Skeletal muscle structure & function
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This document discusses the structure and function of skeletal muscle. It begins with an introduction to skeletal muscle and then covers topics like muscle fiber structure, development of muscle cells, muscle proteins, the sarcomere, sarcoplasmic reticulum, and excitation-contraction coupling. Diagrams are provided to illustrate muscle fiber anatomy, the arrangement of actin and myosin filaments in the sarcomere, and the relationship between the sarcoplasmic reticulum and t-tubules. The document provides definitions of key muscle terms and describes the roles of various muscle proteins.
Skeletal muscle structure & function
- 1. Dr. Dinesh T, Junior resident. 11/19/2011 Jipmer Physiologist 1
- 2. Discussion topics Introduction Myocyte- structure, development Muscle proteins Sarcomere Sarcotubular system Excitation- contraction coupling Molecular basis of muscle contraction Summary Muscle relaxation Conclusion 11/19/2011 Jipmer Physiologist 2
- 3. Introduction Human body contains over 400 skeletal muscles 40-50% of total body weight Functions of skeletal muscle Body movement (Locomotion) Maintenance of posture Respiration Diaphragm and intercostal contractions Communication (Verbal and Facial) Constriction of organs and vessels Peristalsis of intestinal tract Vasoconstriction of b.v. and other structures (pupils) Production of body heat (Thermogenesis) 11/19/2011 Jipmer Physiologist 3
- 4. Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate Striated – have visible banding Voluntary – subject to conscious control Cells are surrounded and bundled by connective tissue = great force, but tires easily Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 11/19/2011 Jipmer Physiologist Slide 6.3 4
- 5. Terms Sarcolemma = Cell membrane Sarcoplasm = Cytoplasm Sarcoplasmic Reticulum = Endoplasmic Reticulum Sarcosomes = Mitochontria 11/19/2011 Jipmer Physiologist 5
- 6. Structure of skeletal muscle: connective tissue covering Epimysium Surrounds entire muscle Perimysium Surrounds bundles of muscle fibers Endomysium Surrounds individual muscle fibers 11/19/2011 Jipmer Physiologist 6
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- 9. Skeletal muscle structure Composed of muscle cells (fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, and multinucleated Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length 11/19/2011 Jipmer Physiologist 9
- 10. •Develop from myoblasts; numbers remain constant •Striated appearance •Nuclei are peripherally located 11/19/2011 Jipmer Physiologist 10
- 11. Embryologic origin: 11/19/2011 Jipmer Physiologist 11
- 12. Muscle fiber anatomy Sarcolemma - cell membrane Surrounds the sarcoplasm (cytoplasm of fiber) Contains many of the same organelles seen in other cells An abundance of the oxygen-binding protein myoglobin Punctuated by openings called the transverse tubules (T-tubules) Narrow tubes that extend into the sarcoplasm at right angles to the surface Filled with extracellular fluid Myofibrils -cylindrical structures within muscle fiber Are bundles of protein filaments (=myofilaments) Two types of myofilaments 1. Actin filaments (thin filaments) 2. Myosin filaments (thick filaments) – At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma – When myofibril shortens, muscle shortens (contracts) 11/19/2011 Jipmer Physiologist 12
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- 15. Muscle proteins Contractile proteins Actin- thin myofiliment Myosin- thick filament Regulatory proteins Tropomyosin Troponin Attachment proteins Titin, nebulin, alpha actinin, dystrophin 11/19/2011 Jipmer Physiologist 15
- 16. Structure of Actin and Myosin 11/19/2011 Jipmer Physiologist 16
- 17. Thin Filament: composed of 3 major proteins 1. F (fibrous) actin 2. Tropomyosin Actin (Thin) 3. Troponin Two strands of fibrous (F) actin form a double helix extending the length Myofilaments of the myofilament; attached at either end at sarcomere. Composed of G actin monomers each of which has a myosin- binding site Actin site can bind myosin during muscle contraction. Tropomyosin: an elongated protein winds along the groove of the F actin double helix. Troponin is composed of three subunits: Tn-A : binds to actin Tn-T :binds to tropomyosin, Tn-C :binds to calcium ions. 11/19/2011 Jipmer Physiologist 17
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- 19. Many elongated myosin molecules shaped like golf clubs. Single filament contains roughly 300 Myosin (Thick) myosin molecules Molecule consists of two heavy myosin molecules wound together to form a rod Myofilament portion lying parallel to the myosin myofilament and two heads that extend laterally. Myosin heads 1. Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site) 2. Attached to the rod portion by a hinge region that can bend and straighten during contraction. 3. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction 11/19/2011 Jipmer Physiologist 19
