Physiology of a Muscle

1. Physiology of a Muscle Unit 4 Part 2 Notes

2. What functional properties allow a muscle to perform its duties? • Irritability • Ability to receive and respond to a stimulus • Contractility • Ability to shorten when adequate stimulus is received

3. What functional properties allow a muscle to perform its duties? • Conductivity • Ability for impulse to travel along plasma membrane of muscle cell • Elasticity • Ability to recoil and resume original length

4. What role does the nervous system play in muscle movement? • Motor Unit – one neuron and all the skeletal muscle cells it stimulates

5. Within a motor unit… • Muscle Fibers • Axons • Axon Terminals (neuromuscular junctions)

7. Nerve endings and muscle fibers don’t physically touch… • Neuromuscular juction – where axon terminals match up with muscle fibers • Snyaptic cleft – space between nerve endings and muscle fibers; chemical impulses travel here between nerve endings and muscle

8. Two Major Steps Of Muscle Contractions(Not in your packet-o-notes) 1st Stimulation of Muscle by Nerve • The brain sending a message through a neuron to the muscles to simply tell that muscle to MOVE! Video #1: http://tinyurl.com/NerveImpulse 2nd Muscle Contraction • Once the muscle has the message from the brain to move, the muscle will move its fibers and contract. Video #2 http://tinyurl.com/aqspwh

9. What steps occur to stimulate muscle movement? • 1. Nerve impulse reaches axon terminals • 2. Chemical Neurotransmitter (ACh – acetylcholine) released • 3. ACh diffuses across synaptic cleft and attaches to receptors

10. What steps occur to stimulate muscle movement? • 4. ACh causes the sarcolemma to become temporarily permeable to Na+ • 5. Na+ rush into the muscle cell • 6. Excess of positive ions creates electric current (action potential) • 7. Muscle contracts (another whole set of steps!)

11. More animations which may help you understand how a muscle contracts! http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.html

12. So, we know how muscle contraction is stimulated… but now we need to know the steps that help muscle contraction to happen! • Called the Sliding Filament Theory

13. The Sliding Filament Theory • Muscle fibers activated by nervous system due to action potential • Calcium ions (Ca+2) happen to be released • Do you remember which structure releases those calcium ions???

14. The Sliding Filament Theory • Muscle fibers activated by nervous system due to action potential • Calcium ions (Ca+2) happen to be released • Do you remember which structure releases those calcium ions??? • THE SARCOPLASMIC RETICULUM

16. The Sliding Filament TheoryNote: this part is worded a little differently than your packet-o-notes • Release of Ca+2 binds to troponin which then moved tropomyosin out of the way and stops it from blocking binding sites on the thin filament (actin). • Once Actin is unblocked, the cross-bridges on the thick filaments(Myosin) attach to the binding sites on the Actin • Let the sliding begin!

17. The Sliding Filament Theory • Energized by energy from ATP, cross-bridges attach and detach from thin filaments • Works like an oar to keep moving thin filaments closer and closer together (Attach, pull, detach!) Check this video to see how it works! http://tinyurl.com/aqspwh

19. The Sliding Filament Theory • As this process is happening in every sarcomere throughout the muscle, the muscle itself is contracting! • The whole series of events (beginning with the nervous system signal) takes just a few thousandths of a second!!!

20. Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Axon terminal Synaptic cleft Synaptic vesicle Sarcolemma T tubule 1 Net entry of Na+ Initiates an action potential which is propagated along the sarcolemma and down the T tubules. ACh ACh ACh Ca2+ Ca2+ SR tubules (cut) SR Ca2+ Ca2+ 2 Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca2+ release from terminal cisternae of SR into cytosol. ADP Pi Ca2+ Ca2+ Ca2+ Ca2+ 6 Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes. 3 Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Ca2+ 5 Removal of Ca2+ by active transport into the SR after the action potential ends. Ca2+ 4 Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process.

