What happens to the membrane potential during the threshold reaching event in the SA node?

What happens to the membrane potential during the threshold reaching event in the SA node?

• A. -70 mV to -50 mV
• B. -60 mV to -40 mV
• C. -50 mV to -20 mV
• D. -75 mV to -35 mV

Answer: (B)

In SA node cells, there isn’t a stable resting potential like in other cardiac cells. They undergo a slow, spontaneous depolarization called the pacemaker potential, driven mainly by inward currents such as If. This gradual depolarization carries the membrane potential from about -60 mV toward the threshold, which sits around -40 mV. The threshold-reaching event is this depolarization from roughly -60 mV up to -40 mV. When -40 mV is reached, voltage-gated L-type Ca2+ channels open, producing the rapid upstroke of the action potential driven by calcium influx rather than sodium. So the membrane potential during the threshold-reaching event moves from around -60 mV to -40 mV.

In SA node cells, there isn’t a stable resting potential like in other cardiac cells. They undergo a slow, spontaneous depolarization called the pacemaker potential, driven mainly by inward currents such as If. This gradual depolarization carries the membrane potential from about -60 mV toward the threshold, which sits around -40 mV. The threshold-reaching event is this depolarization from roughly -60 mV up to -40 mV. When -40 mV is reached, voltage-gated L-type Ca2+ channels open, producing the rapid upstroke of the action potential driven by calcium influx rather than sodium. So the membrane potential during the threshold-reaching event moves from around -60 mV to -40 mV.