12/1/2014 Medical Physiology Cardiac Electrophysiology I Eric Olson Flashcards
Where are the SA and AV nodes located?
The right atrium
Conducting cells are found in what areas of the heart?
SA node atrial internodal tracts AV node bundle of His Purkinje "system"
T/F: Conducting cells can generate APs spontaneously.
True
Conducting cells are specialized muscle cells that do not significantly contribute to the generation of force in the heart. How then does the heart contract?
Contractile cells.
The AP that began at the SA node spreads to the _____ via the atrial internodal tracts.
right and left atria and the AV node
What is the important effect of the AV node having a slow conduction velocity?
This ensures that the ventricles have sufficient time to fill with blood before they are activated and contracted.
Where is the bundle of His?
Near the AV node, almost in the middle of the atria and ventricles.
Once a AP reaches the bundle of His from the AV node, where does it propagate?
common bundle of His –> right and left bundle branches –> smaller bundles in Purkinje system that surround the ventricles –> ventricles.
Conduction velocity is not the same in all myocardial tissue. It is slowest in the __1__ and fastest in the __2__.
- AV node
2. Purkinje fibers
T/F: Skeletal and cardiac muscle have similar AP arcs.
False. The skeletal muscle AP happens over 0-20 msec, and it monophasic (up, down, nothing fancy). The cardiac AP can be 300-400 msec and is triphasic (up, down an bit, up, arc down). See page 245 in notes.
In Phase 0 of the ventricular AP, the upstroke, there is a transient increase in what ion?
Na+
What is dV/dt?
Rate of rise of membrane potential in the upstroke of the ventricular AP.
Phase 1 in the ventricular AP is known as:
initial repolarization
The plateau in the ventricular AP curve is what phase?
2
Repolarization in ventricular AP curve is what phase?
3
Phase 4 in the ventricular AP curve is known as:
resting membrane potential or electrical diastole
What ions are going where in Phase 1 of the ventricular AP curve?
Na+ gates close, no more Na+ going in;
Outward K+ current cause by large driving force of K+ ions.
What ions are going where in Phase 2 of the ventricular AP curve?
Inward Ca2+ current;
Outward K+ current.
What ions are going where in Phase 3 of the ventricular AP curve?
Decrease in inward Ca2+ current;
Increase in outward K+ current, followed by a decrease in outward K+ as membrane potential approaches K+ equilibrium potential.
What is the resting membrane potential of cardiac muscle?
-85 mV
What ions are going where in Phase 4 of the ventricular AP curve?
Na and Ca2+ in, K+ out, all in net equilibrium as cell is repolarized.
The effective refractory period in myocardial muscle is?
Right after the absolute refractory period. Still very unlikely to generate an AP.
T/F: All parts of the heart, ie SA node, common bundle, Purkinje fibers etc. have a characteristic AP curve.
True. They are all a bit different. See page 247 in notes.
The duration of the AP in cardiac muscle determines the duration of what?
The refractory period. 1:1 ratio
The AV node and what other tissue can become pacemakers in a pinch, and develop unstable resting membrane potentials to help the heart pump.
Purkinje fibers
What is the only part of the heart whose upstroke is not characterized by an inward Na+ current?
SA node, has inward Ca2+ current instead.
What tissue has the shortest myocardial AP, and what has the longest?
SA node and atrium both have 150 ms AP duration;
Purkinje fibers have 300 ms AP duration. (Ventricles have 250 ms duration).
What is considered a normal heartbeat, corresponding with the normal range of SA node pacemaker firing?
60-100 bpm
What is a typical AP of the SA node like?
Phase 4 at about -65mV (repolarized) rising, Phase 0 upstroke to 0 mV, Phase 3 falling steeply but no arc, back to Phase 4, from
V-dep and L-type channels are most known for their involvement in what?
SA node calcium channels, open to start upstroke –> massive Ca2+ influx.
What causes pacemaker activity in SA node?
Interaction between I-Ca (T-type channels), I-K and a special current, deemed I-f (funny current).