21. The Sliding Filament Theory • Notice in the contracted muscle, the H zone has disappeared • The I band has shortened significantly (all that’s left is the Z disc) • The A band (the dark striations!) have stayed the same thickness • Check out this animation to see what happens to the sarcomere!! http://tinyurl.com/5u8q25b

22. Where’s all this energy coming from? • As a reminder, energy comes from ATP because of breaking a phosphate bond • Breaking a bond releases energy • When this energy is used by your body, it releases heat • Because ATP is the only energy source that can be used to move the cross-bridges back and forth (which contract the muscle), ATP must be regenerated continuously

23. ATP Regeneration – 3 Sources • Direct phosphorylation of ADP by creatine phosphate • When ATP used, changes to ADP • Creatine phosphate adds that missing phosphorous back on! • PROBLEM: only makes 1 ATP at a time… so not very much. And, only supplies energy for 15-20 seconds of activity! • Your body will always do this, but it’s not very effective. Therefore, we have to have other ways of supplying energy…..

24. ATP Regeneration – 3 Sources • Aerobic respiration • Occurs in the mitochondria • Glucose broken down to pyruvic acid (releasing 2 ATP), and then into carbon dioxide and water (releasing 34 ATP) • 36 ATP made for 1 glucose! A lot of energy! And, can supply energy for hours at a time! • PROBLEM: NEEDS OXYGEN • But what if you’re out of oxygen??? Then your muscles will begin……..

25. ATP Regeneration – 3 Sources • Anaerobic glycolysis and lactic acid formation • Glucose broken down to pyruvic acid, releasing 2 ATP • If oxygen present, process continues to the rest of aerobic respiration… • BUT… if oxygen is inadequate, or muscle activity is intense, pyruvic acid is instead changed to lactic acid • PROBLEM: Buildup of lactic acid is not good… promotes muscle fatigue and soreness. And, only supplies energy for 30 seconds of activity!

26. ATP Regeneration • 95% of ATP produced through aerobic respiration • C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O + ATP • If you don’t have proper blood circulation or breathing, muscles can’t get oxygen needed for aerobic respiration • If they can’t get oxygen, they can’t produce enough ATP, which means muscles can’t contract!

27. Muscle Fatigue • A muscle is fatigued when it is unable to contract even though it is being stimulated – means you don’t have ATP to move the cross-bridges! • Lack of oxygen can cause… • Lactic acid buildup (anaerobic glycolysis) • ATP supply low (production can’t keep up with usage) • Muscle will contract less and less effectively, eventually stopping contraction completely

28. Muscle Fatigue • FYI: When you breathe heavy after physical activity, your muscles are trying to get enough oxygen for aerobic respiration to replace all of the ATP you used!

29. Muscle Contraction • Isotonic contractions • Myofilaments slide, shortening the muscle • Movement occurs – bending knee, rotating arms, smiling

30. Muscle Contraction • Isometric contractions • Myofilaments trying to slide, but can’t – just building up tension (crossbridges are “rowing”, but actin is not moving together) • Movement doesn’t occur – object too heavy to lift, push against wall

31. Muscle Tone • Muscle tone – sustained partial contraction of a muscle; muscle stays firm, healthy, and ready for action • Muscle inactivity can lead to muscle weakness and wasting (this is why Range of Motion exercises on bedridden people is important!)

32. Effect of Exercise on Muscles • Aerobic exercises include… • Running • Jogging • Biking • Elliptical

33. Effect of Exercise on Muscles • Increase endurance of muscles because muscle cells will form more mitochondria and store more oxygen (meaning more energy for the muscles) • Also – improves body metabolism, improve digestion, enhance coordination, strengthens skeleton, heart & lungs more efficient • Muscles do NOT increase in size!

34. Effect of Exercise on Muscles • Resistance exercises include… • Pushing against wall • Contracting muscles (likes gluteus maximus) • Lifting weights • Does increase muscle size! • Due to enlargement of individual muscle cells (makes more myofilaments) • You don’t add more muscle cells – you just bulk up the ones you already have!!! • NEED BOTH TYPES OF EXERCISES IN ANY TRAINING PROGRAM!