Influence of Membrane Resting Potential on Action Potential Properties

• The extent and synchrony of sodium channel activation is dependent on the resting membrane potential.

  • Inactivation gates of sodium channels close in the membrane potential range of -75 to -55 mV (less channels available for sodium ion inward current)

  • For example: less intense sodium current if the resting potential is - 60 mV compared to -80 mV

  • Consequences of reduced sodium activation due to reduced membrane potential (less negative)

    • reduced of velocity upstroke (V_max) [phase 0] (maximum rate of membrane potential change)

    • reduced excitability

    • reduced conduction velocity-- a significant cause of arrhythmias

    • prolongation of recovery:-- an increase in effective refractory period

• Plateau Phase:

  • Plateau phase -- Na channels mostly inactivated

  • Repolarization (h gates reopen)

  • "Refractory period": time between phase 0 and phase 3 -- during this time the stimulus does not result in a propagated response

  • Altered refractoriness may cause or suppress arrhythmias

• Factors that reduce the membrane resting potential & reduce conduction velocity

  • Hyperkalemia

  • Sodium pump block

  • Ischemic cell damage

• Conduction in severely depolarized cells

  • With decreased membrane potentials (e.g., -55 mV), sodium channels are inactivated

  • Under some circumstances, increased calcium permeability or decreased potassium permeability allow for slowly conducted action potentials with slow upstroke velocity

  • Ca2+-inward current-mediated action potentials are normal for the specialized conducting SA nodal and AV nodal tissues, which have resting membrane potentials in the -50 to-70 mV range.

• Ischemia

• Hypoxia

• Acidosis

Alkalosis

• 
  

• Abnormal electrolytes

• Excessive catecholamine levels

• Autonomic nervous system effects (e.g., excess vagal tone)

• Excessive catecholamine levels

• Autonomic nervous system effects (e.g., excess vagal tone)

• Drug effects: e.g., antiarrhythmic drugs may cause arrhythmias)

• Cardiac fiber stretching (as may occur with ventricular dilatation in congestive heart failure)

• Presence of scarred/diseased tissue which have altered electrical conduction properties

Category: Physiology Notes