1. The contraction of the Heart
Reverend Dr. David C.M. Taylor
School of Medicine

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

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 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
Chapter 13 p 147 in Preston and Wilson (2013)
Chapter 9 p 437 in Naish and Court (2014)

9. Then
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…
Chapter 13 p 150 in Preston and Wilson (2013)
Chapter 9 p 440 in Naish and Court (2014)

10. Na+, K+ and Ca2+ The principles are exactly the same as for neurones
But the action potentials last much longer
And Ca2+ ions are 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+
stimulus
Resting membrane potential(-70mV)
-55mV
-70 mV
Fully permeable to K+ (-90mV)
1mS

13. 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 mV
-70 mS
300

14. Factors affecting stroke volume
Preload
Afterload
Contractility

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

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