Mandela 2.5

Test Questions

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Phases of SA Node Depolarization

  • The SA node depolarization consists of three distinct phases:

    • Phase 4 (Slow Depolarization Phase)

    • Phase 0

    • Phase 3

Phase 4: Slow Depolarization Phase

  • Membrane Potential at Phase 4 Start:

    • Begins at -60 mV

  • Permeability Changes:

    • PNA (Permeability of Sodium) is increasing.

    • PK (Permeability of Potassium) is decreasing.

  • Current Generation:

    • The current generated due to sodium influx is referred to as Funny Current (IF).

    • The channel responsible for the influx of sodium ions is the HCN channel.

  • Reason for Naming (Funny Current):

    • Hyperpolarization triggers the opening of the HCN channel, contrary to the norm, which is unexpected (hence the term funny).

    • Requires:

    • Hyperpolarization

    • Cyclic AMP (cAMP)

  • Cyclic Nucleotide Binding Domain (CNBD):

    • Domain where cAMP binds to enable channel opening.

  • Ion Influx During Phase 4:

    • Sodium ions (Na⁺) enter the cell leading to the generation of funny current (IF).

Phase 0: Rapid Depolarization

  • Threshold Potential:

    • Reached at -40 mV, necessary to initiate an action potential in the SA node.

  • Permeability Changes:

    • At threshold:

    • Permeability for L-type Calcium channels increases.

    • Permeability for T-type Calcium channels decreases.

  • Calcium's Role:

    • The driving ion for Phase 0 is calcium ions (Ca²⁺).

    • This contrasts with Phase 0 in ventricular myocytes, which is driven by sodium ions.

  • Action Potential Shape Characteristics:

    • The upward trend of the action potential is smooth; there is no flat line in Phase 4.

Phase 3: Repolarization

  • Key Events in Phase 3:

    • Potassium channels open (K⁺ influx).

    • L-type calcium channels close.

  • Reiteration of Cycle:

    • After Phase 3, the cycle returns to Phase 4.

Summary of Driving Forces and Ion Channels

  • Phase 4 Start:

    • Driving force: Sodium (Na⁺)

    • Channels: HCN channels

  • Phase 0 Initiation:

    • Driving force: Calcium (Ca²⁺)

  • Phase 3 Dynamics:

    • Driving force: Potassium (K⁺)

Differences between SA Node and Ventricular Myocytes

  • True Resting Potential:

    • Ventricular myocytes have a true resting potential of -90 mV, while SA node does not.

  • Ion Channel Activity in Phase 4:

    • Only leaky potassium channels are open during Phase 4 in ventricular myocytes; sodium and calcium channels are closed.

  • Phase 0 Dynamics in Ventricular Myocytes:

    • Rapid depolarization due to sodium influx, in contrast to calcium primarily driving phase in SA node.

Phase 2: Plateau Phase (Ventricular Myocytes)

  • Importance of Phase 2:

    • This phase is crucial for cardiac muscle contraction.

  • Channel Activity:

    • Opening of L-type Calcium channels (Ca²⁺ influx).

    • Potassium ion (K⁺) efflux.

  • Electrical Neutrality in Phase 2:

    • Influx of calcium and efflux of potassium create a flat plateau as the charges neutralize each other.

Phase 3 (Ventricular Myocytes) Recollection

  • Ion Channels Characteristics:

    • All channels closed except delayed rectifier potassium channels (K ext{+}).

    • Involvement of Sodium-Potassium ATPase pump plays a significant role in restoring membrane potential.

On Drugs and Arrhythmia Understanding

  • Knowledge of the differences between SA node and ventricular myocytes assists in understanding the effects of drugs and depolarization processes in arrhythmias.