Action Potentials

Overview

• Action potentials are the primary method neurons use to send signals over long distances quickly.
• They occur briefly in muscles and axons of neurons.
• Represent a rapid change in membrane voltage, potentially by 100 millivolts.
• Unlike graded potentials, action potentials do not decay over time and distance.

Characteristics

• Regeneration: Action potentials are regenerated as they move, maintaining size and shape.
• Nerve Impulses: Term used for action potentials in neurons.
• Voltage Change: Caused by a change in current through specific voltage-gated channels.

Voltage-Gated Channels

• Resting State: All voltage-gated channels are closed.
  • Voltage-gated Sodium (Na+) Channels: Closed.
  • Voltage-gated Potassium (K+) Channels: Closed.
  • Leakage Channels: Open, with more K+ leak than Na+, contributing to resting membrane potential.
• Activation Gates: Closed at rest, open with depolarization allowing Na+ influx.
• Inactivation Gates: Open at rest, block Na+ entry once open, preventing more Na+ influx.

Key Players

• Voltage-Gated Sodium Channels:
  • Closed at rest; open with sufficient depolarization.
  • Allow Na+ influx, leading to further depolarization.
  • Inactivation gate closes after a short time to prevent excess Na+ influx.
• Voltage-Gated Potassium Channels:
  • Closed at rest; open during depolarization.
  • Allow K+ efflux, causing repolarization (voltage becomes more negative).

Events of Action Potential

1. Resting State:
   • Only leakage channels open.
   • K+ leakage keeps voltage negative.
2. Depolarization Phase:
   • Na+ channels open, Na+ influx occurs.
   • Voltage rises from -55 mV to +30 mV.
3. Peak and Repolarization:
   • Na+ channel inactivation gates close, stopping Na+ influx.
   • K+ channels open, K+ efflux occurs, voltage returns towards resting levels.
4. Hyperpolarization:
   • Excessive K+ efflux causes voltage to dip below resting potential.
   • Important for resetting Na+ channels for subsequent activation.

Importance of Sodium-Potassium Pumps

• Maintain concentration gradients essential for action potential.
• Do not directly contribute to action potential but sustain the necessary ionic conditions.
• Use ATP to pump Na+ out and K+ into the cell.

Summary

• Action potentials represent voltage changes, not ionic concentration changes.
• Sodium-potassium pumps ensure gradients are maintained for continued proper neuronal function.