Graded Potentials and Action Potentials

Graded Potentials

  • Definition: A graded potential occurs when chemically gated ion channels on the dendrites or the cell body open, allowing ions to flow in or out of the neuron.

Steps of Graded Potential

  1. Ion Channel Activation:

    • The opening of chemically gated ion channels permits sodium ions (Na⁺) to flow into the cell down their concentration gradient.

  2. Depolarization:

    • The influx of positive charge (sodium ions) causes depolarization of the neuronal membrane.

    • The extent of depolarization is correlated with the amount of sodium that flows in; the greater the sodium influx, the greater the depolarization.

  3. Electrical Current Formation:

    • The movement of positively charged sodium ions into the cell constitutes an electrical current.

    • Notably, current flows a certain distance before the sodium-potassium pumps (Na⁺/K⁺ ATPase) extrude sodium from the cell, leading to a decrease in current strength as distance from the initial stimulus increases.

  4. Voltage Decay:

    • In bare plasma membranes, voltage decays—this is because without voltage-gated channels, current leaks across the membrane, subsequently reducing the strength of the signal.

Action Potentials

  • Definition: Action potentials occur when a specific threshold is reached, a significant change in the membrane potential that propagates along the axon.

Initiating Action Potentials

  1. Threshold Requirement:

    • If electrical current (primarily Na⁺) reaches the axon hillock and changes the membrane potential from -70 mV to -55 mV, this triggers a large depolarization.

  2. Voltage-Gated Channels:

    • Ion channels in the axon hillock are voltage-gated.

    • Each gated sodium channel possesses two gates:

      • Activation Gate: Closes the channel when the membrane potential is at resting conditions.

      • Inactivation Gate: Blocks the channel from opening once it is activated.

    • Both gates must be open for sodium ions to enter the neuron.

  3. Potassium Channel Dynamics:

    • Gated potassium channels possess a single gate that opens and closes more slowly compared to sodium channels.

    • At resting potential, these channels remain closed, preventing potassium ions (K⁺) from exiting the cell until depolarization occurs.

Voltage Levels in Action Potentials

  • Resting Membrane Potential:

    • Approximately -70 mV.

  • Threshold Voltage:

    • Approximately -55 mV (the point at which action potentials are triggered).

  • Peak Depolarization:

    • Approximately +30 mV.

  • Hyperpolarization:

    • Occurs after depolarization, with voltage levels dropping below the resting potential, often around -78 mV.