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