Pacemaker / Noncontractile Cell Action Potentials - Detailed Notes

Pacemaker / Noncontractile Cell Action Potentials

  • Overview

    • Pacemaker action potential acts as the electrical clock that provides automaticity to cardiac pacemaker cells in the SA node.
    • Differs from ventricular action potential: depolarization occurs automatically, regularly, without external triggers.
    • This activity is divided into three phases (phases 4, 0, and 3 in the pacemaker AP model).
    • Key output: sets the heart rate; SA node is the fastest driver, with AV node and Purkinje fibers capable of automaticity if SA node fails.
    • Typical intrinsic rates (in the absence of autonomic tone):
    • SA node: $60$–$100\ \text{bpm}$
    • AV node: $40$–$60\ \text{bpm}$
    • Purkinje/bundle: $20$–$40\ \text{bpm}$

  • Phase 4: Pacemaker (automatic) potential

    • Purpose: slow depolarization that brings the membrane potential toward threshold, constituting automaticity.
    • Main currents: slow inward depolarizing sodium current (funny current) $If$ and a contribution from T-type Ca$^{2+}$ channels $I{CaT}$; both help initiate spontaneous depolarization.
    • At around $Vm \approx -50\ \text{mV}$, transient-type calcium channels (T-type, $I{CaT}$) open and contribute to further depolarization.
    • At $Vm \approx -40\ \text{mV}$, L-type calcium channels ($I{CaL}$) open and provide a larger inward Ca$^{2+}$ current to push toward threshold.
    • Threshold for the action potential: typically between $Vm \approx -40$ to $-30\ \text{mV}$, i.e. V</em>th≈−40 extto−30  mV.V</em>{th} \approx -40\ ext{ to } -30\ \,\text{mV}.
    • Opening/closing dynamics:
    • The funny current $If$ and the $I{CaT}$ are more active during hyperpolarization and early depolarization; they slow/close as the depolarization proceeds toward threshold.
    • The rise in $I_{CaL}$ occurs toward the end of phase 4 and contributes to the upstroke for phase 0.
    • A key point: all these channels are voltage-gated; channels require a change in membrane voltage to activate. Without depolarization, they would remain closed.
    • Important gating concept (illustrated in the transcript): opening is driven by a change in voltage (voltage-gated channels).
    • Note on channel gating terminology:
    • The voltage change during phase 4 toward threshold promotes activation of $If$, $I{CaT}$, and $I_{CaL}$.
    • The channels are not simply open and passively allow ions; they have to be activated by the voltage change.

  • Phase 0: Upstroke (depolarization)

    • Upstroke primarily caused by the increased Ca$^{2+}$ influx through L-type Ca$^{2+}$ channels ($I_{CaL}$) that begin to open toward the end of phase 4.
    • The rapid Ca$^{2+}$ entry via $I_{CaL}$ drives the rapid depolarization, constituting phase 0.
    • In this phase, the funny currents and the T-type Ca channels begin to slow as they close.
    • Calcium influx during phase 0 is crucial as it can contribute to calcium signaling in neighboring cells via gap junctions (see below).
    • The upward phase 0 in nodal tissue happens with a different ionic balance than in ventricular tissue, reflecting the unique pacemaker physiology.

  • Phase 3: Repolarization

    • Main driver: outward potassium currents (voltage-gated K+ channels) that hyperpolarize the cell (move membrane potential toward more negative values).
    • Concurrently, L-type Ca$^{2+}$ channels become inactivated and close, decreasing inward Ca$^{2+}$ currents.
    • Net effect: membrane potential moves toward a more negative value, toward the maximum negative potential around the resting/recovery level for pacemaker cells.
    • The membrane potential moves toward approximately $V_m \approx -60\ \text{mV}$ during this phase.
    • Important correction from the transcript: this is repolarization, not hyperpolarization. Hyperpolarization would take the membrane potential below $-60\ \text{mV}$ (e.g., toward $-70\ \text{mV}$ or lower).

  • End of Phase 3 and start of Phase 4

    • At the end of phase 3, slow inward Na+ funny currents (or funny-channel activity) begin again, signaling the start of the next pacemaker potential (phase 4).
    • This creates the cyclic automaticity characteristic of SA node and other nodal tissues.

  • The funny current $I_f$ and calcium currents in pacemaker cells

    • $I_f$ is a slow inward Na+ current that contributes to the gradual depolarization during phase 4.
    • It is activated by hyperpolarization (more negative membrane potential) and modulated by autonomic inputs in vivo.
    • $I_{CaT}$ provides a transient Ca$^{2+}$ influx contributing to depolarization around $-50$ to $-40$ mV.
    • $I{CaL}$ provides the main Ca$^{2+}$ influx for the phase 0 upstroke around threshold region ($Vm \approx -40$ to $-30$ mV).
    • The interplay of these currents determines the slope of phase 4 and the timing of phase 0.

  • Voltage-driven gating and channel activation

    • All of these channels (funny Na+ channels, $I{CaT}$, $I{CaL}$, K+ channels) are voltage-gated.
    • The opening of each channel requires a change in membrane voltage; otherwise, no ions cross the membrane via these channels.
    • As depolarization proceeds, some Ca$^{2+}$ channels inactivate (e.g., L-type), which reduces inward current and contributes to the transition to repolarization (phase 3).
    • Conversely, during repolarization, the decreased depolarization allows funny channels to open again (recovery toward phase 4).

