Student Reader

Action Potentials

Voltage is the tendency for electrically charged particles such as electrons or ions to move between two points. The principle of voltage is the foundation for action potentials. The action potential is a wave of depolarization of a neuron's plasma membrane. This depolarization requires a resting membrane potential of about -70 millivolts, with the interior of the cell negatively charged. Two proteins are required to establish the resting membrane potential:

  • Na+ ATPase
  • Potassium leak channels
  • Na+ ATPase and potassium leak channels are the two proteins required to establish the resting membrane potential. These proteins are integral membrane proteins in the plasma membrane. The Na+K+ ATPase hydrolizes one ATP molecule to pumps three sodium ions out of the cell and two potassium ions into the cell. The Na+K+ ATPase uses ATP to drive transport against their gradient. This transport of ions against their gradient, while hydrolizing ATP, is an example of primary active transport. The result is a gradient with plentiful sodium outside the cell and plentiful potassium inside the cell.

    Potassium leak channels allow potassium ions, but no other ions, to flow down their gradient. Since Na+K+ ATPase causes an overabundance of potassium ions inside the cell, the potassium ions will flow through the potassium leak channels back outside the cell. Since potassium ions are positive, they will leave the interior of the cell with a net negative charge. The potential across the plasma membrane is about -70mV, the resting membrane potential.

    All cells have a resting membrane potential; neurons and muscle tissues are unique in using the resting membrane potential to generate action potentials. The flow of potassium out of the cell makes the interior of the cell more negatively charged. If the potassium leak channels were blocked, the interior of the cell would have a less negative (more positive) charge. If sodium ions were allowed to flow down their concentration gradient, they would flow into the cell and the interior of the cell would have a less negative (more positive) charge.

    The resting membrane potential establishes a negative charge along the interior of the axons along with the rest of the neuronal interior. An action potential is a disturbance in this membrane potential, a localized depolarization of the plasma membrane that travels in a wave-like manner along an axon. Depolarization is a change in the membrane potential from the resting membrane potential of approximately -70 mV to a less negative or even positive potential. The change in membrane potential during an action potential is caused by movement of ions into and out of the neuron through ion channels. The action potential is not strictly an electrical impulse, like electrons moving in a copper telephone wire, but an electrochemical impulse.

    A key protein in the propagation of action potentials are the voltage-gated soidum channels located in the plasma membrane of the axon. In response to a change in the membrane potential, these ion channels open to allow sodium ions to flow down a gradient into the cell and depolarize that section of membrane. Opening the voltage-gated sodium channels would allow sodium ions to flow into the cell (down the concentration gradient) and make the interior of the cell less negatively, or even positively, charged. These channels are opened by depolarization of the membrane from the resting potential of -70 mV to a threshold potential of approximately -50 mV. Once this threshold is reached, the channels are opened fully; below this threshold, though, they do not allow the pssage of any ions through the channel. When channels open, sodium flows into the cell (down the concentration gradient) and depolarizes that section of the membrane to about +35 mV before inativating. Some of the sodium ions flow down the interior of the axon, slightly depolarizing the neighboring sectino of the membrane. Whn the depolarization reaches -50 mV i the next section of membrane, those voltage-gated sodium channels open as well. This opening of more voltage-gated sodium channels passes the depolarization down the axon. Since action potentials are continually renewed at each point in the axon as they travel, action potentials can't run out of energy before reaching a synapse. Once they begin, they will not stop until that synapse is reahed.

    After depolarization, repolarization returns the membrane potential to normal.

    na+ channels open to gwenerate an action potential
    upstream na+ channels inactivate making membrane refractory...K_ channels open and axon repolarizes
    action potential jumps quickly to new node and contnues from node to node

    Steps in an Action Potential:
    1. At rest the outside of the membrane is more positive than the inside.
    2. Sodium moves inside the cell causing an action potential.
    3. The positive sodium ion influx makes the inside of the membrane more positive than the outside
    4. Potassium ions flow out of the cell, restoring the resting potential net charges.
    5. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions.
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