Lecture 11: Electrophysiology of the Heart Flashcards
(18 cards)
What is the concept of syncytium?
Cells are electrically coupled to
each other (intercalated discs).
Gap junctions allow an action
potential in one cell to causes an
action potential in downstream
cells.
Importance of myocyte bifurcation.
What are intercalated discs?
Specialized cell to cell
junctions that form
mechanical and electrical
connections between cardiac
myocytes.
What is the clinical utility of cardiac troponin?
The test of choice for myocardial
infarction are concentrations of
cardiac troponin I (cTnI) and
troponin T (cTnT).
If there has been damage to the heart tissue, the cardiac troponin will be at higher levels in the blood
What are the two types of Cardiac Myocytes? Describe each one
Contractile and Electrical
Contractile:
▫ Vast majority (> 99%)
▫ Must be stimulated to initiate
contraction.
▫ Unlike skeletal muscle, these
myocytes contract due to
depolarizing currents that enter
through gap junctions.
▫ Atrial and Ventricular
Electrical: Autorhythmicity/Conduction
▫ Concentrated in certain areas of
the heart
▫ Generate spontaneous APs by
undergoing slow depolarization
until they reach threshold.
▫ SA node, atrial internodal tracts,
AV node, Bundle of His, Purkinje fibers
Sinoatrial (SA) node
Usually where AP initiates to
generate heartbeat
(pacemaker).
Atrioventricular (AV) node
Slower conduction velocity
allowing ventricles to fill
with blood.
Bundle of His
AP leaves AV node
Purkinje fibers
Distributes AP to ventricles
Sequence of Activation of the Heart
- The first cells to reach threshold are in the SA node.
- APs are conducted to the AV node through atrial myocytes.
- Conduction is slow through the AV node (allows time for ventr. filling).
- From the AV node APs are conducted through Purkinje fibers (Bundle of His) to the bottom of the ventricles.
- The bundle splits into right and left branches and activates myocytes in the right and left ventricles. Ventricles contract from apex (bottom) to base (top)
for efficient blood ejection.
Where is the fastest conduction velocity?
In the His-Purkinje
fibers
Coordinated contraction of
ventricles and efficient
ejection of blood.
dromotropy
Velocity at which conduction
spreads throughout the HisPurkinje system
How does the action potential of muscle cells compare to that of neurons?
Nerve cells- narrower peaks
Ventricular myocytes- more drawn out
Describe what is happening during phase 0 of the action potentials in Atria, Ventricles and
Purkinje Fibers
When activated Fast Na+ channels (voltage gated) cause depolarization
Mem. potential does not reach ENa+
(+ 65 mv) because Na+ inactivation
gates close due to depolarization
similar to neuronal AP
Describe what is happening during phase 1 of the action potentials in Atria, Ventricles and
Purkinje Fibers
Brief Rapid repolarization
* Na+ inactivation gates have closed –>↓gNa
* Voltage-activated K+ gates open:
Transient Outward (Ito) K channel
Describe what is happening during phase 2 of the action potentials in Atria, Ventricles and
Purkinje Fibers
Plateau
Inward Ca++ current through slow, L-type Ca++ channels
* An outward K+ current, Slow
Rectifier K+ channel.
slow channel = longer AP
2 different currents that balance each other. Since they are matched, we don’t see any change in membrane potential
Describe what is happening during phase 3 of the action potentials in Atria, Ventricles and
Purkinje Fibers
decreased ICaL (until it stops), and ↑IK –> outward positive currents > inward positive currents –>
repolarization.
* K+ slow and fast rectifier
K current increases
Describe what is happening during phase 4 of the action potentials in Atria, Ventricles and
Purkinje Fibers
K+, Na+ and Ca++ all contribute to
maintaining a stable RMP.
Recall, current = conductance X
driving force
K leak channels offset by Na and Ca
high conductance for K, larger driving force for Na & Ca. These balance each other out and membrane gets back to RMP
What are each of the ion channels and characteristics for contractile myocytes?
ALL ARE VOLTAGE GATED
Sodium
-Fast Na+ (INa): Phase 0 of myocytes
Calcium
-L-type (Ica): Slow inward, long-lasting current. Phase 2 of myocytes
Potassium
-Leak channels: Maintains negative potential in phase 4
-Transient Outward (Ito): Contributes to phase 1 in myocytes
-Slow and Fast Rectifier (Ikr): Phase 2 plateau and Phase 3 repolarization
