Pytania i odpowiedzi: Depolaryzacja neuronów, hiperpolaryzacja i zmiana potencjału.

Question:

Answer:

Hyperpolarization and depolarization

• The opening of channels that let positive ions flow out of the cell (or negative ions flow in) can cause hyperpolarization. Examples: Opening of channels that let ${\text{K}}^{+}$‍ out of the cell or ${\text{Cl}}^{-}$‍ into the cell.
• The opening of channels that let positive ions flow into the cell can cause depolarization. Example: Opening of channels that let ${\text{Na}}^{+}$‍ into the cell.

Graded potentials

Action potential

• An action potential begins when a depolarization increases the membrane voltage so that it crosses a threshold value (usually around$-55$‍ $\text{mV}$‍).
• At this threshold, voltage-gated ${\text{Na}}^{+}$‍ channels in the membrane open, allowing many sodium ions to rush into the cell. This influx of sodium ions makes the membrane potential increase very rapidly, going all the way up to about $+40$‍ $\text{mV}$‍.
• After a short time, the sodium channels self-inactivate (close and become unresponsive to voltage), stopping the influx of sodium. A set of voltage-gated potassium channels open, allowing potassium to rush out of the cell down its electrochemical gradient. These events rapidly decrease the membrane potential, bringing it back towards its normal resting state.
• The voltage-gated potassium channels stay open a little longer than needed to bring the membrane back to its resting potential. This results in a phenomenon called “undershoot,” in which the membrane potential briefly dips lower (more negative) than its resting potential.
• Eventually, the voltage-gated potassium channels close and the membrane potential stabilizes at resting potential. The sodium channels return to their normal state (remaining closed, but once more becoming responsive to voltage). The action potential cycle may then begin again.

Transmission of a signal by action potentials

• First, when one patch of membrane (say, right at the axon hillock) undergoes an action potential, lots of ${\text{Na}}^{+}$‍ ions rush into the cell through that patch. These ions spread out laterally inside the cell and can depolarize a neighboring patch of membrane, triggering the opening of voltage-gated sodium channels and causing the neighboring patch to undergo its own action potential.
• Second, the action potential can only travel in one direction – from the cell body towards the axon terminal – because a patch of membrane that has just undergone one action potential is in a “refractory period” and cannot undergo another.
  The refractory period is primarily due to the inactivation of voltage-gated sodium channels, which occurs at the peak of the action potential and persists through most of the undershoot period. These inactivated sodium channels cannot open, even if the membrane potential goes above threshold. The slow closure of the voltage-gated potassium channels, which results in undershoot, also contributes to the refractory period by making it harder to depolarize the membrane (even once the voltage-gated sodium channels have returned to their active state).
  The refractory period ensures that an action potential will only travel forward down the axon, not backwards through the portion of the axon that just underwent an action potential.