Action Potential

Key Concepts

• Action Potential: An electrical signal generated by excitable cells (neurons and skeletal muscle cells).
• Excitable Cells: Cells capable of generating an electrical signal in response to a stimulus.
• Resting Membrane Potential: The difference in electrical potential between the inside and outside of a cell at rest (typically -70 mV in neurons).
• Depolarization: A change in a cell's membrane potential, making it less negative (more positive).
• Hyperpolarization: A change in a cell's membrane potential, making it more negative than the resting potential.
• Threshold: The minimum level of depolarization required to trigger an action potential (approximately -55 mV).
• Gated Ion Channels: Protein channels in the cell membrane that open or close in response to specific stimuli (ligands, mechanical forces, or voltage changes).
• Refractory Period: A brief period after an action potential during which the neuron cannot be restimulated.
• Propagation: The spread of an action potential along the axon of a neuron.

I. Introduction: Reaction Time and Action Potentials

• The video begins by referencing the common "catching a falling ruler" experiment to illustrate reaction time.
• The question is posed: How do brain and muscle cells respond so quickly to stimuli?
• The answer lies in action potentials, electrical signals generated by excitable cells.
• Excitable cells, including neurons and skeletal muscle cells, are crucial for rapid responses.

II. Background: Cell Membranes and Ion Transport

• All cells have membranes that control the movement of ions (charged particles) in and out.
• Ions typically require protein channels for passive flow or protein pumps for active transport against their concentration gradients.
• The sodium-potassium pump is essential for establishing the resting membrane potential.
  • It uses ATP to move 3 sodium ions (Na+) out of the cell and 2 potassium ions (K+) into the cell.
  • This creates a higher concentration of positive ions outside the cell.
  • Combined with negatively charged ions and proteins inside the cell, this results in a negative charge inside relative to the outside.
• Even at rest, some ion channels are "leaky," allowing for passive movement of sodium into the cell and potassium out of the cell, following their concentration gradients.
• The sodium-potassium pump and leaky channels work together to maintain the resting membrane potential.

III. Membrane Potential: Measuring Electrical Differences

• Membrane potential is the difference in electrical potential between the inside and outside of the cell.
• It can be measured using a recording microelectrode inside the cell and a reference electrode outside.
• In a neuron, the resting membrane potential is typically -70 mV, meaning the inside is 70 millivolts more negative than the outside.

IV. The Action Potential: A Step-by-Step Process

• The video uses a graph with membrane potential (mV) on the Y-axis and time on the X-axis to illustrate the changes during an action potential.
• Polarization: The cell is initially polarized at its resting membrane potential (-70 mV).
• Depolarization:
  • Gated sodium channels open, allowing sodium ions (Na+) to rush into the cell, following their concentration gradient.
  • The influx of positive sodium ions makes the cell more positive, reducing the difference in electrical potential.
• Threshold:
  • If the membrane potential reaches a threshold level (approximately -55 mV), an action potential is triggered.
  • The action potential is "all or nothing" – it either occurs fully or not at all.
• Rising Phase:
  • Depolarization causes most sodium channels to open, leading to a rapid influx of sodium.
  • The membrane potential reaches 0 mV and continues to become positive, reaching around +30 mV.
• Repolarization:
  • At approximately +30 mV, sodium channels become inactivated, preventing further sodium entry.
  • Gated potassium channels open, allowing potassium ions (K+) to flow out of the cell, following their concentration gradient.
  • The outflow of positive potassium ions makes the cell more negative, returning it towards the resting potential.
• Hyperpolarization:
  • The membrane potential briefly goes below the resting potential (-70 mV) due to the continued outflow of potassium ions.
• Return to Resting Potential:
  • Gated potassium channels close.
  • The sodium-potassium pump restores the resting membrane potential by actively transporting sodium and potassium ions.

V. Gated Ion Channels: Types and Function

• Gated ion channels are crucial for initiating and propagating action potentials.
• Ligand-gated ion channels: Open when a specific ligand (signal molecule, e.g., neurotransmitter) binds to the channel protein.
  • Example: Neurotransmitters released from a neuron bind to ligand-gated ion channels on a muscle cell, causing them to open and allowing ions to rush in.
• Mechanically-gated ion channels: Open in response to a physical stimulus (e.g., touch).
  • Example: Touching the skin can signal mechanically-gated ion channels on a sensory neuron to open.
• Voltage-gated ion channels: Open or close in response to changes in membrane potential.
  • These are key players in the action potential events.
• Dysfunction of gated ion channels can lead to medical conditions like epilepsy, making them potential targets for medication.

VI. Propagation of Action Potentials

• Action potentials need to spread along the axon of a neuron.
• Depolarization in one segment of the axon reaches the threshold, triggering an action potential.
• The action potential in that segment causes a change in voltage that triggers voltage-gated sodium channels in the neighboring region to open, depolarizing that region.
• Meanwhile, the previous region repolarizes and enters a refractory period, during which it cannot be restimulated.
  • The refractory period is due to the inactivation of sodium channels.
  • It ensures that the action potential moves forward in one direction and sets a maximum firing rate for the neuron.
• The action potential travels along the neuron, with old segments repolarizing and eventually reaching resting membrane potential.
• Myelination: Many neurons are insulated with myelin, which affects the speed and efficiency of action potential propagation (see further reading).

VII. Conclusion

• Action potentials are essential for many daily activities, including movement, thinking, and catching falling rulers.
• Excitable cells rely on action potentials to function.
• The video encourages viewers to "stay curious."