Notes on Cardiac Muscle Structure and Function

Lesson 20.2 Class Activity 3

1. Labeling Cellular Structures in Cardiac Muscle Tissue

  • Nucleus: The nucleus is typically located centrally within the cell and contains the cell's genetic material.

  • Mitochondria: These are organelles responsible for producing energy (ATP) through cellular respiration. They are abundant in cardiac muscle cells due to their high energy demands.

  • Intercalated Disc: Specialized connections between adjacent cardiac muscle cells that allow for synchronized contractions and electrical signaling. They are rich in gap junctions and desmosomes.

  • Myofibril: Long, thread-like structures made up of repeating units called sarcomeres. Myofibrils are responsible for muscle contraction and are composed of thick and thin filaments.

  • Desmosome (found in the bottom image): A type of junctional complex that anchors cardiac muscle cells together, providing structural integrity during the intense mechanical stress of contraction.

  • Sarcomere (found in the bottom image): The functional contractile unit of muscle, defined as the segment between two Z lines. Sarcomeres contain actin (thin) and myosin (thick) filaments.

  • Z Line (found in the bottom image): The boundary of a sarcomere where actin filaments are anchored, serving as the structural framework for the myofibrils.

2. Membrane Potential During Cardiac Cell Contraction

  • When cardiac contractile cells contract, the membrane potential becomes more positive due to the influx of sodium ions (Na+) into the cells. This rapid change causes depolarization, moving the membrane potential toward a more positive value in a process known as action potential initiation.

3. Plateau Period of Cardiac Muscle Contraction

  • During the plateau period of contraction, the membrane potential stays relatively constant because of the prolonged influx of calcium ions (Ca2+) through voltage-gated calcium channels. This influx counteracts the outflow of potassium ions (K+), maintaining a depolarized state for an extended duration, critical for the heart's contraction cycle.

4. Membrane Potential During Cardiac Cell Relaxation

  • When cardiac contractile cells relax, the membrane potential becomes more negative because calcium ions (Ca2+) are actively pumped out of the cell and potassium ions (K+) exit the cell. This efflux of K+ and the exit of Ca2+ contribute to the repolarization of the membrane, restoring the resting membrane potential and allowing the heart muscle to prepare for the next contraction.