HUBS192 Lecture 9: The Excitable Heart

Electrical and contractile cells

• Heart tissue contains two main cell types with different functions: electrical cells (specialized for conduction) and contractile cells (specialized for force generation).
• Proportions: $1\%$ electrical cells and $99\%$ contractile (working myocardial cells).
• Electrical cells include Purkinje cells and AV nodal cells; contractile cells are the muscle cells that actually contract to pump blood.
• Visual descriptions mentioned: electrical cells have a 'pale' striated appearance with low actin and myosin; contractile cells have a striated appearance with high actin and myosin.
• These differences underpin the coordinated electrical activation and mechanical contraction of the heart.

Action potentials and impulse propagation

• Depolarization of the heart starts at the sinoatrial (SA) node (the natural pacemaker).
• The electrical signal spreads to neighboring cells, reaching both electrical and contractile cells.
• In a contractile cell, the impulse increases cytosolic $\mathrm{Ca^{2+}}$ levels, enabling cross-bridge formation (X-bridge attachment) and muscle contraction.
• How the impulse spreads between cell types:
  • Through intercalated disks that connect cardiac cells.
  • Gap junctions within those disks form low-resistance pathways for ionic current.
  • This allows rapid spread of current from one cell to the next, effectively making millions of cells behave as one functional unit (a functional syncytium).
• Key terms: depolarization, repolarization, contraction, relaxation.

Intercalated disks and gap junctions

• Intercalated disks are specialized cell–cell junctions that connect most cardiac cells.
• Contain gap junctions: pores with low electrical resistance that permit current flow between adjacent cells.
• Gap junctions are essential for fast, coordinated spread of the action potential across the heart muscle.
• Result: millions of cardiac cells act as a single functional syncytium, enabling synchronous contraction.

Gap junctions and spreading the impulse

• Pathways for impulse spread:
  • Along the conduction pathway (through specialized conducting tissue).
  • Between electrical and contractile cells (electrical-to-mechanical coupling).
  • Between contractile cells (ensuring rapid propagation).
• Outcome: increased speed and uniformity of impulse, synchronizing the heartbeat.

Electrical wiring of the heart: the conduction pathway

• The classic conduction sequence (as described in Martini et al. and Patton & Thibodeau references):
  1) SA node (sinoatrial)
  2) Internodal bundles
  3) Interatrial bundle
  4) AV node (atrioventricular)
  5) AV bundle (bundle of His)
  6) Bundle branches
  7) Purkinje fibers
• Some sources also emphasize the role of the AV node pause to allow ventricular filling.
• The conduction system ensures that atrial contraction precedes ventricular contraction and that impulses efficiently reach all ventricular regions.

ECG and the cardiac cycle

• Electrocardiogram (ECG) basics:
  • A lead is a virtual line between two surface electrodes; an electrical potential difference between them is recorded.
  • ECG components reflect depolarization and repolarization events:
  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization
  • T wave: ventricular repolarization
• The ECG provides a non-invasive readout of the timing of electrical events, which correspond to mechanical events (contraction/relaxation) in the cardiac cycle.
• Important mapping notes kept in lectures: the electrical events map onto mechanical events during the cardiac cycle (atrial contraction, AV node conduction, ventricular contraction, etc.).

The cardiac cycle: sequence of electrical and mechanical events

• Quiescence ends when excitation spreads from the SA node.
• The atria are fully depolarized and contract early in the cycle.
• Atria repolarize and relax while the AV node conducts the signal to the ventricles.
• The ventricles depolarize and contract.
• The ventricles begin to repolarize and relax.
• The ventricles become fully repolarized and relaxed, returning to quiescence.
• A common cross-reference figure is Marieb & Hoehn, Figure 18.17 (pg. 704) illustrating this sequence.

Putting it all together: sounds and overall flow

• The heart emits a two-part sound: first sound "lub" (mitral/tricuspid closure during the start of ventricular systole) and second sound "dupp" (aortic/pulmonary valve closure at the end of systole).
• The electrical and mechanical events are tightly coordinated by the conduction system and gap junctions, ensuring efficient blood flow.
• The combined electrical conduction pathway and the resulting ECG tracings provide a framework for understanding normal heart function and its disorders.

Exam-style review questions and concepts

• Example exam question (BP curve): In an adjacent figure of a blood pressure curve, which horizontal line most closely corresponds to mean arterial pressure? Options: A, B, C, D. (Referenced in the material as a practice item.)
• Example exam question (ECG and chamber contraction): If the left atrium is contracting, which statement is most likely true?
  • A. The left ventricle is also contracting.
  • B. The right ventricle is also contracting.
  • C. The right atrium is also contracting.
  • D. No other chamber is contracting.
• Example exam question (P wave): Which event occurs during the P wave in an ECG?
  • A. Atrial relaxation
  • B. Repolarization of the ventricles
  • C. Atrial depolarization
  • D. Ejection of blood from the ventricles
• Answers are implied by the lecture: P wave corresponds to atrial depolarization; the QRS complex to ventricular depolarization; the T wave to ventricular repolarization.

Main points to remember

• The heart contains two specialized cell types: electrical (1%) and contractile (99%), with Purkinje and AV nodal cells as key electrical cells.
• Gap junctions in intercalated disks enable rapid, low-resistance current flow, creating a functional syncytium for synchronized contraction.
• The conduction pathway follows a defined sequence to coordinate timing of atrial and ventricular contraction.
• An ECG records depolarization and repolarization of cardiac cells as P, QRS, and T waves, which correspond to specific mechanical events in the cardiac cycle.
• The cardiac cycle comprises a series of electrical and mechanical events that ensure proper heart function; the audible heart sounds provide a practical correlate to the cycle.

Connections to readings and references

• Readings cited: Martini et al. Modules 18.3 (p. 705), 18.10 (p. 719), and 18.12 (p. 722-724).
• Additional references mentioned: Marieb & Hoehn, Figure 18.17 (p. 704) and 18.19 (p. 706); Patton & Thibodeau, Anatomy and Physiology (8th ed.) Figure 22-2 (p. 683).
• Visuals support understanding of the conduction pathway, intercalated disks, gap junctions, and ECG waveforms.

Practical and ethical implications

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Notes on formulas and notation

• Electrical vs contractile cell proportions: $1\%$ vs $99\%$.
• Calcium dynamics in contraction: $\mathrm{Ca^{2+}}$ plays a crucial role in cross-bridge formation and muscle contraction.
• ECG wave identifiers: $P$ (atrial depolarization), $QRS$ (ventricular depolarization), $T$ (ventricular repolarization).
• The conduction pathway can be memorized as a sequence; practice recalling the order to reinforce comprehension of synchronization.

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