Understanding Graded and Action Potentials

Sep 5, 2024

Review flashcards

Graded Potentials and Action Potentials

Graded Potentials

  • Stimulus can cause a small change in membrane potential.
  • Excitatory Stimulus
    • Indicated by a green arrow and causes depolarization.
  • Equilibrium Potentials
    • Green line: Sodium’s equilibrium potential.
    • Purple dashed line: Potassium’s equilibrium potential.
    • Blue dashed line: Threshold.

Threshold and Voltage-Gated Channels

  • If a stimulus changes membrane potential to threshold, new channels open.
  • Voltage-Gated Channels
    • Open at specific voltages, allowing ions to move.

Action Potentials

  • Generated when graded potentials bring membrane potential to threshold.
  • Characteristics
    • Larger than graded potentials.
    • Driven by changes in voltage-gated channels.
    • All-or-none: same size, always excitatory, and do not weaken.
    • Propagated in one direction.

Sodium and Potassium Channels

  • Voltage-Gated Sodium Channels
    • Have activation and inactivation gates.
    • Open (activation gate opens) at threshold, sodium moves in.
    • Inactivation gate closes after ~1 ms, stopping sodium entry.
    • Three conformations:
      • Closed but capable of opening.
      • Open.
      • Inactivated (closed and incapable of opening).
  • Voltage-Gated Potassium Channels
    • Open slower than sodium channels.
    • Potassium moves out, causing repolarization and hyperpolarization.

Membrane Potential Restoration

  • Potassium leaving the cell makes it more negative.
  • Sodium-potassium pump and leak channels restore resting membrane potential (-70 mV).

Refractory Periods

  • Absolute Refractory Period
    • No new action potential can occur.
    • Corresponds to open and inactivated sodium channel states.
  • Relative Refractory Period
    • Stronger than normal stimulus may trigger an action potential.
    • Occurs during hyperpolarization.

Action Potential Propagation

  • Unidirectional due to refractory periods.
  • Axon Hillock
    • High density of voltage-gated sodium channels.
    • Action potential begins here and propagates down the axon.
    • Propagation is prevented in backward direction by absolute refractory period.