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The contraction of the Heart

The contraction of the Heart. Reverend Dr. David C.M. Taylor School of Medicine dcmt@liverpool.ac.uk http://www.liv.ac.uk/~dcmt. Learning outcomes. By the end of this lecture you should be able to discuss The histology of cardiac muscle The role of myosin, actin, troponin and tropomyosin

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The contraction of the Heart

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  1. The contraction of the Heart Reverend Dr. David C.M. Taylor School of Medicine dcmt@liverpool.ac.uk http://www.liv.ac.uk/~dcmt

  2. Learning outcomes • By the end of this lecture you should be able to discuss • The histology of cardiac muscle • The role of myosin, actin, troponin and tropomyosin • The importance of calcium for contraction • Starlings law • Cellular and molecular events underlying cardiac contraction and relaxation • The role of Na+, K+ and Ca2+ in cardiac contractility

  3. Structure of muscle Chapter 13 p 147 in Preston and Wilson (2013) Chapter 9 p 437 in Naish and Court (2014)

  4. Histology

  5. The sarcomere Actin filaments Myosin filaments Z line Chapter 12 p 136 in Preston and Wilson (2013) Chapter 9 p 437 in Naish and Court (2014)

  6. In more detail Troponin-tropomyosin complex actin myosin binding site myosin

  7. In the presence of Calcium • Tropomyosin shifts to expose the myosin binding site • Myosin binds to binding site • ATP is used to provide the energy to flex the myosin head • The muscle shortens

  8. The order of events Chapter 13 p 147 in Preston and Wilson (2013) Chapter 9 p 437 in Naish and Court (2014) • The muscle depolarises • Excitation spreads over the sarcolemma and into the T-tubules (there are fewer T-tubules than in skeletal muscle) • L-type Ca2+ channels open (dihidropyridine receptors), increasing sarcoplasmic Ca2+ levels • Ca2+ induces Ca2+ release from the sarcoplasmic reticulum • Ca2+ binds to tropomyosin • Tropomyosin shifts to expose the myosin binding site • Myosin binds to binding site • ATP is used to provide the energy to flex the myosin head • The muscle shortens

  9. Then Chapter 13 p 150 in Preston and Wilson (2013) Chapter 9 p 440 in Naish and Court (2014) The heart does not remain contracted, but relaxes. This is caused by the activity of the SERCA The SERCA is a Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase So energy is used to draw Ca2+ back into the sarcoplasmic reticulum. And the myosin is released from the actin filaments…

  10. Na+, K+ and Ca2+ The principles are exactly the same as for neurones But the action potentials last much longer And Ca2+ ionsare more important Na+ and K+ regulate the rate of contraction Ca2+ regulates the force of contraction The more Ca2+,for whatever reason, the greater the force of contraction All three are regulated by the autonomic nervous system

  11. The action potential (revision) Fully permeable to Na+(+40mV) +40mV Resting membrane potential(-70mV) -55mV -70 mV Fully permeable to K+ (-90mV) 1mS

  12. The action potential (revision) VANC close Fully permeable to Na+(+40mV) +40mV VANC open gNa+ gK+ Resting membrane potential(-70mV) stimulus -55mV -70 mV Fully permeable to K+ (-90mV) 1mS

  13. 0 mV -70 0 mS 300 Pacemaker activity • The rhythm of the pump is provided by the pacemaker activity of some specialized muscle cells in the wall of the right atrium - the sinoatrial node • There is a steady inward current of both Na+ and Ca2+ • Which causes a gradual depolarisation

  14. Factors affecting stroke volume Preload Afterload Contractility

  15. 100 80 60 40 20 Tension developed % 40 60 80 100 120 140 160 Percentage sarcomere length (100% = 2.2 µm) Preload • increased end-diastolic volume stretches the heart • cardiac muscles stretch and contract more forcefully • Frank-Starling Law of the heart

  16. Starling’s Law 2.2 m 3.8 m 1.8 m 100 80 60 40 20 Tension developed % 40 60 80 100 120 140 160 Percentage sarcomere length (100% = 2.2 m)

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