Muscles Part 1

1. Muscles Part 1

2. By the end of this class you should understand: • The structure and function of whole muscle • The sliding filament model of skeletal muscle contraction • The role of ions in the stimulation and contraction of muscles • The role of different energy sources in muscle contraction

3. Muscles • Muscle is a general term for the tissues and organs of the body that produce force • The organ is sometimes referred to as whole muscle • There are three types of muscle tissue: • Skeletal (voluntary) • Cardiac (heart) • Smooth (hollow organs)

4. Whole Muscle • Whole muscle is an organ composed of skeletal muscle tissue wrapped in areolar connective tissue • Produces force when stimulated by a voluntary nerve fiber • Under conscious control • These are the muscles we think of as muscles

5. Whole Muscle Anatomy • A muscle has an origin and an insertion • Generally the more proximal attachment point is the origin and the distal attachment point is the insertion • Example: Biceps have origin at shoulder and insertion at elbow • Produce force by shortening

6. Whole Muscle Microanatomy • The muscle organ is composed of many cells called muscle fibers which are huge (each the size of a human hair) • Multinucleate due to being many cells fused together • These muscle fibers are arranged into bundles called fascicles

7. Muscle Fiber • A muscle fiber is a single cell composed of many fused cells • Most of the cell is filled with huge organelles called myofibrils • Myo- is the latin root for muscle • The myofibril is made up of many sarcomeres

8. Sarcomeres • The sarcomeres are the actual force-producing organelles of the cell • Composed essentially entirely of protein • This is why protein is essential to the diet of any athlete • Also why animal meat is full of protein

9. Muscle Stimulation • Muscles receive signals from motor neurons • These signals are transmitted by a neurotransmitter called acetylcholine • The signal is very brief so the muscle relaxes as soon as the signal is no longer being sent by the brain • These signals will be explored in more detail next week

10. T Tubule • The motor neuron’s signal reaches the outside of the muscle fiber and then is transmitted throughout the entire fiber through the T tubules • The signal is made up of sodium and potassium ions moving back and forth across the cell membrane • This is also known as an action potential • It moves much faster than diffusion

12. T Tubule and SR • When a neurotransmitter is released onto a muscle fiber, the electrochemical signal is transmitted through the T tubules through the entire fiber • As the signal moves through the T tubules, it stimulates the sarcoplasmic reticulum to release calcium • Modified version of the endoplasmic reticulum

13. Calcium? • Calcium ions are being constantly pumped into the SR • When the muscle is not being stimulated, all calcium ions are in the SR • When the muscle is stimulated, calcium is released but is still being pumped back into the SR • This means once the signal ends the muscle will stop contracting quickly • This requires a lot of constant ATP expenditure

14. Sarcomere • A sarcomere is made of two different kinds of protein fibers • Myosin filaments • Actin filaments • The myosin filaments have many myosin heads with two features: • They can attach to actin filaments • They try to bend

15. Sliding Filament Model • The myosin heads of the myosin filament attach to the actin filaments and attempt to move them together • There are two things that are required for this: • The actin can only be attached to when it has a calcium ion • The myosin head needs ATP to detach after it bends

16. Sliding Filaments • The actin filament has two molecules called troponin and tropomyosin • Together they block the myosin head from attaching to the actin filament • When a calcium ion attaches to the troponin, the tropomyosin is moved out of the way • The myosin head may now attach to the actin filament

17. ATP Usage • Clearly contracting our muscles requires energy • The actual use of the ATP is to detach the myosin head from the actin filament • Once the myosin head is detached it can reattach at a new point and pull again • Imagine if you had to stay in place and help play tug-of-war!

18. ATP Sources • All muscle cells have some free ATP when relaxed but this is depleted almost immediately during contraction • Once the ATP supply is depleted one of two things will happen: • More ATP must be produced rapidly • ATP will only be produced slowly by the mitochondria and the force produced will be less

19. Sources of ATP: • Creatine Phosphate (anaerobic) • Rapidly converted to an ATP, runs out after a few more seconds • Glycogen (anaerobic) • Stored glucose in the muscle fiber • Blood Glucose (aerobic) • Only absorbed and metabolized slowly • Fatty acids (aerobic) • Stored in muscle, metabolized slowly

20. ATP Depletion • Once the muscle’s main ATP supply is depleted, the available force produced is much lower • Fatigue • If all ATP is depleted, the muscle may lack the ATP supply to detach the myosin heads once the nerve signal ends • Cramp! • Once the muscle is relaxed, blood glucose and O2 are used to restore the ATP supply • Oxygen debt

21. Oxygen Debt • If more energy is used than can be produced aerobically, anaerobic production of energy can sustain activity • Produces by products such as lactic acid • Lactic acid can be reabsorbed and processed aerobically using more oxygen • This is why people pant and become winded if they have exerted themselves anaerobically

22. Rigor Mortis • When a person dies, they are initially limp • Once all ATP has been depleted from the cells, calcium begins to diffuse into the muscles • Calcium allows myosin head to attach to actin filament • Lack of ATP means the myosin head cannot be detached • Ultimately the entire body becomes incredibly stiff and will stay that way until the proteins physically break down from rotting (rigor mortis)

23. See you Wednesday!