Cardiac muscle Flashcards
What are the intercalated discs of cardiac muscle?
Mechanical connections - adherens junctions, desmosomes
Electrical connections - gap junctions
What cells use fast response action potentials?
Atrial and Ventricular cells
What is the cause of the rapid depolarization in a FR AP?
Fast sodium channels
Open for brief period, close to inactive state,
Return to closed resting state when membrane repolarizes
What causes the initial partial repolarization of the FR AP?
Efflux of K through transient outward channels
Inactivation of Na channels
What causes the plateau phase?
Combination of decreased K efflux because of channel closure and increased calcium influx
L-type Ca channels gradually open during depol. and stay open for a relatively long time
What causes the repolarization for the FR AP?
K efflux occurs through delayed rectifier channels, and starts to exceed Ca influx
Eventually calcium channels close and it becomes pure K efflux
No undershoot
What are the three differences between slow response and fast response cells?
Resting potential is less negative
Causes Na channels to be constantly inactivated
Spontaneous gradual depolarization
What causes the spontaneous gradual depolarization of slow response cells?
Pacemaker current, If or funny current
Influx of Na through slow channel
Why is the depolarization of pacemaker cells slow?
No fast sodium channels
Depends on L and T type calcium channels
What is the effective refractory period?
Phases 0-2 and part of 3
Action potentials cannot be generated, due to inactivated Na channels
What is the relative refractory period?
Second part of phase 3 as membrane repolarizes and channels transition back to closed state
AP can be generated with large stimulus but its weak and slow
Describe the excitation-contraction coupling in cardiac cells
Calcium enters during phase 2 of the AP
This calcium activates RyR2 receptors that release calcium from the SR, which is necessary to induce contraction
What is the contraction mechanism for cardiac muscle?
Ca binds to Tropinin C, changing the troponin/tropmyosin conformation, allowing the sliding filament model of contraction to occur
What is phospholamban?
Protein found in SR membrane, inhibits SERCA when unphosphorylated
What occurs when phospholamban is phosphorylated?
Phosphorylated by cAMP-dependent kinase, inhibition removed
SERCA more active, less Ca available in cytoplasm to initiate contraction, promotes immediate relaxation
More Ca released after next AP
How is contraction regulated (inotropy)?
Amount of intracellular Ca determines the force of contraction
Cannot increase force by adding more cells
What is the result of catecholamines binding to B1-adrenergic receptors?
Activation of PKA, which phosphorylates L-type calcium channels and causes more Ca to enter the cell
Causes stronger contraction
PKA also phosphorylates phospholamban, which increases calcium uptake by SERCA and promotes relaxation
NET result is to increase heart rate
What is the result of cholinergic signaling?
Inhibits kinases, which slows contractions by reducing intracellular calcium levels
Reduces heart rate
What is the effect of cardiac glycosides?
Inhibits ATPase, which causes a rise in intracellular sodium
Leads to greater contractions
What is the Frank-Starling law of the heart?
Stretching the sarcomere increases the force of contraction
Increasing preload, increases stretch, allows greater contraction
