Neural Communication and Action Potentials

Neuron Structure

• Dendrites: Receive incoming signals.
• Axon: Sends outgoing signals to the nerve terminal.

Electrical Signaling

• Nerve Impulses/Action Potentials:
  • Rapid communication along the axon.
  • Brief reversal of electric polarity across the cell membrane.

Resting Membrane Potential

• Cells are polarized with a typical voltage of -70mV.
• Concentration Gradients:
  • More sodium (Na+) outside, more potassium (K+) inside.
  • Maintained by the sodium-potassium pump.

Depolarization Process

• Stimulus at Dendrites:
  • Excitatory signals open ligand-gated sodium channels.
  • Sodium influx reduces negative charge inside (depolarization).
• Axon Hillock:
  • Known as the "trigger zone" for action potentials.
  • Action potentials initiated if the membrane reaches a threshold of -55mV.

Action Potential Phases

1. Rising Phase:
   • Rapid sodium influx causes further depolarization.
   • Positive feedback: More sodium channels open.
   • Polarity across the cell membrane reverses.
2. Falling Phase:
   • Sodium channels close; potassium channels fully open.
   • Potassium efflux returns voltage to resting value.
   • Causes hyper-polarization due to slow potassium gate closure.

Refractory Period

• Absolute Refractory Period:
  • From start of action potential to when voltage returns to resting value.
  • Sodium channels are inactivated, preventing new action potential.
• Relative Refractory Period:
  • Follows absolute; ends with hyper-polarization.
  • Some potassium channels remain open; stronger signal needed to depolarize.

Action Potential Propagation

• Sodium influx depolarizes adjacent membrane sections.
• Unidirectional Propagation:
  • Ensured by refractory properties of ion channels.
  • Only unfired patches respond with new action potentials.

Conclusion

• Action potentials travel from the axon hillock to the nerve terminal.
• Higher concentration of voltage-gated ion channels in the axon facilitates this directionality.