Overview
This lecture explains how pacemaker cells in the heart generate action potentials on their own, drive the heartbeat, and the phases and ion channels involved in this process.
Pacemaker Cells and Automaticity
- Pacemaker cells set the heart's rhythm and pace independently of signals from other cells.
- The property that allows pacemaker cells to fire action potentials without external triggers is called automaticity.
- Three main groups of pacemaker cells are located in the sinoatrial (SA) node, atrioventricular (AV) node, and the bundle of His/Purkinje fibers.
Membrane Potential and Ion Permeability
- Membrane potential is measured in millivolts (mV), with different ions influencing this voltage.
- Calcium influx alone would set a potential around +123 mV, sodium around +67 mV, and potassium around -92 mV.
- Real cells are permeable to multiple ions; the balance determines the actual membrane potential.
Action Potential Phases in Pacemaker Cells
- Pacemaker cells typically start at -60 mV and are mostly permeable to sodium, causing a gradual depolarization.
- At -40 mV (threshold), voltage-gated calcium channels open, causing rapid depolarization (phase 0).
- Calcium channels close, and potassium channels open, causing repolarization (phase 3) as potassium exits the cell.
- Once potassium channels close, sodium permeability dominates again and the cycle repeats.
- Pacemaker cells do not have a true resting potential; their membrane potential constantly changes.
Action Potential Cycle and Phases
- The cycle involves three main phases: phase 4 (slow depolarization), phase 0 (rapid depolarization), and phase 3 (repolarization).
- Unlike other heart cells, pacemaker cells lack phases 1 and 2 in their action potential.
- Phase 0 in pacemaker cells is slower compared to the 'fast' phase 0 of cardiac muscle cells.
Depolarization and Repolarization Definitions
- Depolarization is when the membrane potential becomes less negative.
- Repolarization is when the membrane potential becomes more negative.
Key Terms & Definitions
- Pacemaker cell — a heart cell that initiates its own action potential to control heart rhythm.
- Automaticity — the ability to generate action potentials without external stimulation.
- Threshold — the membrane potential level where voltage-gated ion channels open.
- Depolarization — process of membrane potential becoming less negative.
- Repolarization — process of membrane potential becoming more negative.
- Voltage-gated channel — an ion channel that opens or closes in response to changes in membrane potential.
Action Items / Next Steps
- Review the phases of the pacemaker cell action potential: 4, 0, and 3.
- Understand which ions (Na⁺, Ca²⁺, K⁺) are responsible for each phase.