action potentials

• • 3 sodium ions out
  • 2 potassium ions in
  • leads to -60mv inside
• • not all neurones myelinated - could be short and thick
• • 1/3 of all peripheral neurones in vertebrates are myelinated. Myelin is a fatty white substance that is produced by schwann cells. schwann cells wrap themselves around axons and prevent movement of ions across the membrane of the neurone.
• • myelin sheath is not continuous and gaps exist (nodes of ranvier) means nerve impulse can only be conducted where there are gaps. This leads to nerve impulse 'jumping' from node to node - known as saltatory conduction.
• • The function is to insulate the neurone and increase the speed at which nerve impulses are conducted
• • unmyelinated neurones are not individually wrapped but are associated with myelin
  • myelin helps maintain the neurone and keep it alive
• an action potential is the brief reversal of the potential across the membrane of a neurone
• stimulus detected


  • there are also sodium channels which open when a stimulus such as touch is detected
  • sodium channels open and sodium ions enter at generator region. This is because there is an electrical and chemical gradient - the electrochemical gradient. This produces a generator potential and will either go no further or combine with other generator potentials to become a larger depolarisation. This will happen if the potential difference reaches a threshold of -50mv.
• action potential (depolarisation) 1


  • in addition to sodium-potassium pumps, there are voltage-gated sodium and potassium channels. They open and close depending on the potential difference (voltage). They are closed when the potential difference is -60mv (resting potential)
  • neighbouring voltage-gated sodium channels also open, causing more sodium to enter the cell and lead o depolarisation - positive feedback
• action potential (depolarisation) 2


  • inside of the cell because more positively charge because of the influx of sodium ions
  • this is known as depolarisation and causes an action potential (brief reversal of the potential across the membrane of a neurone). The potential difference reaches +40mv
• resting potential restored (following repolarisation) 1

  • soon after, sodium channels close and then the voltage-gated potassium channels open so potassium ions leave the cell. They diffuse down their electrochemical gradient.
  • This is called repolarisation - the charge across the membrane starts to return the normal.
• resting potential restored (following repolarisation) 2


  • this dip (hyperpolarisation) is caused because so many sodium and potassium ions are leaving. Sodium - potassium channels close and the sodium-potassium pumps restore the resting potential (pumps out sodium and brings in potassium ions). eventually, resting potential is restored
  • there is a refractory period, during which it is impossible to generate another action potential