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- 21. Sarcomere - repeating functional units of a myofibril About 10,000 sarcomeres per Sarcomeres: Z Disk myofibril, end to end Each is about 2 µm long to Z Disk Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance. A bands: a dark band; full length of thick (myosin) filament M line - protein to which myosins attach H zone - thick but NO thin filaments I bands: a light band; from Z disks to ends of thick filaments Thin but NO thick filaments Extends from A band of one sarcomere to A band of the next sarcomere Z disk: filamentous network of protein. Serves as attachment for actin myofilaments Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment 11/19/2011 Jipmer Physiologist 21
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- 24. Sarcoplasmic Reticulum (SR) SR is an elaborate, smooth endoplasmic reticulum runs longitudinally and surrounds each myofibril Form chambers called terminal cisternae on either side of the T-tubules A single T-tubule and the 2 terminal cisternae form a triad SR stores Ca++ when muscle not contracting When stimulated, calcium released into sarcoplasm SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction 11/19/2011 Jipmer Physiologist 24
- 25. Sarcoplasmic Reticulum (SR) Figure 9.5 11/19/2011 Jipmer Physiologist 25
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- 28. Muscular Contraction The sliding filament model Muscle shortening occurs due to the movement of the actin filament over the myosin filament Formation of cross-bridges between actin and myosin filaments Reduction in the distance between Z-lines of the sarcomere 11/19/2011 Jipmer Physiologist 28
- 29. Sliding Filament Theory Rest – uncharged ATP cross-bridge complex Excitation-coupling – charged ATP cross-bridge complex, “turned on” Contraction – actomyosin – ATP > ADP & Pi + energy Recharging – reload cross-bridge with ATP Relaxation – cross-bridges “turned off” 11/19/2011 Jipmer Physiologist 29
- 30. Sliding Filament Model of Contraction Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree In the relaxed state, thin and thick filaments overlap only slightly Upon stimulation, myosin heads bind to actin and sliding begins 11/19/2011 Jipmer Physiologist 30
- 31. Cross-Bridge Formation in Muscle Contraction 11/19/2011 Jipmer Physiologist 31
- 32. Myosin ATPase Cycle ADP Pi From: Stryer 1988. Biochemistry. Freeman eds. Based on: Lymn and Taylor. 1971. Biochemistry 10: Jipmer Physiologist 11/19/2011 4617 32
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- 34. EM Shows Different TM positions along the Actin Filament Relaxed Ca-Activated Rigor Ca-Activated X-bridge From: Craig & Lehman.. 2001. J. Mol. Biol 311: 1027. 11/19/2011 Jipmer Physiologist 34
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- 36. Excitation-Contraction Coupling Mechanism where an action potential causes muscle fiber contraction Involves Sarcolemma Transverse or T tubules Terminal cisternae Sarcoplasmic reticulum Ca2+ Troponin 11/19/2011 Jipmer Physiologist 36
- 37. Sources of ATP for Muscle Contraction 11/19/2011 Jipmer Physiologist 37
- 38. Energy Sources ATP provides immediate energy for muscle contractions from 3 sources Creatine phosphate During resting conditions stores energy to synthesize ATP Anaerobic respiration Occurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acid Aerobic respiration Requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water More efficient than anaerobic 11/19/2011 Jipmer Physiologist 38
- 39. Energy for Muscle Contraction Direct phosphorylation Muscle cells contain creatine phosphate (CP) CP is a high-energy molecule After ATP is depleted, ADP is left CP transfers energy to ADP, to regenerate ATP CP supplies are exhausted in about 20 seconds 11/19/2011 Jipmer Physiologist Figure 6.10a Slide 6.24 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 39
- 40. Energy for Muscle Contraction Anaerobic glycolysis Reaction that breaks down glucose without oxygen Glucose is broken down to pyruvic acid to produce some ATP Pyruvic acid is converted to lactic acid 11/19/2011 Jipmer Physiologist Figure 6.10b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Slide 40
- 41. Energy for Muscle Contraction Aerobic Respiration Series of metabolic pathways that occur in the mitochondria Glucose is broken down to carbon dioxide and water, releasing energy This is a slower reaction that requires continuous oxygen 11/19/2011 Jipmer Physiologist Figure 6.10c Slide 6.25 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 41
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