Muscle Anatomy. Muscle Fiber

Transcription

1 Muscle Anatomy Muscle is made up of a group of fasiculi Muscle is surrounded by deep fascia and epimysium (connective tissue) Fasiculus is made up of a group of muscle fibers (cells) Fasiculus is surrounded by perimysium 1 Muscle Fiber 2 Each fasiculus is made up of groups of muscle fibers/cells Each muscle fiber is surrounded by endomysium Plasma membrane is called sarcolemma Cytoplasm is called sarcoplasm One muscle fiber contains Numerous nuclei, which regulate protein synthesis Numerous mitochondria for ATP production

2 Myofibrils 3 Each muscle fiber contains myofibrils Each myofibril is made up of myofilaments myofilament: proteins that actually move for contraction thin filament thick filament 4 Muscle Fiber

3 Connective Tissues=Movement 5 Endomysium, perimysium, and epimysium all connect to the muscle s tendon (dense regular connective tissue which attaches to bone) Contraction of muscle therefore pulls on tendon, which moves bone Myofibril 6 Made up of groups of myofilaments Thin filaments contain actin, troponin, tropomyosin (proteins) Thick filaments contain myosin (protein) Elastic filaments help hold thin and thick filaments in place Myofilaments interact with one another to produce muscle contraction

4 7 Myofibril 8 Myofibril structure Made up of many contractile subunits (sarcomeres) placed in series

5 Sarcomeres Ends of a sarcomere are called Z lines (disk), and anchor the thin filaments Thick filaments are anchored to each other at the center of a sarcomere (M line) Thick filaments are also indirectly anchored to the Z line through an elastic filament (titin/connectin) 9 Sarcomeres 10 I (isotropic) band: contains only thin filaments Contains Z band

6 Sarcomeres 11 A (anisotropic band): contains both thin and thick filaments, but border is the outside edge of thick filaments Anything that is not I band is part of A band Contains M (myosin) line and H (heavy) zone [thick filaments only] Sliding Filament Model 12 H.E. Huxley and A.F. Huxley, 1954, explained how muscles contract (shorten) to achieve the function of movement

7 Sliding Filament Model 13 Muscle contracts because thin (actin) and thick (myosin) filaments bind to one another The myosin twists like a ratchet to pull the actin toward the middle of the sarcomere (power stroke) Myosin releases and reattaches to the actin at a point closer to the Z line Sliding Filament Model 14 Pulling the actin in moves the Z lines closer together, shortening the overall length of the sarcomere(s) and thus shortening the length of the muscle = Contraction!

8 Sliding Filament Model 15 Muscle contraction is the result of the sarcomere shortening as thin filaments slide over thick filaments, drawing thin filaments closer together Myofilaments do not change their overall length H zones and I bands narrow A bands do not narrow Sliding Filament Model 16

9 Sliding Filament Model Contraction occurs when myosin binding sites on actin are exposed Binding site hidden by tropomyosin at rest Tropomyosin and troponin are bound to one another on the thin filament 17 Sliding Filament Model 18 Binding of calcium to troponin causes troponin to change shape, moving tropomyosin, and exposing myosin binding sites on actin

10 Sliding Filament Model 19 Myosin (already in energized, elongated position) binds to a binding site on actin, and myosin undergoes power stroke Sliding Filament Model 20 Power stroke requires energy, which was stored in elongated myosin Using that energy for stroke leaves myosin head in shorter, low energy configuration

11 Sliding Filament Model For myosin to release the thin filament, another ATP must bind to myosin head broken down to ADP + Pi + energy myosin releases actin and elongates (back to high energy configuration) 21 From where does calcium come? 22 Sarcoplasmic reticulum: a tubular system within the muscle fiber designed to store and release calcium upon muscle stimulation

12 Muscle Contraction & SR 23 When muscle fiber is stimulated by nervous system (motor unit) Electrical impulse moves down the T tubules, which contact the terminal cisterns on either side T tubule carries the impulse deep into the muscle myofibrils very quickly so that the myofibrils contract together Muscle fibers are all or none Neuromuscular Junction 24 Site of communication between neuron and muscle Gap=synapse Communication achieved by neuron releasing neurotransmitter (acetylcholine) which binds to receptors on sarcolemma

13 Sarcoplasmic Reticulum Calcium leaves the SR through calcium release channels in the cisterns bind to calcium binding sites on troponin 25 Muscle Relaxation 26 Relaxation occurs when muscle is no longer stimulated by the nervous system Calcium is taken up by calcium active transport pumps in the SR Without calcium available to bind to troponin, tropomyosin no longer pulls tropomyosin away from myosin binding sites Myosin therefore cannot bind with actin

14 Muscle Tone 27 At any given time in a resting muscle, some of the fibers within a muscle are contracting The fibers which are contracting alternate within a muscle to prevent fatigue of any particular fiber Muscle Contraction Two potential limits on muscle contraction Availability of calcium Calcium feedback (bones) No real loss from muscle cell Availability of ATP Body has multiple sources of ATP, each fine tuned to deliver ATP in a specific time frame and for a specific duration ATP depletion can occur 28

15 Muscle Metabolism 29 Muscle Metabolism 30 Two major categories of energy sources: aerobic and anaerobic Systems are linked together Aerobic capabilities can speed up recovery from anaerobic bursts of energy

16 31 Muscle Metabolism Aerobic Metabolism 32 Aerobic energy delivery Virtually limitless energy delivery Multiple sources of energy substrates (fat, carbohydrates, proteins) Relatively slow rate of energy delivery Produces only CO 2 and H 2 O as byproducts

