Overview
This lecture covers the electrical and mechanical activity of the heart, focusing on pacemaker function, action potentials in different cardiac cells, ionic mechanisms, and the importance of the refractory period.
Structure and Function of the Heart
- The heart is a muscle made of cardiac myocytes specialized for contraction and blood pumping.
- Cardiac myocytes contract in response to self-generated electrical impulses called action potentials.
- Unlike skeletal muscle, the heart generates its own impulses without nervous system input.
Pacemaker and Conduction System
- Pacemaker cells are specialized myocytes that initiate and conduct action potentials.
- The SA node is the primary pacemaker, controlling heart rate and initiating heartbeats.
- If the SA node fails, secondary pacemakers in the conduction system can take over.
Action Potentials in Cardiac Cells
- Action potentials start in pacemaker cells and spread to contractile myocytes via gap junctions.
- Electrical coupling through gap junctions ensures rapid and synchronous contraction.
- All myocytes must depolarize in unison for effective heartbeat.
Ionic Basis of Action Potentials
- Resting membrane potential is negative due to ion gradients: more Na⁺ and Ca²⁺ outside, more K⁺ inside.
- Depolarization means the cell becomes less negative; repolarization means it becomes more negative again.
- Threshold depolarization is required to trigger an action potential.
Pacemaker Cell Action Potentials
- Pacemaker cells lack a true resting potential and spontaneously depolarize due to "funny" currents.
- Depolarization (pacemaker potential) starts at -60mV and reaches -40mV threshold.
- At threshold, Ca²⁺ channels open for rapid depolarization; repolarization follows as K⁺ channels open.
Contractile Myocyte Action Potentials
- Contractile myocytes have a stable resting potential of -90mV and depolarize only when stimulated.
- Rapid Na⁺ influx causes sharp depolarization; slow Ca²⁺ influx via L-type channels creates a plateau phase.
- Plateau phase maintains contraction long enough to expel blood from the chambers.
Excitation-Contraction Coupling
- Ca²⁺ influx triggers further Ca²⁺ release from sarcoplasmic reticulum (SR), leading to muscle contraction.
- Contraction occurs during and ends with the plateau phase as Ca²⁺ channels close and K⁺ efflux repolarizes the cell.
- Ionic pumps restore original gradients after each action potential.
Refractory Period and Heart Function
- The cardiac absolute refractory period is about 250 msec, much longer than skeletal muscle's 1 msec.
- This long period prevents tetanus, ensuring the heart relaxes between contractions.
Key Terms & Definitions
- Cardiac myocytes — muscle cells of the heart specialized for contraction.
- Pacemaker cells — myocytes that initiate electrical impulses independently.
- SA node — sinoatrial node, the primary pacemaker of the heart.
- Action potential — a brief reversal of membrane voltage that propagates along cells.
- Depolarization — process where cell membrane voltage becomes less negative.
- Repolarization — return of cell membrane voltage to a more negative value.
- Gap junctions — connections allowing electrical signals to pass between cells.
- Plateau phase — sustained depolarization unique to cardiac action potentials.
- Sarcoplasmic reticulum (SR) — organelle storing calcium in muscle cells.
- Absolute refractory period — time when a cell cannot be re-excited, ensuring separate contractions.
Action Items / Next Steps
- Review the phases of action potentials in both pacemaker and contractile myocytes.
- Study the role of ion channels and pumps in generating heart action potentials.