Download Cardiovascular system Cardiac muscle

Cardiac muscle
Cardiovascular system
Gap junctions
SR
• Sources of calcium
1. Extracellular
2. Intracellular – sarcoplasmic reticulum
Ryanodine Receptor
T-tubule My
Dihydropyridine
receptor
SR
myoplasm
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Ca++
Ca++
Ca++
Ca++
SR
Ca++
pump
Depolarization of plasma
membrane
Voltage sensitive Ca++ channels
open (dihydropyridine receptors)
Myoplasm
(intracellular)
Ca++
Ca++
Ca++
Ca++ interacts with troponin
& causes contraction
↑ Cytosolic Ca++
Ca++
Opens Ca++ channels of
sarcoplasmice reticulum
(ryanodine receptors)
Opened by depolarization
Ca++
Things to understand:
1. Electrical activity of each muscle cell
2. Coordination of activity across the heart
Cell Types
1. Contractile
a) Ventricular
b) Atrial
↑ Cytosolic Ca++
T-tubule
(extracellular)
Electrical activity of the heart
Ca++ activates receptors on
sarcoplasmic reticulum
Muscle contraction
Electrical activity of the heart
The heart’s
pacemaker
and
conducting
system are
shown in
bright yellow.
2. Pacemaker
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Ionic basis of ventricular myocyte action
potential
Action potential of ventricular myocyte
Early repolarization
Plateau
depolarization
repolarization
•
•
•
rest
•
The rapid opening of voltagegated sodium channels is
responsible for the rapid
depolarization phase.
Rest - membrane potential due to K+
efflux
Depolarization – Na+ influx via voltagegated Na+ channels
Plateau – balance of Ca++ influx and K+
efflux through voltage-gated ion channels
Repolarization – more K+ efflux through
voltage-gated ion channel
The prolonged “plateau” of
depolarization is due to the slow
but prolonged opening of
voltage-gated calcium channels
PLUS
reduced potassium channel
permeability
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Channel events during ventricular myocyte
This is the L-type
action potential
Ca++
Na+
voltage-gated Ca++
channel
Opening of potassium
channels results in the
repolarization phase.
Na+
K+
Ca++
K+
Please note this sequence is a little simplified, as there are at least 3
different K+ channels that contribute
Ventricular
Electrical activity of the heart
Atrial
The heart’s
pacemaker
and
conducting
system are
shown in
bright yellow.
4
Pacemaker Cells
depolarization
repolarization
Pacemaker potential
Ionic basis of automaticity
1. Pacemaker potential:
a) Na+ channel that opens with negative
potential
b) Brief Ca++ channel opening (T-type)
Depolarization of the pacemaker cells is:
• Automatic
• Rhythmic
• Sinoatrial node is the natural pacemaker
of the heart
• Pacemaker cells do not have a steady
resting potential, rather it gradually
depolarizes.
The action potential of an
autorhythmic cardiac cell.
Na+ in through hyperpolarization
activated channels
PLUS
calcium ions moving in through
the T channels cause a
threshold graded depolarization.
2. Depolarziation by voltage-gated Ca++
channel opening (L-type)
3. Repolarization by voltage-gated K+
channels
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Reopening of potassium channels
PLUS
closing of calcium channels
are responsible for the
repolarization phase.
The rapid opening of voltage-gated
calcium channels is responsible
for the rapid depolarization phase.
Repolarizing the membrane leads
to the next opening of the Na+
channels
Coordination of contraction in the
whole heart
Faster depolarization = fast HR
Slower depolarization = slower HR
The rate of the pacemaker potential depolarization
sets the heart rate
1. SA node depolarization initiates
atrial contraction
2. atrial depolarization spreads
and activates AV node
3. Purkinje fibers carry excitation
to the bottom of the heart
4. Ventricular contraction sweeps
up from the bottom
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Ventricle depolarization
Atrial depolarization
• Important points about coordination
1. Spread through the atrial muscle is by
way of gap junctions
2. The only electrical connection between
atria and ventricles is the AV node and
conducting fibers
3. Conduction through the AV node is
slow→ delay between atrial and
ventricular excitation
Ventricle repolarizatio
The relationship between
the electrocardiogram
(ECG), recorded as the
difference between currents
at the left and right wrists,
and
an action potential typical of
ventricular myocardial cells.
Normal
ECG: P waves (atrial depolarization) are followed faithfully by
QRS (ventricular depolarization) and T waves (ventricular repolarization).
Abnormal
ECG: every other P wave fails to evoke QRST (partial atrioventricular block).
Abnormal
ECG: P waves and QRST occur independently (full atrioventricular block).
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