Learning Objective
Explain why the resting membrane potential (RMP) of ventricular, atrial, and Purkinje cells is close to the potassium equilibrium potential, and describe the roles of ungated K⁺ channels and inward-rectifier K⁺ channels (IK₁) in stabilizing the RMP.
Ventricular, atrial, and Purkinje myocytes all share a common electrical characteristic: very high potassium conductance at rest. Because potassium permeability dominates, the resting membrane potential (~ –85 to –90 mV) lies close to the K⁺ equilibrium potential (~ –95 mV).
This high resting K⁺ conductance comes from two major channel types:
- Ungated (Leak) K⁺ Channels
- Inward-Rectifying K⁺ Channels (IK₁)
Together, they stabilize the RMP and prevent spontaneous depolarization, distinguishing non-nodal cells from pacemaker tissues.
Ungated Potassium Channels (“Leak channels”)
- Always open at rest
- Permit continuous K⁺ efflux because the intracellular K⁺ concentration is high
- Continue to carry current unless membrane potential becomes equal to Eₖ (~ –95 mV)
- Their constant outward K⁺ leak is a major determinant of the negative RMP
Key Point: Leak K⁺ channels maintain a steady outward potassium movement that pulls the membrane potential toward Eₖ.
Activity
Inward-Rectifying Potassium Channels (IK₁)
IK₁ channels behave differently from typical voltage-gated channels:
- Open at rest → large K⁺ conductance stabilizes RMP
- Close with depolarization
- This closure reduces K⁺ efflux during phase 0 and phase 2 of the action potential
- Reopen during late repolarization
- Help return the membrane potential to Eₖ
- Contribute strongly to phase 3 repolarization
Concept: IK₁ channels “rectify” by allowing more K⁺ flow at negative potentials (rest) and less flow at positive potentials (depolarization).
Activity
Functional Significance
- Prevents spontaneous depolarization → non-nodal cells do not exhibit automaticity
- Keeps RMP stable and negative
- Helps shape the plateau and repolarization phases of the ventricular AP
- Ensures electrical stability and coordinated contraction
