Lecture: Reaction Time and Action Potentials

Introduction to Reaction Time

• Popular lab exercise: catching a falling ruler
• Purpose: measure response time from stimulus to reaction
• Key Question: How do brain cells and muscle cells respond quickly?
• Answer: Action Potential

Action Potentials

• Found in excitable cells (neurons, skeletal muscle cells)
• Action Potential: Electrical signal that can be generated and transmitted
• Excitable Cells: Can generate electrical signals from a stimulus

Cellular Membranes and Ions

• Cell Membranes: Control ion movement in and out
• Ions: Charged particles; need proteins to pass through membranes
  • Sodium-Potassium Pump: Moves sodium (Na+) and potassium (K+) against gradients with ATP
  • Resting Membrane Potential: More positive ions outside than inside; inside of cell is more negative

Ion Channels and Membrane Potential

• Leak Channels: Allow passive ion movement
  • Sodium moves into the cell
  • Potassium moves out of the cell
• Membrane Potential: Electric potential difference between inside and outside of cell
  • Recording Microelectrode: Measures cell's internal charge vs. outside reference

Resting Membrane Potential

• Example of a Neuron: -70 mV at rest
• Polarization: Inside is negatively charged compared to outside

Process of Action Potential

1. Gated Sodium Channels Open: Sodium rushes in, depolarizing the cell
2. Threshold Level: At -55 mV, action potential is triggered (all-or-nothing)
3. Rising Phase: More sodium enters, membrane potential reaches +30 mV
4. Repolarization: Sodium channels inactivate, potassium channels open, potential returns to rest
5. Hyperpolarization: Membrane potential briefly goes below resting potential
6. Return to Rest: Sodium-potassium pump and closing of potassium channels restore resting potential

Types of Gated Ion Channels

• Ligand-Gated Ion Channels: Open upon ligand binding (e.g., neurotransmitter)
• Mechanically-Gated Ion Channels: Open in response to physical stimuli (e.g., touch)
• Voltage-Gated Ion Channels: Open in response to voltage changes

Propagation of Action Potentials

• Neurons: Action potentials spread along axons
• Refractory Period: Prevents restimulation, ensuring one-directional signal travel
• Myelinated Neurons: Propagation is different due to insulation

Conclusion

• Excitable cells rely on action potentials for many everyday functions
• Stay curious about the mechanisms behind physiological processes