Asynchronous Lecture for CPR 20 - Cardiac Action Potentials Flashcards
What is the resting membrane potential of a cardiac myocyte? What are the equilibrium potentials for sodium and potassium?
Vm = -85 mV
Ek = -90 mV
Ena = +60 mV
In which directions does the cardiac myocyte Vm drive Na+, K+, and Ca++? What are the ion channels relevant to cardiac action potentials? What is each channel’s purpose in a cardiac action potential?
Vm drives Na+ & Ca+ into the cell and K+ out of the cell
V-gated Na+ channels - initial depolarization
L-type Ca++ channels - maintain depolarized state
Kir channels - K+ inward rectifiers - maintain Vm in between APs
Kto channels - K+ transient outward - prevent excessive depolarization and begin repolarization
Kdr channels - K+ delayed rectifiers - repolarize membrane
Draw out the cariac action potential and label the different phases. Also draw out the influx/outflux of Na+, K+, and Ca++ occurring through the different channels at different times.
What are the three important aspects of cardiac action potentials
- A long absolute refractory period prevents spatial and temporal summation from creating tetany in heart muscle
- The long absolute refractory period allows for electrical defibrillation to easily reset the ventricles to the same AP frequency
- Cardiac action potentials are easily manipulated with drugs
List the steps to a SA node pacemaker action potential.
- The Na+ funny channel is open at a hyperpolarized state which starts to slightly depolarize the membrane (but not to threshold). This causes the transient Ca++ channels (t-type) to open which will depolarize the membrane past threshold. This whole step is considered phase 4.
- L-type Ca++ channels now open to rapidly depolarize the membrane and then close. This is phase 0
- At the same time the L-type Ca++ channels close the K+dr channels open to repolarize the membrane. This is phase 3
NOTE - there are only phases 0, 3, and 4
Draw out the SA node action potential.
Describe the mechanism through which the sympathetic nervous system increases heart rate.
- Sympathetic nerves releases NE at the SA nodal cells where it binds to ß1 receptors.
- This leads to the activation of PKA and the phosphorylation of the funny Na+ and T-type Ca++ channels. This increases their probability of opening.
- This will shorten the time it takes for phase 4 of SA node APs to reach threshold.
- This leads to an increased frquency of APs and therefore an increased heart rate
Describe the mechanism through which the parasympathetic nervous system decreases heart rate.
- Parasympathetic nerves release ACh onto the SA nodal cells where is binds to muscarinic M2 receptors which leads to the activation of both Gßγ and Giα proteins.
- Gßγ will open KACh channels which increases K+ coductance and hyperpolarizes SA nodal cells
- Giα activation will lead to less PKA activation and, therefore, less phosphorylation of the funny Na+ and T-type Ca++ channels.
- Both of these things will lead to a slower depolarization rate of the SA nodal cells which leads to a decreased AP frequency which leads to a decreased heart rate
What is the bowditch effect?
When the SNS causes an increased HR the SERCA pump cannot clear Ca++ from the cytoplasm fast enough leading to a slightly more increased intracellular Ca++ level. This will lead to more forceful contractions. As HR increases so does the Ca++ increase. Therefore, the contractions become more and more forceful as HR increases. This is also known as positive inotropy
What structure in the heart has the slowest speed of electrical conductance? Why is this important?
The AV and SA nodes. Since the SA node is what gives off the initial AP, it doesn’t matter that is has slow conductance. It is important that the AV node has slow conductance because this gives the atria time to contract before the AP reaches the ventricles and causes them to contract
Why is atrial contraction needed? Wouldn’t gravity just pull the blood down into the ventricle when the AV valves opened?
Gravity would not pull all of the blood into the ventricle before the AV valves closed. Under normal conditions, atrial contraction is only needed to eject about 10% of the blood into the ventricles. During increased heart rate, however, atrial contraction could be required to eject as much as 40% of the blood.
List the different bpms that would occur if the pacemaker of the heart was only the SA node, AV node, Bundle of His, or Purkinje fibres. Which structure is the actual pacemaker of the heart under normal conditions?
