Action Potential and Excitability in the Heart

Action Potential and Excitability Change

• Absolute Refractory Period: coincides with the period of depolarization till the end of the plateau, where excitability is lost.
• Relative Refractory Period: coincides with the period of rapid repolarization, where excitability is weak.

Rhythmicity

• Defined as the ability of the heart to beat regularly and initiate its own regular impulse independently of any nerve supply.
• Rhythmic cells characterized by:
  • Discharge spontaneously.
  • Membrane potential is unstable with no resting membrane potential (RMP).
  • No plateau.
  • Membrane is more permeable to Na and Ca.
  • Firing level = -40 mV.
  • Peak of AP = +10 mV.
• SAN (Sinoatrial Node) discharge rate: 90/min.
• AVN (Atrioventricular Node) discharge rate: 60/min.
• Purkinje discharge rate: 30/min.

Ionic Bases of Rhythmicity

• Membrane potential decreases spontaneously from -60 mV to -50 mV due to Na influx.
• At -50 mV, transient Ca channels open, and Ca enters the cell, causing depolarization to the firing level (-40 mV).
• At -40 mV, long-lasting Ca channels (L-type) open, and Ca enters the cell, causing depolarization and closure of transient Ca channels and Na channels till +10 mV.
• Repolarization phase occurs due to K outflow through K channels and closure of L-type Ca channels.
• At -60 mV, inactivation of K channels and K efflux stops.

Action Potential and Mechanical Change

• Systole: coincides with the period of depolarization till the end of the plateau.
• Diastole: coincides with the period of rapid repolarization phase and continues in the rest phase, because the mechanical response lasts one and a half times as long as the action potential.
• Increase in slope: increases heart rate (tachycardia).
• Decrease in slope: decreases heart rate (bradycardia).

Conductivity

• Defined as the ability of cardiac muscle to transmit cardiac excitation waves.
• Excitation wave generated in SAN, then passes through the intermodal pathway to the far part of atria.
• Excitation wave reaches AVN, where there is a delay in transmission at a rate of 0.05 m/sec.
• Once excitation wave enters the A-V bundle, it spreads rapidly down to the branches and Purkinje fibers at a rate of 4-5 m/sec.
• Cardiac impulse is then conducted through ventricular muscle itself at a rate of 1 m/sec.

Significance of Conduction Rates

• Rate of conduction in different parts:
  • Atrial pathway: 1 m/sec.
  • AV node: 0.05 m/sec.
  • Bundle of His: 1 m/sec.
  • Purkinje fibers: 4-5 m/sec.
  • Ventricular muscle: 1 m/sec.
• Significance of slow conduction at AVN:
  • Delays in conduction allow for both atria to be excited and contract to empty their blood into ventricles.
  • Prevents abnormal rhythms originating in atria from reaching ventricles.
• Significance of fast conduction at Purkinje:
  • Rapid conduction allows for all ventricles to contract as one unit to eject blood.

Contractility

• Defined as the ability of the cardiac muscle to contract to pump blood.
• Ca release from sarcoplasmic reticulum initiates contraction.