Lecture Notes: Action Potential and Ion Channels

Overview of Action Potential

• Brief change in electrical potential (difference in charge between inside and outside of the cell).
• Begins at a negative resting membrane potential, becomes positive, then returns to resting state.
• Propagates from axon hillock to presynaptic terminal, releasing neurotransmitters.
• Driven by ion channels in the membrane.

Types of Ion Channels

Non-Gated (Leak) Channels

• Discussed in previous lessons; involved in resting and postsynaptic potentials.

Voltage-Gated Ion Channels

• Located at axon hillock, along axon, and terminal.
• Necessary for action potential propagation.
• Open when membrane potential reaches a specific threshold value.

Mechanism of Action Potential

• Threshold: EPSPs summate causing depolarization; reaching threshold opens voltage-gated channels.
• Rising Phase: Depolarization due to sodium influx through open voltage-gated sodium channels.
• Falling Phase: Inactivation of sodium channels and opening of potassium channels; potassium efflux repolarizes the cell.
• Undershoot: Membrane potential hyperpolarizes past resting potential.
• Return to Resting Potential: Sodium channels de-inactivate and close; potassium channels eventually close.
• Sodium-Potassium Pump: Reestablishes ionic concentrations post-action potential.

Refractory Periods

• Absolute Refractory Period: No second action potential possible; sodium channels are open or inactivated.
• Relative Refractory Period: Action potential possible with stronger stimulus; potassium channels still open causing hyperpolarization.

Characteristics of Action Potentials

• Consistent within a neuron; varies between different neurons or altered environments.
• Stimulus strength encoded by frequency of firing, not action potential height.

Propagation of Action Potential

• Sodium influx depolarizes adjacent axon segments, moving action potential forward.
• Directionality: Moves from cell body to presynaptic terminal due to refractory period.

Myelination and Conduction Speed

• Myelinated Axons: Action potential "jumps" between Nodes of Ranvier (saltatory conduction).
• Unmyelinated Axons: Continuous wave propagation.
• Myelination prevents charge loss, increasing speed.
• Axon Diameter: Larger diameter reduces resistance, increasing action potential speed.

Conclusion

• Understanding the mechanisms of action potentials and the roles of different ion channels is crucial for explaining how neurons communicate and process information.