Lecture 10: Skeletal Muscle Fiber Types (PDF)

Lecture 10 Are all skeletal fibers created equal? - No How many different fiber types do humans have? - 3 What are the 3 types of fibers that a human has? - Slow fibers: type I - Fast fibers: type IIx and type IIa Are there different fibers within each muscle? - Yes, there isn’t just...

Lecture 10 Are all skeletal fibers created equal? - No How many different fiber types do humans have? - 3 What are the 3 types of fibers that a human has? - Slow fibers: type I - Fast fibers: type IIx and type IIa Are there different fibers within each muscle? - Yes, there isn’t just one type of muscle fiber within a given skeletal muscle - Also driven by genetics How can you tell each fiber apart? - How fast the contraction is - The force of the contraction - The fatigue resistance - Sarcoplasmic reticulum proteins, how fast calcium is being shot out - The amount of ATP being generated - The size of the fiber and how it is stimulated - How well the fiber utilizes fats, carbohydrates and proteins - How many types of enzymes that are associated with glycolysis and the Krebs cycle - Blood flow related properties, outside the muscle fiber - Capillary density around the fiber, if it needs a lot oxygen or not What is myoglobin? - binds oxygen, stored in muscle, links to oxygen dropped off by the capillaries, reserved oxygen stored in muscle fiber - Stores and carries oxygen What makes up a type I muscle fiber? (slow twitch) - High capillary density, using a lot of oxygen, are aerobic, using krebs cycle and ETC - High myoglobin concentration - High mitochondria (uses a lot of oxygen) - High resistance to fatigue - Uses ETC, mitochondria, and krebs cycle - Low contraction rate - High efficiency, uses little ATP but still efficient - Moderate force - Long periods of exercise with low energy - Ex: distance running, cross country skiers What makes up a type IIa muscle fiber? (fast twitch) - Intermediate fiber - Moderate capillary density  - Moderate myoglobin concentrations - high/moderate mitochondria - high/moderate fatigue resistance - Both anaerobic and aerobic - High ATPase activity, breaks down ATP really fast - Intermediate contraction - Moderate efficiency - High force - Ex: cross-fit, soccer What makes up a type IIx muscle fiber? (fast twitch) - Low capillary density - Low myoglobin concentrations - Low mitochondria (glycolysis) - Low resistance to fatigue - Anaerobic energy source - Highest ATPase (breaking down a lot of ATP) - Highest contractions, really fast - Low efficiency - Highest force generation - Ex: divers, powerlifters Can fiber type be altered by endurance and resistance exercise training? - Yes but can only go from fast to slow twitch - Can’t go from slow to fast twitch How can a fiber change from fast to slow twitch muscle fiber type? - Mitochondrial biogenesis which is similar to converting fast twitch to slow twitch fibers - More capillaries since it needs more oxygen Can a fiber type go from a slow to a fast phenotype? - No What happened in the experiment with Buller and Eccels? - Did surgeries on cats - Surgically switched slow and fast twitch nerves - Turned slow to fast, and redid the same surgery to make sure it did actually work - Changed the alpha motor neuron signal - You can use surgically intervention to change slow to fast neurons How does resistance training increase strength if there is little evidence of slow to fast conversion? - Hypertrophy and changing in diameter What happens to muscle adaptations in resistance training? - There is a steep increase - Learn the techniques during the first couple of weeks  - After the first couple of weeks it plateaus, and the growth is coming from hypertrophy What is hypertrophy? - Increase of muscle size - Laying down more actin and myosin What causes hypertrophy? - The signal is the muscle stretch or the force being generate by the fiber to overcome the resistance - Satellite cells that are housed within the fiber, are changed into nuclei to increase myonuclear domain - Insulin-growth factors (IGF) is the primary signaling pathway - ↑ IGF = hypertrophy What is the signal for IGF to be engaged in resistance? - The stretch against the force What happens to skeletal muscle when endurance and resistance training are done together? - TSC ½ increases from endurance training - And it blocks mTOR which is in resistance training - You won’t get as much hypertrophy since it is being blocked from endurance training What is hyperplasia? - The increase of muscle fibers Can hyperplasia be a result of exercise training? - Hyperplasia doesn’t occur with resistance training What causes hyperplasia to happen? - When satellite cells in skeletal muscle potentially form into new muscle cells - Myostatin is knocked out - Can’t split apart and make new cells - Myostatin isn’t restricted - Comes from a gene mutation What is myostatin? - Is a protein that restricts muscle growth - Less myostatin = more muscle growth Does myostatin regulate muscle