Video 3.2 The Action Potential - Tagged

Exam Overview

• Covers: All material from Weeks 1 to 3 including:

  • Video lectures

  • Packets, worksheets, quizzes

  • No surprises from the textbook

• Key Topics:

  • Homeostasis: Negative and Positive Feedback

  • Basic Chemistry: Notes, PowerPoints, reading packets

  • Cell Structure: Molecules of life, plasma membrane (lipid bilayer)

  • Tonicity, osmosis, cell transport

  • Nervous System: Nerve Cell Packet, Membrane Transport, RMP, Action Potential, basic function and structure, neuronal pathway

Electrochemical Gradients and Neuron Function

Sodium Potassium Exchange Pump

• Function: Sodium-Potassium ATPase; maintains/restores resting conditions

• Mechanism: 2 K+ ions are moved in, while 3 Na+ ions are moved out.

• Importance: Constantly combats leak channels in the neuronal cell membrane.

Resting Membrane Potential (RMP)

• Intracellular and Extracellular fluid environments:

  • ICF: High Na+ ions, Low K+, more positive environment

  • ECF: Low Na+, High K+, Anions (Proteins), more negative environment

• Diffusion:

  • Na+ moves into the cell due to both chemical and electrical gradients.

  • K+ moves out of the cell, with the chemical gradient being stronger than the electrical gradient.

Graded Potentials and Action Potentials

• Neuron Communication: Graded potentials must sum to reach the threshold of -60 to -55 mV at the axon hillock to initiate an action potential.

Action Potential Characteristics

• All-or-None Principle: Once the threshold is reached, the response is consistent (can't be stopped).

• Continuous activation of voltage-gated channels results in a chain reaction along the axon.

• Refractory Period: Follows action potential, during which the neuron cannot fire.

Ion Movements During Action Potential

Potassium (K+)

• Chemical Gradient: High concentration of K+ inside the cell drives diffusion out.

• Electrical Gradient: Positive charge outside the cell drives K+ into the cell.

• Net movement of K+ during resting is out of the cell, but influenced by current potential.

Sodium (Na+)

• Chemical Gradient: High concentration outside the cell pushes Na+ into the cell.

• Electrical Gradient: Positive charge outside assists in Na+ moving into the cell.

• Net Effect: Sodium moves into the cell during action potential generation.

Drugs and Neuron Function

• Example: Carvidelol

  • Inhibits K+ leak channels in heart, leads to increased action potentials.

  • More positive intracellular environment.

• Example: Lidocaine

  • Binds to voltage-gated Na+ channels, preventing Na+ flow and thereby inhibiting action potentials.

Action Potential Generation Steps

1. Threshold Reached: Graded potential triggers voltage-gated sodium channels (at -60 to -55 mV).

2. Depolarization: Sodium rushes into the cell, changing the membrane potential to more positive values.

3. Repolarization: Sodium channels inactivate, while potassium channels open allowing K+ to exit the cell.

4. Hyperpolarization: Overshoot occurs as K+ channels close slowly after repolarization, briefly making the cell more negative.

5. Return to Resting Potential: Sodium-potassium pump and leak channels stabilize the membrane potential back to ~-70 mV.

Neuronal Communication

• Graded Potentials: Must reach a specific threshold at hillock to trigger action.

• Action Potential Visual Representation: Shows changes in membrane potential across time during an action potential, illustrating depolarization and repolarization phases.