Exercise-induced mitochondrial biogenesis in muscle 1

My name is David Hood and I work at York University in Toronto. I'm the director of the Muscle Health Research Center at York University. And the topic today is "Exercise-Induced Mitochondrial Biogenesis in Muscle".

A subset of the topics include the discussion of mitochondrial morphology in muscle fiber types and mitochondrial adaptations to exercise training.

As an introduction, it's useful to know that human skeletal muscle has a variety of different muscle fiber types. It's a heterogeneous tissue. That image shows a histochemical stain with some arrows that point to the distinction we can make using histochemistry of the different fiber types that exist in a muscle like the quadriceps or the gastrocnemius or the deltoid or muscles like that. So, fiber types exist in three types. There's the Type I, the slow twitch-red type of fiber. It has a high oxidative capacity which means it has a lot of mitochondria and a good capillary network around it. The Type IIa fibers also called fast twitch-red or FTR have a moderate-high oxidative capacity so a little bit less than Type I fibers and the Type IIb fibers also called fast twitch-white have the lowest oxidative capacity of the three types of muscle fibers. They have a high glycolytic capacity and they are big fibers and very powerful, but those fibers have the lowest mitochondrial content or lowest oxidative capacity. And that's reflected physiologically in the type of endurance that you see and that's shown in the graph showing the percent of the initial force generated and as a function of time, minutes of contractile activity along the bottom. So when you force at these different types of fibers that all belong to different types of motor units, you get different fatigue responses. So, the Type I motor units in response to contractile activity don't lose force very much. They are fatigue resistant. Now, the Type II motor units, on the other hand, that consist largely of the Type IIb or Type IIa fibers, they lose force quite considerably. That is they fatigue much more rapidly. When you take the average of those, of course, you'll get a whole muscle average of fatigue resistance that is a reflection of how many Type I fibers there are and how many Type II fibers or motor units there are. So, Type I fibers have a high endurance, they are not very strong because those fibers are the smallest. So, they have a low force generating capacity, low total strength. On the other hand, the Type IIb fibers can produce a great deal of force because they are bigger fibers, but they have very poor endurance capacity because of their low mitochondrial content. So in this talk, we're going to speak about mitochondria and how it contributes to muscle physiology, how mitochondria adapts in response to exercise and the molecular signalling involved in maintaining mitochondria in muscle.