17 Anaerobic Metabolism 33 Anaerobic energy delivery Very limited energy supply Rate of energy delivery is very fast Produces relatively harmful by- products (acid) ATP Available in Cell 34 Sources of energy Enough ATP in muscle for about 1-2 seconds Phosphocreatine (PC) good for a few seconds more Transfers its one P i group to ADP

18 35 Anaerobic Metabolism Glycolysis 36 Breakdown of sugar (glucose) Glucose converted to 2 pyruvic acid + 2 ATP Muscle s source of glucose is glycogen or newly digested glucose in blood Glycogen is the storage form of glucose in liver and muscle

19 37 Glycolysis 38 Rate limiting enzyme is phosphofructokinase (PFK) which converts fructose- 6- phosphate into fructose- 1,6- biphosphate

20 Muscle Metabolism 39 Two major fates of pyruvic acid Pyruvic Acid 40 If adequate mitochondrial capacity available, pyruvic acid is converted into acetyl- CoA and enters the Krebs Cycle [and then electron transport]

21 Pyruvic Acid 41 Krebs Cycle is the preferred pathway because it is much more efficient at producing energy and the only waste products are water and CO 2 Limits: number of mitochondria, availability of oxygen Enzyme for conversion to acetyl- CoA is pyruvate dehydrogenase Pyruvic Acid 42 If adequate mitochondrial capacity is not available, pyruvic acid is converted to lactic acid Enzyme: lactate dehydrogenase (LDH)

22 Muscle Fatigue 43 Lactic acid is disassociated into lactate ion (La - ) and hydrogen ion (H + ) The excess H + (increased acidity of muscle environment) inhibits muscle contraction Inhibits PFK (shutting down glycolysis) Reduces the effect of Ca +2 binding on troponin (actin and myosin cannot bind) Fates of Lactate 44 Cori Cycle Liver can convert lactate into glucose, which can them by stored as glycogen or reused (glyconeogenesis) Essentially the reverse of glycolysis and glycogenolysis

23 Fates of Lactate 45 Cardiac muscle and skeletal muscle can convert lactate into pyruvate (LDH) Pyruvate can then enter the mitochondria for aerobic energy production (pick up where you left off) Still limited by availability of oxygen and mitochondria Aerobic Metabolism 46 Krebs cycle (citric acid cycle, TCA) and electron transport (ET) / oxidative phosphorylation

24 Aerobic Metabolism 47 Glucose + 6O ADP + 38 P i 6CO 2 + 6H 2 O + 38 ATP Krebs Cycle 48 Produces electrons for ET which uses electrons to produce ATP (max. 34 electrons/glucose) Produces 2 CO 2 for each acetyl CoA (thus, 4 CO 2 for each glucose)

25 Krebs Cycle 49 Electron Transport Uses 3 O 2 for both acetyl CoA s to produce 32 or 34 ATP (+ 2 GTP) 50

26 Sources of Acetyl CoA Carbohydrates Proteins Lipids 51 Sources of Acetyl CoA Amino acids 52

27 Sources of Acetyl CoA Lipids TG converted to FA β- oxidation to acetyl- CoA 53 Changes in Muscle 54 Strength training Damage to muscle cells after overload produces soreness Damage also is stimulus for muscles to build more thin and thick filaments within a muscle cell (hypertrophy) Response to perceived inadequate muscle supply Positive effect of negative (soreness)

28 Changes in Muscle 55 Aerobic training More and larger mitochondria Increased capillarization (shorter distances for O 2 and CO 2 to travel) More myoglobin within the cell O 2 Consumption Dynamics 56 Exercise at or below 50% of maximal aerobic capacity (VO 2 max) Brain is using mostly glucose for ATP metabolism True under almost all conditions Muscles (and rest of body) using mostly lipid metabolism for ATP production

29 O 2 Consumption Dynamics 57 Exercise between 50-70% of (VO 2 max) Above 50%, we must use more carbohydrates (glycogen and recently consumed glucose) for additional energy required Breakdown of fats cannot supply energy fast enough to meet demand O 2 and mitochondria sufficient, so acidity doesn t increase, as pyruvate goes through preferred pathway O 2 Consumption Dynamics 58 Exercise above 70% (VO 2 max) Accumulate excess H + in blood Called anaerobic or lactate threshold Serious limitations in length of time you can perform at this level Acidity inhibits PFK Acidity inhibits Ca +2 and troponin binding Individual s lactate threshold can be changed

30 59 Neuromuscular Junction 60 Muscle Fiber Types Motor unit: one motor neuron and all the muscle fibers that it innervates (the group of muscle fibers that contract when stimulated by a single neuron) Fibers contract on all or nothing basis All muscle fibers of a motor unit are of the same type

31 Muscle Fiber Types 61 SO (slow oxidative) Type I or red muscle Contain large amounts of myoglobin, mitochondria, and capillaries Contain to primarily metabolize energy aerobically Muscle Fiber Types 62 FG (fast glycolytic) Type II or white muscle Largest fibers Contain little myoglobin, less mitochondria and fewer capillaries than SO More glycogen Enzymes to primarily go through glycolysis Powerful contractions but limited endurance

32 Muscle Fiber Types 63 FOG (fast oxidative glycolytic) Type IIA or intermediate fibers Contains characteristics of both SO and FG Most trainable type of muscle Will behave like SO or FG, depending on training Muscle Fiber Types 64 Most human skeletal muscle is a combination of types Triceps about 75% FG Soleus about 85% SO Vastus lateralis is about mix of slow, intermediate, and fast

33 Muscle Fiber Types 65 Motor units are recruited in the order of SO, then FOG, then FG Spares glycogen as much as possible Try to save the most powerful motor units for greatest muscular demand Use only SO motor units for delicate movements requiring light force