I-f is also known as:
the pacemaker current.
Why is I-f “funny?”
It activates in response to hyperpolarization, not depolarization. It has both an inward Na+ component and an outward K+ component.
The effects of the autonomic (S and PS) nervous system on the heart are called:
chronotropic effects - S stimulation –> positive chronotropic effect; PS stimulation –> negative chronotropic effect.
The SA node has neuronal input
What part of the SA node AP changes with sympathetic stimulation?
Phase 4 rate is increased (meaning Phase 4 is shorter) leading to frequency of APs
What part of the SA node AP changes with parasymmpathetic stimulation? As with the opposite of stimulation of the sympathetic nervous system, Phase 4 rate is decreased, but somthing else also happens to create the negative chronotropic effect. What is it?
PS activation also hyperpolarized the maximum diastolic potential to decrease the frequency of APs.
Describe the vagal brake signaling pathway to reduce the heart rate.
Vagal activity (Cn X) –> ACh released at SA node –> ACh binding to and activation of M2 receptors –> Activation of Gi –> decrease in cAMP –> Phase 4 rate is slower
In parasympathetic inhibition of the SA node, Phase 4 steepness is reduced by a number of factors. What are they?
- Reducing inward I-f
- Reducing inward I-CaT
- Increasing outward I-KACh
Also, threshold of I-CaL is at a more positive value.
Describe the sympathetic stimulation pathway that leads to increased heart rate.
Norepi binds to B-1 adrenegic receptors (metabotropic) –> Gs activated –> AC activated –> cAMP increases –> pacemaker rate increases
In sympathetic stimulation of the SA node, does Phase 4 get steeper or less steep?
Steeper, leading to faster rates of depolarization.
Steeper due to increasing inward I-f and increasing inward I-CaT; threshold of I-CaL at more negative value.
Cardiac APs are propagated through gap junctions. In this way, the atria and ventricles are both _____ of cells.
Syncytia
What proteins make up gap junctions
Connexins
Like SA node cells, AV node cells have:
an instrinsic pacemaker activity
T/F: Sympathetic and parasympathetic influence can act on the AV node as well as the SA node.
True
The intrinsic firing rate of the AV node is:
40 bpm
The intrinsic firing rate of the Purkinje fibers is:
< 20 bpm and irregular
What is the source of the pacemaker current in the Purkinje fibers?
The funny current
What is the order of ventricular muscle activation?
Ventricular apex (endo to epicardium) –>via Purkinje fibers –> ventricular base (top) (endo to epicardium) = coordinated contraction
What are the refractory periods of the ventricular AP?
- Absolute (biggest)
- Effective (Absolute + a bit longer)
- Relative (Overlaps slightly with Effective)
- Supranormal period
See page 265 in notes for graph.
When does the supranormal period occur?
In ventricular AP, it begins last, at about -70, going to -85 mV. The cell is more excitable than normal during this period.
What is the intrinsic firing rate of the Bundle of His?
About 40 impulses/min
What happens if the intrinsic firing rate of one of the pacemakers should become faster than that of the SA node?
The latent pacemaker takes over
Parasympathetic effects on the heart always involve what kind of receptor?
Muscarinic
Alpha-1 and Beta-2 both act on the vasculature during SNS stimulation. What are their effects?
Alpha-1 –> Constriction (vascular smooth muscle ie splanchnic)
Beta-2 –> Dilation (skeletal)
Name two beta blockers and say what they do:
Slow heart rate by blocking Beta-1 receptors in heart, “cardioselective”
Propranolol
Atenolol
Name an agent that affects muscarinig ACh receptors and when it would be administered.
Atropine, during CPR
Blocks M2 receptors leading to tachycardia
A nerve agent which inhibits acetylcholinesterase has what effect on the heart?
Bradycardia, ie sarin gas (also causes convulsions and death, think Japanese terrorist attack)
Some antidepressants affect heart rate, how?
They block the reuptake of norepi, ie Prozac and elavil, leading to increased heart rate
