Cardiac Excitation/Contraction Coupling Flashcards
What is the resting membrane potential in phase 4?
-85 mV
What sets the resting membrane potential in phase 4?
3 potassium channels (K1, K-ATP, K-Ach)
What are the statuses of the sodium gates in phase 4?
inactivation gates are open, voltage gates are closed
What occurs in phase 0?
depolarization
At what membrane potential do sodium voltage gates open?
-40 mV
What causes the inactivation gates to close?
positive membrane potential
What channels are closed during depolarizaiton?
potassium
What occurs in phase 1?
slight repolarization due to the transient outward potassium channel
What occurs in phase 2?
plateau
What channels function during the plateau?
calcium channels open and there is an inward current to balance the outward K+ currents of (I-Kur, I-Kr, I-Ks)
What happens in phase 3?
repolarization
What gates are reactivated in phase 3?
potassium channels (K-ATP, K-Ach)
At what voltage do sodium channels reset?
around -85 mV
What is the lowest membrane potential that SA node cells reach?
around -65 mV
Do SA node cells have fast Na channel activity?
NO, because their membrane potential never gets low enough to “reset” the sodium gates
What makes the resting membrane potential of SA node cells unstable?
“funny” Na current
What activates the Na “funny current”?
negative membrane potential
Why is the slope of the AP in SA node cells less steep?
because L-type calcium channels are slower in conducting current than fast Na channels
What parts of the heart have the highest conduction rate?
Bundle of His and Purkinje cells
What parts of the heart have the slowest conduction rates?
SA node and AV node
What part of the heart has the highest spontaneous discharge rate?
SA node
What transmits sympathetic efferent nerves to heart?
sympathetic cardiac nerve
What do sympathetic efferent nerves influence?
SA node, AV node and VENTRICULAR CELLS
What are the affects of the sympathetic efferent nerves on the heart?
Increase HR
Increase conduction velocity in the AV node
Decreases threshold for calcium channels
Increases excitability of latent pacemakers
What transmits parasympathetic efferent nerves?
vagus nerve
What do parasympathetic efferent nerves influence?
SA node and AV node only
What are the affects of the parasympathetic efferent nerves on the heart?
Decreases HR, decreases conduction velocity, decreases excitability of latent pacemakers
Name 3 ways you can decrease HR by altering parameters in the SA node?
1) decrease funny current (so you don’t get depolarization started)
2) increase potassium channel activity (repolarization lasts too long)
3) raise threshold (less negative) for calcium channels to open by decreasing cAMP levels
What is the resting HR? What is this without sympathetic and parasympathetic tone?
60-70
without tone it is around 100
What is the critical activator of cardiac muscle contraction?
calcium
When are calcium levels high enough for contraction?
In phase 2 (plateau) AND Ca must be released from the SR
What regulates the strength of muscle contraction?
1) intracellular calcium levels
2) length of cardiac fibers (this determines their sensitivity to calcium)
How do catecholamines affect calcium signaling in myocytes?
they are involved in the GPCR pathway that culminates in the phosphroylation of:
1) Troponin I
2) PLB
3) Ica Channel
4) Ryanadine receptor
What is the result of phosphorylating troponin I?
decreases its affinity for calcium and releases it leading to cardiac relaxation (lusitrophy)
What is the result of phosphorylating PLB?
decreases intracellular calcium quickly and packs it in the SR
What is the result of phosphorylating the calcium channel?
increases calcium intake
What is the result of phosphorylating the ryanadine receptor?
greater calcium release from the SR so greater cardiac contraction (inotrophy)
What is another term for increasing HR?
chronotrophy
Does acetylcholine have an affect on ventricular contraction?
NO- parasympathetic activity is only on the SA and AV node
What is the equation for SV according to the Frank-Starling effect?
SV= EDV-ESV
What does the Frank Starling effect postulate?
increased sarcomere length results in increased active tension
What are the 2 ways that increased length corresponds to increased cardiac force?
1) Fast response- increased Ca sensitivity to myofilaments
2) Slow response- activation of stretch-activated calcium channels
