Lecture on Neurons and Action Potentials

Structure of Neurons

• Neurons: Cells that make up the nervous system.
• Main Parts of Neurons:
  • Dendrites: Branch-like structures that receive signals from other neurons.
  • Soma (Cell Body): Contains the neuron’s main organelles like the nucleus.
  • Axon: Often wrapped in fatty myelin; transmits electrical signals.

Signal Transmission

• Neurotransmitters: Chemical signals that bind to receptors on dendrites.
• Ion Channels: Open to allow ions in/out, converting chemical signals to electrical signals.
• Action Potential: Electrical signal triggered if enough dendrites change the cell's charge.
  • Travels down the axon, reaching speeds of up to 100 meters/second.
• Neuron Communication: Neurotransmitters relay signals; action potentials propagate within the cell.

Ion Distribution & Resting Membrane Potential

• Ion Concentrations:
  • Outside: More Na+ (sodium), Cl- (chloride), and Ca2+ (calcium).
  • Inside: More K+ (potassium) and A- (negatively charged anions).
• Resting Membrane Potential: Net negative charge (~-65mV) inside the cell.

Mechanism of Action Potential

• Ligand-Gated Ion Channels: Open in response to neurotransmitter binding.
  • E.g., Na+ channels allow Na+ to flow in, causing depolarization.
• EPSP (Excitatory Postsynaptic Potential): Net influx of positive charge.
• IPSP (Inhibitory Postsynaptic Potential): Net influx of negative charge.
• Threshold: If membrane potential reaches ~-55mV, voltage-gated Na+ channels open at the axon hillock.
• Depolarization: Sodium influx makes the cell positive (~+40mV).
  • Inactivation Gate: Blocks Na+ influx soon after depolarization.
• Repolarization:
  • Voltage-Gated K+ Channels: Open, allowing K+ out.
  • Sodium-potassium pump helps restore resting potential.
  • Results in hyperpolarization and refractory periods (absolute and relative).

Myelin and Signal Propagation

• Myelin: Fatty layer from glial cells (Schwann cells or oligodendrocytes).
• Nodes of Ranvier: Gaps between myelin where ion channels are present.
• Saltatory Conduction: Electrical signal “jumps” from node to node, speeding up propagation.

Summary

• Action Potential: Happens when EPSPs open voltage-gated Na+ channels.
• Propagation: Rapid depolarization transmits electrical charge down the axon.
• Myelination: Increases the speed of signal transmission via saltatory conduction.

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