Cardiac Muscle and Heart Function

Atrioventricular (AV) Bundle (Bundle of His)

  • The AV bundle is the only pathway between the atria and ventricles.

    • It forks into two main components:

    1. Right bundle branch

    2. Left bundle branch

    • These branches descend down the interventricular septum, which is the wall separating the left and right ventricles.

Purkinje Fibers

  • The Purkinje fibers are large diameter nerve fibers that branch from the bundle branches.

    • They turn upward to spread throughout the ventricular myocardium.

    • Their primary function is to distribute electrical excitation upward throughout the ventricles, facilitating coordinated contraction.

Structure of Cardiac Muscle vs. Skeletal Muscle

Cardiac Muscle

  1. Myocytes (cardiac muscle cells) can pass signals from cell to cell, allowing for synchronized contraction.

  2. Cardiac muscle is striated in appearance due to the organization of muscle fibers.

  3. Each myocyte contains a centrally placed nucleus.

  4. The sarcoplasmic reticulum (SR) is less developed compared to skeletal muscle.

  5. Cardiac muscle lacks terminal cisternae, which are found in skeletal muscle.

  6. Transverse tubules are larger in cardiac muscle.

  7. Cardiac muscle cells contain larger mitochondria, reflecting their energy needs.

  8. Intercalated discs are present in cardiac muscle, comprising:

    • Gap Junctions: Facilitating cell-to-cell communication.

    • Desmosomes: Providing structural integrity.

Skeletal Muscle

  1. Skeletal muscle fibers cannot pass signals from cell to cell like cardiac muscle.

  2. Muscles are also striated.

  3. Skeletal muscle fibers are multinucleated, containing multiple nuclei per fiber.

  4. The sarcoplasmic reticulum in skeletal muscle is more developed.

  5. It contains terminal cisternae for calcium storage.

  6. Transverse tubules are larger in skeletal muscle but function differently than in cardiac muscle.

  7. Skeletal muscle has smaller mitochondria compared to cardiac muscle.

  8. There are no intercalated discs in skeletal muscles.

Metabolism of Cardiac Muscle

  • The metabolism of cardiac muscle strongly depends on aerobic respiration for ATP production.

    1. Cardiac muscle is rich in myoglobin, a protein that binds oxygen, enhancing aerobic activity.

    2. Energy Sources:

    • 60% of energy primarily comes from fatty acids derived from adipose tissue.

    • 35% of energy is derived from glycogen (stored sugar).

    • 5% of energy comes from other metabolic fuels.

    1. Cardiac muscle is particularly vulnerable to oxygen debt due to its high metabolic demands.

Electrical and Contractile Activity of the Heart

  • Cardiac rhythm is governed by several components:

    1. Systole: The phase of contraction and emptying of the heart chambers.

    2. Diastole: The phase of relaxation and filling of the heart chambers.

    3. Sinus Rhythm: The normal heartbeat generated by the SA node, typically ranging between 60-100 beats per minute (bpm).

    4. Arrhythmia: Refers to any abnormal heartbeat, which can include:

    • Premature Ventricular Contraction (PVC): An extra heartbeat originating from the ventricles, firing independent of the SA node.

      • Factors such as caffeine, nicotine, and certain chemicals/drugs can induce PVC.

Electrical Behavior of Myocardium

Depolarization

  • The myocardial cells have a resting potential of approximately -60 mV.

    1. A stimulus opens voltage-gated sodium (Na+) channels, leading to an influx of Na+ and resulting in depolarization until a threshold is reached.

    2. At the threshold, additional Na+ gates open:

    • Action potential peaks at around +30 mV.

Plateau Phase

  1. As the action potential continues:

    • Na+ gates close, and the action potential spreads along the plasma membrane.

    • Slow calcium (Ca2+) channels open:

      • A small amount of Ca2+ enters from the extracellular fluid (ECF) into the cell.

    • Ca2+ binds to ligand-gated Ca2+ channels on the sarcoplasmic reticulum (SR):

      • This triggers the opening of more Ca2+ channels, causing further release of Ca2+ into the cell.

    • A second wave of Ca2+ binds to troponin, initiating muscular contraction that is prolonged.

Repolarization

  • Following the plateau:

    1. Ca2+ channels close.

    2. Potassium (K+) channels open, allowing K+ to diffuse out of the cell, returning the intracellular fluid (ICF) back to negative potential.

    3. Ca2+ is transported back to the ECF and the SR.

    4. Muscle tension follows the electrical events in the myocardium.

Refractory Periods

  • There are phases of refractory periods in the cardiac cycle:

    1. Absolute Refractory Period: The phase during which no new action potential can be initiated, providing time for the heart to relax and refill.

    2. Relative Refractory Period: A phase in which a stronger-than-normal stimulus can initiate a new action potential, making the myocardium temporarily excitable.

Pacemaker (SA Node) Physiology and Action Potential Formation

  1. The membrane potential of the pacemaker cells is around -60 mV.

    • A threshold of -40 mV triggers certain ion channels to open:

      • Na+ and Ca2+ Channels: Influx of Na+ and Ca2+ leads to depolarization.

  2. Following depolarization:

    • Potassium (K+) channels open, allowing K+ to leave the cell, leading to repolarization.

    • Ultimately, K+ channels close, returning the membrane potential to its baseline.

  3. Each complete cycle of electrical conduction through the SA node corresponds to one heartbeat.

Impulse Conduction to Myocardium

  1. The firing of the SA node excites the atrial myocytes, leading to contraction of the atria.

    • The electrical signal then reaches the AV node, where it slows down due to:

      • Smaller nerve fibers.

      • Fewer gap junctions, which allow electrical signaling between adjacent cells.

      • This delay is crucial for ventricular filling before contraction begins.

  2. After the AV node, the signal accelerates through the AV bundle (bundle of His) and Purkinje fibers:

    • The ventricles contract almost simultaneously starting at the apex.

  3. Papillary Muscles:

    • Contract before the ventricular myocardium, taking up slack in the chordae tendineae, thereby keeping the atrioventricular (AV) valves closed to prevent backflow.

Ectopic Focus

  1. An ectopic focus occurs when there is firing from sites other than the SA node:

    • Firing can originate from:

      • AV node (nodal rhythm): typically between 40-50 bpm.

      • Purkinje fibers in cases where both the SA and AV nodes are damaged, resulting in a heart rate of 20-40 bpm.

  2. When the SA and AV nodes are not functioning correctly or are damaged, an artificial pacemaker may be needed to maintain a regular cardiac rhythm.

  3. Fibrillation:

    • This is characterized by uncoordinated, out-of-phase contractions among heart muscle cells, leading to ineffective pump function.

Heart Block

  1. Heart block refers to the failure of the cardiac conduction system and may involve:

    • The AV node or bundle branches, which can disrupt normal conduction pathways.

  2. In heart block, the ventricles may beat independently and at their own pace.