Neuronal Action Potentials and Signal Transmission

Structure of Neurons

• Neurons: Cells that make up the nervous system, composed of three main parts:
  • Dendrites: Branches that receive signals from other neurons.
  • Soma: The cell body that contains the nucleus and main organelles.
  • Axon: Intermittently wrapped in fatty myelin.

Signal Reception and Transmission

• Neurotransmitters: Chemicals that bind to receptors on dendrites to convert chemical signals into electrical ones.
• Ion Channels: When neurotransmitters bind, ion channels open allowing ions to flow, changing the cell's overall charge.
• Action Potential: If combined input from dendrites changes the cell charge enough, an action potential (electrical signal) is triggered, propagating down the axon at up to 100 meters/second.
• Neuron Communication: Uses neurotransmitters to communicate with other neurons and action potentials to propagate signals within the cell.
• Long Neurons: Important for neurons spanning from the spinal cord to the toes.

Resting Membrane Potential

• Ion Concentration: Different ions inside vs. outside the cell create an electric charge:
  • More Na+, Cl-, and Ca2+ outside.
  • More K+ and negatively charged anions (A-) inside.
• Net Charge: Inside of the cell has a net negative charge (~-65 mV) relative to the outside.

Steps of Action Potential

Depolarization

• Ligand-Gated Ion Channels: Neurotransmitters open these channels, altering ion flow.
• Example: Na+ channels open, Na+ flows in, reducing the cell's negative charge (depolarization).
• EPSPs: If there’s a net influx of positive charge, it’s called an excitatory postsynaptic potential.
• IPSPs: Net influx of negative charge (e.g., Cl- channels) creates inhibitory postsynaptic potential.

Threshold and Propagation

• Threshold Value: Enough EPSPs push membrane potential to ~-55 mV, triggering action potential.
• Voltage-Gated Na+ Channels: Open at axon hillock, Na+ rushes in, setting off a chain reaction down the axon.
• Neuron 'Firing': Cell becomes positively charged (~+40 mV).
• Inactivation: Na+ channels stop Na+ influx.

Repolarization and Hyperpolarization

• Voltage-Gated K+ Channels: Open after Na+ channels, K+ flows out, repolarizing the cell.
• Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ in to help repolarize.
• Absolute Refractory Period: Na+ channels are inactivated; no response to stimuli, preventing action potentials from happening too close together and ensuring one-direction propagation.
• Hyperpolarization: Overcorrection due to K+ outflow.
• Relative Refractory Period: Na+ channels can be activated, but require strong stimulus due to hyperpolarized state.
• Return to Resting Potential: All channels close, and the neuron returns to ~-65mV.

Role of Myelin and Nodes of Ranvier

• Speed of Transmission: Myelin from glial cells speeds up signal transmission.
• Nodes of Ranvier: Gaps in myelin where ion channels are located.
• Saltatory Conduction: Action potential appears to 'jump' from node to node, speeding up signal propagation.

Summary

• Neuronal Communication: Dendrites receive signals, which can lead to action potentials if thresholds are met.
• Propagation Mechanism: Depolarization through Na+ channels, repolarization via K+ channels and sodium-potassium pump, including refractory periods.
• Myelination: Enhances speed of signal transmission through saltatory conduction.

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