In a neuron cell that is at rest, the ion channels are closed. When ion channels open, sodium or potassium will diffuse rapidly across the membrane from the region of high concentration to the region of low concentration. If a few sodium ion channels open, then a few sodium ions will flow from outside of the cell to the inside of the cell, lessening the charge difference at that location. This is known as [ Select ] ["hyperpolarization", "repolarization", "depolarization"] . If more sodium ion channels open, more and more sodium ions will flow into the cell, reducing the difference in charge even further, to approach zero. As soon as the difference reaches [ Select ] ["+30 mV", "-70 mV", "-55 mV"] as measured using a voltmeter, that is the trigger to open voltage-gated sodium ion channels, and a lot of sodium ions rush into the cell, rapidly changing the potential difference and [ Select ] ["depolarizing", "repolarizing", "hyperpolarizing"] the membrane. This measured voltage that triggers the action potential is known as [ Select ] ["superthreshold", "threshold", "subthreshold"] .
As soon as the potential difference reaches [ Select ] ["-70 mV", "30 mV", "-55 mV"] , the voltage-gated sodium ion channels close, and the voltage-gated potassium ion channels open. This allows potassium ions to rapidly flow from inside the cell to the outside of the cell. As this happens, the potassium ions make the outside of the cell to be positively charged again, a process known as [ Select ] ["hyperpolarization", "repolarization", "depolarization"] . Potassium ions continue to flow to the outside of the cell until the potential difference passes the resting membrane potential and reach [ Select ] ["-85 mV", "- 55 mV", "-70 mV"] . At this point, we say the membrane is [ Select ] ["repolarized", "depolarized", "polarized", "hyperpolarized"] . The voltage-gated potassium ion channels then close at this voltage, and the [ Select ] ["calcium-chloride pump", "sodium-calcium pump", "potassium-chloride pump", "sodium-potassium pump"] then resumes work, returning the sodium back to the outside of the cell and potassium ions back into the cell. The membrane then returns to resting membrane potential, to wait for another stimulus.