Action Potentials - Key Vocabulary

Action Potentials

Neuron Function and Signaling

  • Neurons change membrane potential via:
    • Action potentials (within neurons)
    • Graded/local/postsynaptic potentials (between neurons)

Neuron Structure

  • Dendrites: Receive inputs.
  • Cell body: Integrates inputs.
  • Axon: Sends output signals and generates action potentials.

Membrane Potential Changes

  • Depolarization: Membrane becomes more positive (usually due to Na^+ influx).
  • Repolarization: Return to resting membrane potential.
  • Hyperpolarization: Membrane becomes more negative (usually due to K^+ efflux).

Graded Potentials

  • Initial trigger for membrane potential change.
  • Variable in size.

Action Potentials

  • Triggered by a sufficiently large graded potential (threshold ~ -50 mV).
  • Initiated in the axon hillock.
  • "All or nothing" - magnitude is independent of graded potential size.
  • Fast (1-2 ms) and travel long distances without diminishing.

Ion Flow in Action Potentials

  • Voltage-gated Na^+ and K^+ channels mediate ion flow.

Voltage-Gated Channels

  • Na^+ channels:
    • 3 states: closed, open, inactivated.
    • Opening triggered by depolarization, return to closed via repolarization.
    • Open and close quickly.
  • K^+ channels:
    • 2 states: closed, open.
    • Opening triggered by depolarization, return to closed via repolarization.
    • Open and close slowly.

Positive Feedback of Sodium Channels

  • Depolarization leads to Na^+ influx, further decreasing membrane potential and opening more voltage-gated Na^+ channels.

Action Potential Propagation

  • Action potentials depolarize the membrane as Na^+ flows in.
  • Electrical current flows to adjacent axon part, depolarizing it.
  • Na^+ channels open, triggering another action potential.

Refractory Periods

  • Absolute: No further action potentials can be generated (Na+ channels are inactivated).
  • Relative: An action potential can be generated with a large stimulus (some Na^+ channels are closed).
  • Limit firing frequency and ensure unidirectional travel.

Factors Affecting Propagation Speed

  1. Axon Diameter: Thicker axons = faster propagation.
  2. Temperature: Higher temperature = faster reaction.
  3. Myelination: Insulates membrane, speeds up propagation (saltatory conduction).

Saltatory Conduction

  • Voltage-gated ion channels at nodes of Ranvier.
  • Action potentials "jump" between nodes.
  • Faster than non-myelinated fibers.

Multiple Sclerosis

  • Immune system attacks myelin.
  • Impulse conduction slows/ceases.

Action Potential Information Conveyance

  • Signaled through firing frequency.
  • High frequency = strong stimulus
  • Low frequency = weaker stimulus