Lecture Notes: Understanding Action Potential

Introduction to Action Potential

• Action potential is a key concept in neuroscience related to nerve signal transmission.
• Neurons send signals via action potentials, which are electrical chemical signals.
• These involve ions (charged atoms/elements) moving in/out of neurons.

Key Ions in Neurons

• Sodium (Na⁺): Positively charged, primarily outside the neuron.
• Potassium (K⁺): Positively charged, primarily inside the neuron.
• Other ions like Chloride (Cl⁻), Magnesium (Mg²⁺) are mentioned but not the focus.

Resting Membrane Potential

• Excitable tissues (muscles, neurons) have a resting membrane potential.
• For neurons, inside is negatively charged compared to outside.
• Typical resting potential: ~ -70 millivolts.

Initiation of Action Potential

• Excitation: Requires neurotransmitters like glutamate binding to receptors.
• Sodium Influx: Glutamate opens receptors allowing Na⁺ to enter neuron, increasing positive charge inside.
• If enough Na⁺ enters to reach -55 millivolts (threshold), a voltage-gated sodium channel opens.

Propagation of Action Potential

• Voltage-Gated Sodium Channels: Open sequentially, allowing Na⁺ influx.
• Results in a domino effect, propagating the electrical signal.
• Spikes graphically from resting potential to +30 millivolts.

Repolarization

• At +30 millivolts, sodium channels close and potassium channels open.
• Potassium Efflux: K⁺ exits the neuron, lowering internal charge back to negative.
• Hyperpolarization occurs as potential drops below -70 millivolts to ~-90 millivolts.

Hyperpolarization and Implications

• Prevents immediate re-excitation of neurons.
• In epilepsy, lack of hyperpolarization leads to repeated neuron firing.

Phases of Action Potential

• Depolarization: Na⁺ enters, reducing polarity (more positive inside).
• Repolarization: K⁺ exits, restoring negative internal charge.
• Hyperpolarization: Excess K⁺ exits, creating a more negative internal environment.

Resetting the Neuron

• Membrane embedded pumps restore original ion positions.
• Na⁺/K⁺ Pump: Exchanges 3 Na⁺ out for 2 K⁺ in.
• Resets the neuron for subsequent action potentials.

This action potential process is crucial for understanding how neurons communicate electrically and how various neurological conditions might arise due to malfunctions in this signaling pathway.