2. Action potential

• When does action potential occur?

  When an impulse is being transmitted along an axon
• What is the value of action potential?

  +40mV
• How is action potential reached?

  • a stimulus causes a change in voltage, opening the voltage gated Na+ channel (membrane is permeable to Na+)
  • Na+ move into the axon rapidly by facilitated diffusion. This is DEPOLARISATION
  • Na+ move within the axon to the next node of Ranvier (this is a local circuit)
  • the presence of Na+ stimulates the opening of the voltage gated channel for Na+, so they move in
  • the action potential jumps along the axon from one node of Ranvier to the next, this is SALTATORY CONDUCTION
• Why are low level background stimuli filtered out?

  The amount of depolarisation that they cause does not achieve the threshold value (-55mV)
• If there is not enough depolarisation to reach threshold potential, no action potential is generated
• What is the all or nothing law?

  The value of action potential is always +40mV, regardless of the nature or size of the stimulus.
• A stronger stimulus can increase the frequency of an action potential
• What factors can affect the speed of conduction of an impulse along an axon?

  -myelination

  -temperature

  -axon diameter
• How does temperature affect the speed of conduction of an impulse?

  -increase in temperature increases the kinetic energy of molecules (e.g. Na+)

  -they move faster, so speed of conduction increases

  -but at higher temperatures, proteins denature (e.g. channel proteins), so speed decreases
• How does axon diameter affect the speed of conduction of an impulse?

  -increase in diameter increases speed of conduction

  -wider axons have less internal resistance, easier for Na+ to move

  -wider axons have a larger surface area, so more protein channels are present
• Summarise what happens during action potential.?

  -the axon membrane is depolarised

  -when sodium ion channels open, Na+ diffuse into the axoplasm

  -a stimulus causes Na+ to leak into the axoplasm

  -when threshold is reached (-55mV), all sodium ion channels open

  -if threshold is not reached, sodium ion leakage stops and no action potential is generated

  -the all or nothing law means that action potentials are always the same size