Overview
This lecture reviews the histology of cardiac muscle, focusing on structural characteristics, contraction mechanisms, and the autorhythmic nature of the heart.
Cardiac Muscle Histology
- Cardiac muscle cells are branched, with one centrally located nucleus per cell.
- These cells contain numerous mitochondria to meet high energy demands.
- Cardiac muscle fibers connect at intercalated discs, which include desmosomes (strong cell junctions) and gap junctions (allowing rapid communication).
- Cardiac muscle is striated, indicating the presence of sarcomeres—the contractile units also found in skeletal muscle.
Comparison: Cardiac vs. Skeletal Muscle
- Cardiac muscle has less sarcoplasmic reticulum than skeletal muscle, so it stores less intracellular calcium.
- Cardiac muscle relies partly on extracellular calcium, which enters via large T tubules filled with extracellular fluid.
- T tubules carry action potentials deep into the cell and help deliver extracellular calcium for contraction.
- Calcium binds to troponin, moving tropomyosin and exposing binding sites for muscle contraction.
Autorhythmicity & Conduction System
- The heart is autorhythmic, meaning it can contract on its own without external stimulation.
- Autorhythmic cells (about 1% of cardiac muscle) form the heart’s conduction system and can spontaneously depolarize.
- These cells generate action potentials by reaching threshold spontaneously, creating the heart’s pacemaker potential.
Mechanism of Pacemaker Activity
- Autorhythmic cells contain special voltage-gated cation (mainly calcium and sodium) channels called pacemaker channels.
- Pacemaker channels open when the cell membrane is below threshold, letting positive ions in and slowly depolarizing the cell.
- Once threshold is reached, channels close, and an action potential fires due to the all-or-nothing principle.
- The cycle repeats automatically, producing a regular rhythm (pacemaker potential) without outside signals.
Key Terms & Definitions
- Intercalated discs — Specialized connections between cardiac muscle cells containing desmosomes and gap junctions.
- Sarcomere — The basic contractile unit of striated muscle tissue.
- Autorhythmic cells — Modified cardiac muscle cells capable of spontaneous action potentials.
- Pacemaker channels — Voltage-gated ion channels in autorhythmic cells that enable spontaneous depolarization.
- Pacemaker potential — The gradual depolarization leading to threshold and action potential in autorhythmic cells.
Action Items / Next Steps
- Review the process of cardiac muscle contraction and the role of calcium.
- Prepare to discuss how the heart's rhythm can be modified to meet physiological demands in the next lecture.