Cardiac Ion Channels & Action Potentials

Question 1

Fact cardiac action potentials cell types

Answer 1

occur in myocardial cells and cells of rapid conduction pathways

Question 2

Slow cardiac action potentials cell types

Answer 2

occur in pacemaker cells of SA and AV nodes

Question 3

Channels involved in fast cardac action potentials (5)

Answer 3

1. I_Na = voltage-gated sodium channel
2. I_Ca-L = L-type calcium channel; activate in response to to depolarization and undergo VDI (voltage-depend. inactiv.) and CDI (calc.-depend. inact.)
3. I_Kto = Potassium channel; activates/inactivates slower than Na channels upon depolarization
4. I_Kr + I_Ks = “delayed rectifiers”; activated by depolarization
5. I_k1 = “inward rectifier current”; holds cells near E_k between action potentials

Question 4

Channels involved in slow cardiac action potential

Answer 4

1. I_Ca-T + I_Ca-L = I_Ca-T is T-type channel (LVA=low voltage activated); I_Ca-L is L-type channel (HVA).
2. I_f = “funny” current; induced at hyperpolarization, permeable to Na⁺ and K⁺
3. I_Kr + I_Ks = “delayed rectifiers”; activated by depolarization

Question 5

Ionic mechanisms behind spontaneous firing of pacemaker cells

Answer 5

• balance between ICa and delayed rectifier current (IKr and IKs) –> repolarization occurs shortly after the peak of the action potential.
• repolarization is followed by a slow depolarization (the pacemaker potential) which brings the cell back to threshold for the generation of another action potential.
• Funny current (If): induced by hyperpolarization. Induction of If allows cation fluxes which drive voltage potential towards the reversal potential of If (-30 mV).

Question 6

Role of I_K1 in fast action potentials

Answer 6

• IK1=inward rectifier potassium channel.
• IK1 has a strong, “instantaneous” rectification: readily conduct inward K+ current at potentials below EK and only weakly pass outward K+ current at potentials slightly positive to EK
• responsible for HOLDING CELLS NEAR E_K BETWEEN ACTION POTENTIALS without producing an outward current.

Question 7

Role of I_f(I_h) in slow action potentials

Answer 7

• I_f generates the “funny” current—a current induced by hyperpolarization.
• Induction of If in the context of the “slow” cardiac action potential allows cation fluxes which drive voltage towards the reversal potential of If and may play a role in generation of the pacemaker potential—which is critical to allowing pacemaker cells generate rhythmic firing in the absence of neuronal input.

Question 8

Absolute refractory period definition

Answer 8

period of time following a “fast” cardiac action potential a second action potential cannot be initiated until most of the inactivation of INa is removed (during the repolarizing phase).

Question 9

Relative refractory period definition

Answer 9

period of time following a “fast” cardiac action potential during which the threshold for a second action potential remains elevated until after repolarization is complete (complete removal and inactivation of INa and deactivation of IKr and IKs has occurred).

Question 10

Mechanism of overdrive suppression

Answer 10

• while other myocardial cells in the heart (including AV node), are capable of spontaneous activity, they fire at a lower frequency than those of the SA node
• usually, these cells are driven by action potentials originating in the SA node:
• an action potential will spread to them from the SA node before they reach threshold on their own.

Question 11

Ectopic pacemakers definition

Answer 11

abnormal circumstance under which another cell besides those of the SA node take over initiation of contraction/heartbeat