10 Minute Neuroscience: Action Potentials

Introduction

• Action Potentials: Electrical impulses in neurons that lead to neurotransmitter release.
• Essential for neural communication and nervous system function.

Neuron at Rest

• Cell Membrane: Separates intracellular from extracellular environments.
• Ions: Charged particles in intracellular and extracellular fluids.
  • Sodium Ions (Na⁺): Higher concentration outside the neuron.
  • Potassium Ions (K⁺): Higher concentration inside the neuron.
• Ion Channels: Allow ions to pass through the cell membrane.
  • Leak Channels: Always open; more potassium than sodium leak channels.
• Sodium-Potassium Pump: Enzyme that pumps Na⁺ out and K⁺ in.
  • Maintains higher K⁺ inside and higher Na⁺ outside.
• Diffusion and Electrostatic Forces:
  • Diffusion: Movement from high to low concentration.
  • Electrostatic Forces: Attraction/repulsion between charged particles.
• Membrane Potential: Difference in electrical charge across the membrane.
  • Resting Membrane Potential: ~ -70 millivolts.

Action Potential Initiation

• Depolarization: Membrane potential becomes less negative, closer to 0.
  • Caused by influx of positively charged ions due to neurotransmitter binding.
• Threshold: Point at which depolarization triggers an action potential.
• Voltage-Gated Ion Channels:
  • Open in response to changes in membrane potential.
  • Sodium Channels: Allow Na⁺ to rush into the cell, further depolarizing it.

Rising Phase and Peak of Action Potential

• Rapid depolarization as more Na⁺ enters the cell.
• Membrane potential may reach around +40 millivolts.
• Signal moves down the axon.

Repolarization and Ending

• Voltage-Gated Potassium Channels: Open to allow K⁺ to exit, repolarizing the neuron.
• Potassium Leak Channels: Also contribute to K⁺ exiting.
• The sodium-potassium pump helps restore ionic balance.

Propagation of Action Potential

• Axon: Long extension where the action potential travels.
• Myelin Sheath: Insulates axon, increasing speed and efficiency.
  • Nodes of Ranvier: Gaps in myelin, rich in voltage-gated sodium channels.
  • Saltatory Conduction: Action potential appears to "jump" from node to node.

Refractory Periods

• Absolute Refractory Period: Sodium channels unresponsive; no new action potential.
• Relative Refractory Period: Neuron hyperpolarized; stronger stimulus needed for action potential.

All-Or-None Law

• Action potentials are consistent in size; stimulus intensity affects firing frequency.

Conclusion

• Neurons can fire many action potentials per second, even with refractory periods.
• Action potentials are crucial for neural communication.

• Summary: This lecture covered the initiation, propagation, and termination of action potentials, highlighting key components such as ion channels, sodium-potassium pumps, and the role of myelin in speeding conduction. The principles of refractory periods and the all-or-none law were also discussed.
• Thanks for watching!