Muscle: Anatomy Physiology

1. Muscle: Anatomy & Physiology Lecture 20

3. Motor Pool all a motor neurons that innervate a single muscle An a motor neuron and all the muscle fibers that it innervates 1:3 to 1:100 fewer muscle fibers ---> finer control 3 types based on speed of contraction & fatigue ~ Motor Pools & Motor Units

4. Type 1 Fibers Slow fibers dark red slow, sustained contraction slow to fatigue Aerobic metabolism many capillaries & mitochondria oxygen required for ATP synthesis myoglobin gives dark red appearance ~

5. Type 2b Fibers Fast fatigable fibers white fibers rapid, brief contraction fast to fatigue produce about 10x force of Type 1 Anaerobic metabolism fewer capillaries & mitochondria ATP generated by glycolysis lactic acid buildup ~

6. Type 2a Fibers Fast fatigue-resistant fibers pale red properties intermediate to types 1 & 2b rapid, brief contraction slow to fatigue produce least force Aerobic & Anaerobic metabolism many capillaries & mitochondria ~

7. Types of Motor Units Most muscle contain both slow- & fast-twitch fibers ratio depends on function e.g. ankle extensors Soleus active during standing hi ratio of slow fibers Medial Gastrocnemius: active during running & jumping hi ratio of fast fibers ~

8. Neuromuscular Junction Synapse between neuron & effector Cholinergic (ACh) nicotinic receptors Motor end-plate postsynaptic membrane folds packed with receptors increased surface area ~

9. Motor end-plate

10. Graded Control of Muscle Contraction Highly reliable synapse 1 presynaptic AP ---> 1 postsynaptic AP ---> 1 twitch (contract/relax) temporal summation � tension & sustained contraction Recruitment � # motor units ---> � tension order: smallest ---> largest ~

11. Muscle Contraction AP generated in muscle fiber (cell) Ca++ released from internal stores Muscle fiber contracts continues while Ca++ & ATP available Relaxation Ca++ sequestered by active transport ~

12. Muscle Fiber Structure Multinucleated fusion of multiple precursor cells Sarcolemma Excitable membrane Myofibrils: contractile units Sarcopasmic reticulum (SR) sequesters Ca++ T tubules AP from sarcolemma to SR like inside-out axons ~

14. Myofibril: structure & function Sarcomeres repeating sections Z lines dividers between sarcomeres thin filaments anchored to Z lines actin & troponin Thick filaments between thin filaments myosin Contraction:filaments slide by each other ~

17. Contraction Excitation-contraction coupling Myosin �heads� crossbridges w/ actin Ca++ dependent binds to troponin, reveals binding site Myosin head rotates �ratchets� actin inward ~

18. Contraction ATP binds to myosin ---> detachment cocks myosin ---> binds again rigor mortis: no ATP fibers remain crosslinked Repeats as long as Ca++ present sequestered via active transport ~

23. Movement Disorders of Muscle

24. Duchenne�s Muscular Dystrophy Muscular Dystrophies wasting away of muscles metabolic / structural abnormalities Duchenne�s best understood young boys ~

25. Duchenne�s Muscular Dystrophy Cause hereditary - maternal X chromosome single gene ---> protein dystrophin maybe involved in Ca++ regulation Treatment Inject dystrophin or mRNA Gene therapy promising for muscles ~

26. Myasthenia Gravis Severe muscle weakness rapid fatigue following exercise Develops in people of all ages Most common: women in 30s Risk of respiratory paralysis Autoimmune disorder body develops antibodies for ACh-R reduces synaptic transmission ~

27. Myasthenia Gravis: Treatment AChE inhibitors � degradation of ACh narrow therapeutic window too much ACh ---> paralysis Reduce immune response remove thymus filtering antibodies from blood temporary ~