Cardiac Action Potential and Heart Cells

Cardiac Action Potential

• A complete cycle of depolarization and repolarization makes up the Cardiac Action Potential.

Cardiac Cells

• There are three types of cells in the heart: Pacemaker cells, Electrical conducting cells, and Myocardial cells.
• Pacemaker cells:
  • Are small, about 5 to 10 μm long.
  • Can depolarize spontaneously over and over again.
  • Have no true resting potential due to “leaky K channels”.
  • Are located in the upper Right Atrium (SA Node).
• Electrical conducting cells:
  • Carry current rapidly and efficiently to distant regions of the heart.
  • The AV Node is a slow conductor.
  • Atrial conducting systems can vary anatomically, while the ventricular conducting system forms distinct pathways.
• Myocardial cells:
  • Make up the largest part of heart tissue.
  • Are about 50 to 100 μm long.
  • Contain an abundance of actin and myosin (contractile proteins).
  • Depolarization wave reaching a myocardial cell causes calcium release → cells contract (excitation–contraction coupling).

Pacemaker Cells

• The SA Node spontaneously fires 60 to 100 times per minute, and varies based on:
  • Autonomic nervous system (Sympathetic vs. parasympathetic).
  • Increased demand (exercise vs. rest).
• Autonomic regulation of heartbeat:
  • Vagal Nerve slows HR → ↑Acetylcholine release = ↓ rate of rise of Phase 4 depolarization.
  • Accelerant nerve speeds HR → ↑Norepinephrine release = ↑ rate of rise of the cardiac action potential during phase 0.
  • ↑Epinephrine release = ↑ rate of rise and acetylcholine.

Cardiac Conduction System

• The cardiac conduction system consists of:
  • Native/dominant pacemaker: Sinoatrial or Sinus node (60-100 BPM).
  • Ectopic/latent pacemaker: Atrial Pacemaker (60-75 BPM), AV Node (Junctional) (40-60 BPM), Myocardial (Ventricular) cells (30-45 BPM), and Conducting (Purkinje) cells (30-40 BPM).
• The cardiac conduction system works as follows:
  • SA node → Bachman’s Bundle (depolarization of LA) → SA node → AV node (short delay) → Bundle of His (splits) → L&R Bundle Branches (depolarization of ventricles) → Purkinje System (depolarization of rest of ventricles).

Cardiac Electrophysiology

• Physiological properties of myocardial cells:
  • Automaticity: ability to initiate an impulse.
  • Excitability: ability to respond to a stimulus.
  • Conductivity: ability to transmit an impulse.
  • Contractility: ability to respond with pumping action.
• Cardiac cells are electrically polarized (negatively charged) in the resting state.
• Depolarization: the loss of negativity in cardiac cells.
• Repolarization: return to resting polarity (negatively charged).

Action Potentials

• An action potential is a brief change in voltage (membrane potential) across the cell membrane of heart cells.
• Comprised of one complete cycle of depolarization and repolarization.
• Initiates a wave of depolarization that propagates throughout the heart.
• ECG is the composite representation of action potentials of all cardiac cells.
• Phases of action potential:
  • Phase 0: Rapid depolarization.
  • Phase 1: Slight repolarization.
  • Phase 2: Plateau phase.
  • Phase 3: Rapid Repolarization.
  • Phase 4: Diastolic resting membrane potential.

Pacemaker Cell Action Potential

• Similar to depolarization in the myocardium, except:
  • Depolarization results from the influx of Calcium (rather than Na).
  • There is no plateau phase (no Phase 1 and 2).