Nerve structure

• some neurons are myelinated , their axons are coated in myelin sheath. made of Schwann cells. between Schwann cells are called nodes of Ranvier.
• (1) stimulus:

  • neurone cell membrane is excited
  • NA+ ion channels open
  • membrane is more permeable to Na+ ions , so they diffuse down the gradient.
  • membrane becomes less negative
• (2) Depolarisation:

  • if a neurone is stimulated and the generator potential is large enough to reach the threshold.
  • Voltage-gated Na+ ion channels open
  • Even more Na+ ions flood into the cell making more positive than outside the cell.
• (3) Repolarization:

  • When the potential difference of the membrane reaches +30mv the Na+ ions channels close and the K+ channels open.
  • The membrane is more permeable to K+ ions , so they diffuse out of the cell down their gradient.
  • This starts to bring membrane back to resting potential.
• (4) Hyperpolarization:

  • K+ ions channels are too slow to close so the inside of the axon has too many K+ ions and cannot diffuse out of neurone.
  • the membrane can become negative than usual resting potential.
• Refractory period:

  • time period after when its impossible for further action potential to be generated.
  • Membrane is impermeable to Na+ ions channels are closed during hyperpolarization.
  • Act as a time delay between each action potential , makes sure they dont overlap.
• (5) Resting potential:

  • when a neurone is at rest, there is a gradient as there are more Na+ ions outside the cell than inside
  • in total more positive ions move out than in so inside the cell remains more negatively charged than the outside.
• Myelination:

  in a myelinated neurone depolarisation only occurs at the node of Ranvier , the sodium ions diffuse through the cytoplasm and depolarize the membrane at the next node so that the impulse jumps from node to node. this is known as saltatory conduction.