Download Cardiovascular response to extreme circumstances

The contraction of the Heart
Reverend Dr. David C.M. Taylor
School of Medicine
[email protected]
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
 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
Structure of muscle
Chapter 13 p 147 in Preston and Wilson (2013)
Chapter 9 p 437 in Naish and Court (2014)
Histology
The sarcomere
Actin
filaments
Z line
Myosin
filaments
Chapter 12 p 136 in Preston and Wilson (2013)
Chapter 9 p 437 in Naish and Court (2014)
In more detail
Troponin-tropomyosin complex
actin
myosin
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 order of events
 The muscle depolarises
 Excitation spreads over the sarcolemma and into the Ttubules (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
Chapter 13 p 147 in Preston and Wilson (2013)
• The muscle shortens
Chapter 9 p 437 in Naish and Court (2014)
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)
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
The action potential (revision)
Fully permeable
to Na+(+40mV)
+40mV
Resting
membrane
potential(-70mV)
-55mV
-70 mV
1mS
Fully
permeable to
K+ (-90mV)
The action potential (revision)
VANC
close
+40mV
Fully permeable
to Na+(+40mV)
VANC
open
gNa+
gK+
stimulus
-55mV
-70 mV
1mS
Resting
membrane
potential(-70mV)
Fully
permeable to
K+ (-90mV)
Pacemaker activity
 The rhythm of the pump is
provided by the pacemaker
0
activity of some specialized
muscle cells in the wall of the
mV
right atrium - the sinoatrial
node
 There is a steady inward
current of both Na+ and Ca2+ -70
 Which causes a gradual
0
depolarisation
mS
300
Factors affecting stroke volume
Preload
Contractility
Afterload
 increased enddiastolic volume
stretches the heart
 cardiac muscles
stretch and contract
more forcefully
 Frank-Starling Law of
the heart
Tension developed %
Preload
100
80
60
40
20
40 60 80 100 120 140 160
Percentage sarcomere length
(100% = 2.2 µm)
Tension developed %
Starling’s Law
2.2 m
1.8 m
3.8 m
100
80
60
40
20
40
60
80
100
120
140
160
Percentage sarcomere length (100% = 2.2 m)