Action Potentials and Membrane Potentials

Resting Membrane Potential

• Resting membrane potential is -70 millivolts. This means the inside of the cell is slightly more negative than the outside when the neuron is at rest (not sending an action potential).
• The membrane potential is not fixed; it can change. It can become:
  • Less negative (even positive)
  • More negative

Threshold

• -55 millivolts is a critical value called the threshold.
• If the membrane potential reaches -55 mV, an action potential is generated.
• A sufficient excitatory stimulus can shift the membrane potential to -55 mV, triggering an action potential.
• Action potentials are "all or nothing." If the threshold is reached, an action potential is sent; otherwise, it is not.

Ion Distribution and Voltage-Gated Channels

• Extracellular Fluid:
  • High sodium (Na+) concentration
• Intracellular Fluid:
  • Low sodium (Na+) concentration
  • High potassium (K+) concentration
• Voltage-Gated Channels:
  • These channels open in response to specific membrane potential voltages.
  • At -55 mV (threshold), voltage-gated sodium (Na+) channels open.

Depolarization Phase

• Opening of Sodium Channels:
  • At -55 mV, voltage-gated sodium channels open, allowing sodium to move into the cell.
  • Sodium influx stimulates more voltage-gated sodium channels to open (positive feedback).
  • Sodium rushes into the cell because of its positive charge, making the inside of the cell less negative.
  • The membrane potential rapidly increases towards zero and becomes positive (e.g., +30 mV). At this point, the inside of the cell is more positive than the outside.
• Membrane Permeability:
  • During depolarization, the membrane becomes much more permeable (leaky) to sodium.
  • This is because many sodium channels are open, allowing sodium to rush down its concentration gradient.
• Sodium channels close at +30mV. The permeability for sodium begins to drop.

Repolarization Phase

• Potassium (K+) Channels Open:
  • Voltage-gated potassium channels open as sodium channels close.
  • Potassium floods out of the cell down its concentration gradient.
• Membrane Potential Changes:
  • As positive potassium ions leave the cell, the inside becomes more negative again.
  • The membrane potential moves back towards the resting membrane potential (-70 mV).
• Membrane Permeability:
  • The membrane becomes more permeable to potassium as potassium channels open.

Hyperpolarization Phase

• Slow Closure of Potassium Channels:
  • Potassium channels are slow to close, resulting in too much potassium leaving the cell.
  • The membrane potential drops below the resting membrane potential (below -70 mV).
  • This makes the neuron further away from the threshold, reducing the likelihood of sending another action potential immediately.

Ion Restoration

• Sodium-Potassium Pump:
  • Restores the concentration gradients of sodium and potassium.
  • Pumps sodium (Na+) out of the cell and potassium (K+) into the cell using ATP.
  • Restores the membrane potential back to its resting state.

Action Potential Propagation

• The process occurs along the entire length of the axon.
• The influx of sodium at one location initiates the action potential and triggers the process in adjacent areas if the threshold is reached.

Ion Movement vs. Electron Movement

• Ions as Charged Particles:
  • Action potentials are generated by the movement of charged ions (sodium and potassium).
• Electrical Currents:
  • In neurons, electrical currents are generated by the movement of ions, not electrons like in household electrical circuits.