Chapter 20 Flashcards: Heart Anatomy and Cardiac Muscle

Additional Internal Structures

• Pectinate Muscles
  • Predominantly in the right atrium.
  • Some sources suggest they increase the force of contraction without increasing muscle size or thickness.
• Trabeculae Carneae
  • Found in both ventricles.
  • Exhibit a weaving-like pattern in the myocardium.
  • Assist the papillary muscles in valve stability.
• Fossa Ovalis
  • Only found in the Right Atrium
  • A scar resulting from the closing of the foramen ovale.
    • The foramen ovale was a "hole" in the interatrial septum during fetal development.
    • It closes shortly before birth, becoming the fossa ovalis.

Ventricular Myocardium Differences

• Significant differences exist in the outer ventricular wall between the right and left ventricles.
  • The right ventricle is thinner than the left ventricle.
  • The right ventricle pumps blood to the pulmonary circuit (lungs), which are close to the heart, requiring less force; hence, the thinner wall.
  • The left ventricle pumps blood to the systemic circuit (the entire body), necessitating a thicker wall for greater contraction force.

Heart Wall Construction

The heart wall consists of three layers:

• Epicardium
  • The surface layer covers the heart.
• Myocardium
  • The middle layer.
  • Myo = muscle.
• Endocardium
  • The internal layer lines the heart.

Epicardium

• Covers the surface of the heart muscle.
• Also known as the visceral pericardium and is part of the serous membrane.
• Composed of a single layer of simple squamous epithelium called a mesothelium.
• Helps produce serous fluid, which is secreted into the pericardial cavity.

Endocardium

• Inner lining of the heart chambers.
• Composed of a single layer of simple squamous epithelium.
• Produces an endothelium, which makes the interior surface of the ventricles extremely smooth.
• Reduces friction between blood cells and the chamber wall and prevents clotting of cells inside the ventricles.
• Easily observable in lab settings on sheep hearts, where the interior surface appears waxed.

Myocardium

• The majority of the heart tissue.
• Middle, muscular layer forming the atria and ventricles.
• Contains cardiac muscle tissue, blood vessels, and nerves.
• Features concentric muscle tissue layers.
• Layers form a figure-eight around the atria.
• Superficial muscle layers wrap both ventricles.
• Deep muscle layers form a figure-eight around the ventricles.
• During muscle contractions, this pattern squeezes the heart, "like twisting a wash rag to get out all the water," to eject the blood.

Myocardium: Cardiac Muscle Tissue

• Cardiac muscle tissue is exclusively found in the heart.
• Characterized by:
  • Small cell size.
  • Single, centrally located nucleus.
  • Branching interconnections.
  • Specialized intercellular connections.

Intercalated Discs

• Intercalated discs contain:
  • Desmosomes: Stabilize the relative positions of adjacent cells.
  • Gap junctions: Allow ions and molecules to move directly between cells, creating a direct electrical connection for action potential passage.
• Enable cells to "pull together" for maximum efficiency.
• Allow all cells to function "as one" (functional syncytium).

Cardiac Muscle Cell vs. Skeletal Muscle Fiber

Comparison of Action Potentials (AP) in Skeletal and Cardiac Muscle

• Skeletal Muscle

  1. Depolarization phase: Voltage-gated Na^+ channels open, and voltage-gated K^+ channels begin to open.
  2. Repolarization phase: Voltage-gated Na^+ channels close, voltage-gated K^+ channels continue to open, and voltage-gated K^+ channels close at the end of repolarization, returning the membrane potential to its resting value.
  3. Refractory period effect on tension: Maximum tension is obtained after the refractory period is completed, allowing for increased tension with additional stimulation.

• Cardiac Muscle

  1. Depolarization phase: Voltage-gated Na^+ channels open, and voltage-gated K^+ channels close; voltage-gated Ca^{2+} channels begin to open.
  2. Early repolarization and plateau phases: Voltage-gated Na^+ channels close, some voltage-gated K^+ channels open, causing early repolarization, and voltage-gated Ca^{2+} channels are open, producing the plateau by slowing further repolarization.
  3. Final repolarization phase: Voltage-gated Ca^{2+} channels close, and many voltage-gated K^+ channels open.
  4. Refractory period effect on tension: Cardiac muscle contracts and relaxes almost completely during the refractory period.

Pacemaker Potential

• Permeability changes in pacemaker cells

  1. Pacemaker potential
     • A small number of Na^+ channels are open.
     • Voltage-gated K^+ channels that opened in the repolarization phase of the previous action potential are closing.
     • Voltage-gated Ca^{2+} channels begin to open.
  2. Depolarization phase
     • Voltage-gated Ca^{2+} channels are open.
     • Voltage-gated K^+ channels are closed.
  3. Repolarization phase
     • Voltage-gated Ca^{2+} channels close.
     • Voltage-gated K^+ channels open.

Review Questions Answered

• From superficial to deep, name the layers of the heart wall.
  • Epicardium, Myocardium, Endocardium
• What is a function of the epicardium.
  • Covers surface of heart muscle.
• Why is it important that cardiac tissue be richly supplied with mitochondria and capillaries?
  • Cardiac cells need a great deal of energy to function, hence the large number of mitochondria.
• List five differences between cardiac and skeletal muscle
  • See the differences in the "Cardiac Muscle Cell vs. Skeletal Muscle Fiber" Table
• What is the function of the intercalated discs?
  • Contain desmosomes and gap junctions. Allow ions and molecules to move directly between cells. Allow cells to “pull together” for maximum efficiency
• Do you have a foramen ovale in your right atrium? Why or why not.
  • No, if it remains open after birth its called a patent foramen ovale
• What is a functional syncytium?
  • All cells to function “as one"