At what membrane potential have we reached saturation of sodium channels?

At what membrane potential have we reached saturation of sodium channels?

• A. -70 mV
• B. -60 mV
• C. -50 mV
• D. -80 mV

Answer: (C)

Sodium channel activation depends on how depolarized the membrane is. As the cell voltage rises from rest, more fast Na+ channels flip from closed to open, increasing inward current. Reaching a membrane potential around -50 mV, the population of channels that can activate is essentially maximized—most of the available channels are open. At this point, further depolarization doesn’t recruit many more channels into the open state, so the activation "saturation" is reached. If the membrane becomes even more positive, inactivation gates begin to close some channels, which reduces the net inward Na+ current even though the voltage continues to rise. That’s why -50 mV is the point where activation saturation occurs.

Sodium channel activation depends on how depolarized the membrane is. As the cell voltage rises from rest, more fast Na+ channels flip from closed to open, increasing inward current. Reaching a membrane potential around -50 mV, the population of channels that can activate is essentially maximized—most of the available channels are open. At this point, further depolarization doesn’t recruit many more channels into the open state, so the activation "saturation" is reached. If the membrane becomes even more positive, inactivation gates begin to close some channels, which reduces the net inward Na+ current even though the voltage continues to rise. That’s why -50 mV is the point where activation saturation occurs.