CVS – Excitation

SA Node

Ventricle

Resting membrane potential

• No set RMP, however approx. -60mV
• βadrenergic stimulation causes a less negative RMP and increased slope of the initial upstroke
• muscarinic stimulation causes a more negative RMP and a decreased slope of the initial upstroke

Resting membrane potential

• Approx -90mV

Threshold potential

• Approx 40mV

Threshold potential

• Approx. -50mV

Phase 0

• Funny Na⁺ current allow leakage of Na⁺ into the the cell slowly increasing membrane potential to the threshold potential

T and L-type Ca²⁺ channels (voltage gated) open when threshold potential reached

• Ca²⁺ influx
• Depolarization
• Shallow upstroke compared to ventricular myocyte

Phase 0

• Rapid depolarization
• Opening of fast Na⁺ channels
• Influx of Na⁺
• Overshoot to +20mV

Phase 1

• Early repolarization
• Closure of fast Na⁺ channels and opening of K⁺ channels (transient outward, inward rectifier)
• Efflux of K⁺

Phase 2

• Plateau
• Opening of intially T-type Ca²⁺ channels and subsequently L-type Ca²⁺ channels
• Ca²⁺ influx balances K⁺ efflux
• Na⁺ channels in closed state à absolute refractory period
• No action potential can be generated in this state

Phase 3

K⁺ channels open and Ca²⁺ channels slowly close

• K⁺ efflux
• Repolarization
• No plateau phase

Phase 3

• Repolarization
• Closure of Ca²⁺ channels while K⁺ channels remain open (delayed rectifier)
• K⁺ efflux
• Returns potential towards RMP
• Na⁺ channels transition to inactive state à relative refractory period
• Another action potential can be generated with greater adequate stimulus

Phase 4

• Na⁺/K⁺ ATPase and Na⁺/Ca²⁺ ATPase maintain ionic gradients

Phase 4

• RMP
• Na⁺/K⁺ ATPase and Na⁺/Ca²⁺ ATPase maintain ionic gradients