growth? - Yes What happened in the case report with the baby with hyperplasia? - At birth the child had an increase of muscle size in the thighs and arms - The child had a decrease of myostatin gene expression - It can impact smooth muscle, cardiac muscle, and skeletal muscle - Took MRI to compare muscle mass and saw an increase in the child - The bone size didn’t change - More muscle cells can form At birth in the case report, what happened to muscle size in the thighs and arms? - Increased Which best explains the results of the case report about the child with big muscles? - Decreased myostatin gene expression Lecture 11 What is the definition of VO2? - The volume of oxygen consumed by the body at any given point Why is oxygen important for exercise? - To generate ATP What is the Fick equation and what does it measure? - VO2 = CO x a-vO2 diff - Measures the volume of oxygen being consumed - CO is cardiac output (Q) - a is arterial - v is venous What is cardiac output? - The amount of blood leaving the heart in a minute Where are capillaries located? - They fall between the arterial and venous What is a-vO2 diff? - It measures the amount of oxygen extracted or utilized - The difference in oxygen content of blood between the arterial blood and the venous blood What happens to a-vO2 diff when going from rest to exercise? - Increases - More oxygen is being extracted from blood, to go into mitochondria to make more ATP - The body needs more oxygen when exercising, therefore it will extract more from the blood - Has a greater ATP demand when exercising How does chronic aerobic exercise training affect a-vO2 diff? - Rest - No change - Submaximal - No change - Maximal - Increase - Can contract more oxygen - It takes the same amount of oxygen for a sedentary and trained person to do the same amount of exercise - A trained person go longer since they can contact more oxygen What happens to venous oxygen contact with aerobic exercise? - Decreases - There is less oxygen since there are more capillaries - The venous content stays the same as a sedentary person, they are just able to pull off more oxygen making the venous levels go down What happens to arterial oxygen contact with aerobic exercise? - No change - You can’t increase something that is already at 100% What changes occur to the skeletal muscle after chronic aerobic exercise training that would facilitate oxygen extraction? - Mitochondrial biogenesis (more mitochondria, utilize more oxygen) - Angiogenesis (amount of capillaries surrounding it) - Will work together to get more oxygen into that skeletal muscle What increases oxygen extraction? - Mitochondrial biogenesis - Angiogenesis What is angiogenesis? - Formation of new capillaries from existing capillaries What are the two types of angiogenesis? - Capillary intussusception - Sprouting angiogenesis What is intussusception angiogenesis? - Taking a preexisting capillary and making it into two by splitting down the middle creating a wall in between What is sprouting angiogenesis? - A bud growing from the side of the capillary forming into another capillary What is the exercise stimulus in angiogenesis? - The mechanical forces on preexisting capillaries from exercise, which is the signal - Shear forces (friction of RBC bumping into the walls of the capillaries) - Compression (surrounded by skeletal muscle fibers, when it contracts the blood vessel compresses on the muscle) - Stretch (force from the blood pressure increases) What is molecular signaling in angiogenesis? - Vascular endothelial growth factors (VEGF) being released into the cells, in the capillaries for angiogenesis to occur - Only occurs when muscle is contracting How does increasing capillary numbers facilitate the diffusion of oxygen from red blood cells to contracting skeletal muscle? - During exercise more capillaries open so there is more oxygen - Increase transit time  - Decrease diffusion distance of oxygen to mitochondria since there are more capillaries - Better at making ATP, exercising longer How does resistance training impact angiogenesis? - Doesn’t increase - Not a stimulus strong enough to do, doesn’t reach threshold - Don’t see an increase because of skeletal hypertrophy, it is pushing the capillaries out of the slide How does resistance training impact mitochondrial biogenesis? - Expansion of density - It can accommodate more oxygen - Expand network for a trained person How does endurance training lead to a greater a-vO2 diff? - The greater capillary networks that surround that muscle fiber which allows blood to move slower for the diffusion to occur into that muscle fiber - A smaller distance for that blood to diffuse into the mitochondria - The mitochondria gets bigger so it can utilize the oxygen What type of exercise training has been associated with the greatest increases in capillary numbers? - Endurance exercise training Endurance exercise training results in what in the blood flow of capacity to contracting skeletal muscle? - Increase