The contraction of the Heart

1
The contraction of the Heart

• Reverend Dr. David C.M. Taylor
• School of Medicine
• dcmt_at_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)
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In more detail
Troponin-tropomyosin complex
actin
myosin binding site
myosin
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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

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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
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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
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Preload

• increased end-diastolic volume stretches the
  heart
• cardiac muscles stretch and contract more
  forcefully
• Frank-Starling Law of the heart

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Starlings 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)