Repolarization and refractory period

• Nerve impulses one direction diagram

  
• Very shortly after an action potential in a section of axon membrane is generated, all the sodium ion voltage-gated channel proteins in the section close
• Stopping any further sodium ions diffusing into the axon
• Potassium ion voltage-gated channel proteins

  Proteins that allow the diffusion of potassium ions out of the axon
• Potassium ion voltage-gated channel proteins in this section of axon membrane are now open allowing the diffusion of potassium ions out of the axon, down their concentration gradient
• Repolarization

  The process that returns the potential difference to normal (about ~70V)
• There is a short period of hyperpolarization
• Hyperpolarization

  When the potential difference across section of axon membrane briefly becomes more negative than normal resting potential
• Potassium ion voltage-gated channel proteins then close
• The sodium ion channel proteins in this section of the membrane become responsive to depolarization again
• Until this occurs, this section of membrane is in a period of recovery and is unresponsive
• Refractory period

  The period when the membrane is unresponsive
• Membrane potential changes during an impulse

  
• Refractory period

  Important for ensuring discrete action potentials, preventing merging, and allowing efficient nerve impulse transmission
• Reasons for the importance of the refractory period

  • Ensures action potentials are discrete events
  • Ensures changes in membrane potential are generated ahead
  • Means impulses can only travel in one direction
  • Means there is a minimum time between action potentials
  • Determines maximum frequency of impulses transmitted
• The refractory period ensures that action potentials are discrete events, stopping them from merging into one another
• The refractory period ensures that changes in membrane potential are generated ahead rather than behind original action depolarization
• The region behind the original action depolarization is recovering from repolarization that has just occurred
• The refractory period means that impulses can only travel in one direction, which is essential for the successful and efficient transmission of nerve impulses along neurons
• The refractory period also means there is a minimum time between action potentials occurring at any one place along a neuron
• The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurons (500 – 1000 per second)