the most powerful things you can do to maintain health as you get older, and you can see gains from exercise long into life. A handful of studies have shown that adults between age sixty and eighty can improve their aerobic fitness by twenty to thirty percent with exercise. One study even showed that seniors in their nineties could get stronger and see thigh muscle hypertrophy with strength training. This becomes a huge deal when it comes to staying independent and avoiding falls. Studies use different styles or prescriptions of exercise, but they usually include some kind of weight lifting component. Those treatments help reduce muscle loss by improving those satellite cells, preventing further loss of nervous connections, and making more mitochondria. But I doubt you're really gonna care how many mitochondria you have when you're that old. You're probably more concerned with stuff like balance and day to day tasks. Thankfully, strength training helps with that too. Going into the future, researchers are working on identifying the finer details behind sarcopenia and how to best treat it, maybe with drugs or other strategies. In the meantime, I'm strangely motivated to lift all of a sudden. Thinking about frailty and mortality can make people feel uneasy, I know, but part of what makes life so special is the fact that it's fragile. Ultimately, it's what makes our lives human. Thanks for watching this episode of Seeker Human, I'm Patrick Kelly.