The contraction of the Heart Reverend Dr. David C.M. Taylor School of Medicine [email protected]...
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Transcript of The contraction of the Heart Reverend Dr. David C.M. Taylor School of Medicine [email protected]...
The contraction of the Heart
Reverend Dr. David C.M. TaylorSchool of Medicine
http://www.liv.ac.uk/~dcmt
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
Learning outcomes
Structure of muscle
Chapter 13 p 147 in Preston and Wilson (2013)Chapter 9 p 437 in Naish and Court (2014)
Histology
The sarcomere
Chapter 12 p 136 in Preston and Wilson (2013)Chapter 9 p 437 in Naish and Court (2014)
Z line
Actinfilaments
Myosinfilaments
In more detail
myosin
actin
Troponin-tropomyosin complex
myosin binding site
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
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
The order of events
Chapter 13 p 147 in Preston and Wilson (2013)Chapter 9 p 437 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…
Then
Chapter 13 p 150 in Preston and Wilson (2013)Chapter 9 p 440 in Naish and Court (2014)
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
Na+, K+ and Ca2+
The action potential (revision)
-70 mV-55mV
+40mV Fully permeable to Na+(+40mV)
Fully permeable to K+ (-90mV)
1mS
Resting membrane potential(-70mV)
The action potential (revision)
-70 mV-55mV
+40mV
VANC open
VANC close Fully
permeable to Na+(+40mV)
Fully permeable to K+ (-90mV)
1mS
stimulus
Resting membrane potential(-70mV)
gNa+
gK+
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
0
mV
-70
0 mS 300
Factors affecting stroke volume
Preload Afterload
Contractility
Preload
increased end-diastolic volume stretches the heart
cardiac muscles stretch and contract more forcefully Frank-Starling Law of
the heart 40 60 80 100 120 140 160
Percentage sarcomere length (100% = 2.2 µm)
100
80
60
40
20T
ensi
on d
evel
oped
%
Starling’s Law
40 60 80 100 120 140 160Percentage sarcomere length (100% = 2.2 m)
100
80
60
40
20
Ten
sion
dev
elop
ed %
1.8 m2.2 m
3.8 m