  • Calcium handling and excitation-contraction coupling in noncontractile vs contractile cells

    • Pacemaker (noncontractile) cells rely on Ca$^{2+}$ entry for upstroke and for signaling to neighboring cells.
    • The Ca$^{2+}$ that enters nodal cells (through $I{CaL}$ and $I{CaT}$) can diffuse through gap junctions to adjacent contractile cells.
    • In contractile cells, the extracellular Ca$^{2+}$ and the Ca$^{2+}$ that diffuses into the cell can trigger ryanodine receptor (RyR)–mediated Ca$^{2+}$ release from the sarcoplasmic reticulum (SR) into the cytosol, a process known as calcium-induced calcium release (CICR).
    • This released Ca$^{2+}$ can bind to troponin in the contractile myocytes, enabling actin-mosin cross-bridge cycling and contraction.
    • Thus, the nodal cells help initiate the calcium signaling cascade that triggers contraction in neighboring contractile cells.

  • Gating and interpretation of the video content

    • The video emphasizes that the opening of channels is driven by voltage changes; a stable membrane potential would keep channels closed.
    • It also notes that channels may be ‘open but inactivated’ during certain phases (e.g., L-type Ca channels during phase 4 and phase 3) and then recover.
    • The transcript includes an instructional emphasis that the channels require both being open and activated; a channel that is merely open but not activated would not conduct ions.

  • Common corrections and clarifications from the transcript discussion

    • The statement that phase 3 causes “hyperpolarization” to $-60$ mV is corrected to say it is repolarization toward $-60$ mV, not hyperpolarization.
    • Hyperpolarization would imply a membrane potential more negative than the resting value (e.g., below $-60$ mV).
    • The correct sequence: phase 0 upstroke via $I{CaL}$, phase 3 repolarization via outward K+ currents, end of phase 3 sets the stage for phase 4 via $If$ and $I_{CaT}$ rebound.

  • Connections to foundational principles and real-world relevance

    • Automaticity of SA node sets the heart’s rhythm; autonomic inputs (not discussed in depth here) modulate rate by shifting phase 4 slope and threshold.
    • Understanding nodal APs is crucial for interpreting antiarrhythmic drugs that target specific channels (e.g., funny current modulators, calcium channel blockers) and for understanding pacemaker implants.
    • The intercellular coupling between nodal and contractile cells via gap junctions underpins the propagation of the impulse and synchronized contraction of the heart.

  • Equations, numbers, and constants (LaTeX)

    • Threshold and membrane potentials: Vth≈−40 to −30 mVV_{th} \approx -40 \text{ to } -30\ \text{mV}
    • Action potential cadence (typical intrinsic rates): f<em>SA≈60–100 bpm,  f</em>AV≈40–60 bpm,  fPurkinje≈20–40 bpmf<em>{SA} \approx 60\text{--}100\ \text{bpm},\; f</em>{AV} \approx 40\text{--}60\ \text{bpm},\; f_{Purkinje} \approx 20\text{--}40\ \text{bpm}
    • Key membrane potentials mentioned:
    • V<em>m≈−50 mVV<em>m \approx -50\ \text{mV} (when $If$ and $I_{CaT}$ begin to contribute)
    • V<em>m≈−40 mVV<em>m \approx -40\ \text{mV} (when $I{CaL}$ opens)
    • Vth≈−40 to −30 mVV_{th} \approx -40 \text{ to } -30\ \text{mV}
    • Vm≈−60 mVV_m \approx -60\ \text{mV} (end of phase 3 / repolarization target)
    • Conceptual equations (gating and currents; qualitative): channel opening requires a voltage-dependent change in membrane potential; the net inward current during phase 4 is the sum of $If$ and $I{CaT}$ (and to a lesser extent, $I_{CaL}$ as it starts to activate).

  • Summary of the sequence (quick reference)

    • Phase 4: Slow depolarization through $If$ (Na+) and $I{CaT}$; hyperpolarization-activated opening; reaches $V_{th}$ around $-40$ to $-30$ mV.
    • Phase 0: Upstroke driven by Ca$^{2+}$ entry through $I_{CaL}$; rapid depolarization.
    • Phase 3: Repolarization via outward K+ currents; $I_{CaL}$ inactivates; membrane returns toward $-60$ mV.
    • End of phase 3: $I_f$/funny currents begin again, restarting phase 4.

  • Practical implications and why this matters

    • The SA node’s pace sets heart rate; disorders of automaticity can cause bradycardia or tachyarrhythmias.
    • Drugs that block L-type Ca channels (e.g., certain calcium channel blockers) or modulate funny current can alter heart rate and rhythm by changing Phase 4 slope and threshold.
    • Understanding CICR and intercellular calcium signaling helps explain excitation-contraction coupling in the heart and how impulses from nodal tissue trigger coordinated contraction in the ventricles.

  • Notes on terminology and interpretation

    • The transcript occasionally blends terms (e.g., calling repolarization hyperpolarization); the correct distinction is important for accurate understanding:
    • Repolarization: membrane potential becomes more negative, toward the resting level (here around $-60$ mV).
    • Hyperpolarization: membrane potential becomes more negative than the resting value (e.g., below $-60$ mV).

  • Ethical, philosophical, and practical implications

    • From a clinical ethics perspective, devices that regulate heart rhythm (pacemakers) have significant patient quality-of-life implications and require careful consideration of risks, benefits, and autonomy in decision-making.
    • In practice, understanding the biology of pacemaking informs safer drug development and better patient education about how medications can influence heart rate and rhythm.