Download Cardiovascular System 2 Cardiac muscle

Term Test Tuesday Oct 17
Cardiovascular System 2
Surname A-Q → South 2074
R-Z → CCIT 2150
Test time 90 minutes
~12:15-1:45
Cardiac muscle
• Sources of calcium
1. Extracellular
2. Intracellular – sarcoplasmic reticulum
Gap junctions
1
Ca++
Ca++
Ca++
Ca++
SR
Ca++
pump
SR
Ryanodine Receptor
T-tubule My
Dihydropyridine
receptor
Myoplasm
(intracellular)
Ca++
Ca++
Ca++
Ca++ interacts with troponin
& causes contraction
Ca++
SR
Opened by depolarization
myoplasm
Ca++
Electrical activity of the heart
Depolarization of muscle
cell plasma membrane
Voltage sensitive Ca++ channels
open (dihydropyridine receptors)
↑ Cytosolic Ca++
T-tubule
(extracellular)
Ca++ activates receptors on
sarcoplasmic reticulum
Opens Ca++ channels of
sarcoplasmice reticulum
(ryanodine receptors)
↑ Cytosolic 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
2. Pacemaker
Muscle contraction
2
Electrical activity of the heart
Action potential of ventricular myocyte
Early repolarization
Plateau
depolarization
The heart’s
pacemaker
and
conducting
system are
shown in
bright yellow.
repolarization
rest
Ionic basis of ventricular myocyte action
potential
•
•
•
•
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 rapid opening of voltagegated sodium channels is
responsible for the rapid
depolarization phase.
3
The prolonged “plateau” of
depolarization is due to the slow
but prolonged opening of
voltage-gated calcium channels
PLUS
reduced potassium channel
permeability
Opening of potassium
channels results in the
repolarization phase.
Channel events during ventricular myocyte
This is the L-type
action potential
Ca++
Na+
Na+
K+
Ca++
Ventricular
voltage-gated Ca++
channel
Atrial
K+
Please note this sequence is a little simplified, as there are at least 3
different K+ channels that contribute
4
Electrical activity of the heart
Pacemaker Cells
depolarization
The heart’s
pacemaker
and
conducting
system are
shown in
bright yellow.
repolarization
Pacemaker potential
Ionic basis of automaticity
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.
1. Pacemaker potential:
a) Na+ channel that opens with negative
potential (called “funny” Na channel)
b) Brief Ca++ channel opening (T-type)
2. Depolarization by voltage-gated Ca++
channel opening (L-type)
3. Repolarization by voltage-gated K+
channels
5
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.
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.
Faster depolarization = fast HR
Slower depolarization = slower HR
The rate of the pacemaker potential depolarization
sets the heart rate
Repolarizing the membrane leads
to the next opening of the Na+
channels
6