Depolarisation

• When do neurone cell membranes become depolarised?

  When they are stimulated
• What does a stimulus trigger?

  A stimulus triggers sodium ion channels to open
• What will happen if the stimulus is big enough?


  It will trigger a rapid change in potential difference

  The sequence of events is known as an ACTION POTENTIAL
• What are the steps of an action potential?

  1. Stimulus
  2. Depolarisation
  3. Repolarisation
  4. Hyperpolarisation
  5. Resting Potential
• Describe the first step of an action potential: stimulus?


  This excites the neurone cell membrane, causing sodium ion channels to open

  The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient

  • THIS MAKES INSIDE OF NEURONE LESS -IVE
• Describe the SECOND step of an action potential: DEPOLARISATION?


  If potential difference reaches the threshold, more sodium ion channels open. More sodium ions diffuse rapidly into the neurone
• What is the threshold for step 2- depolarisation?

  around -55mV
• Describe the third step of an action potential: REPOLARISATION?


  At a potential difference of around +30mV the sodium ion channels close & potassium ion channels open

  The membrane = more permeable to potassium = potassium ions diffuse out the neurone down the potassium ion conc gradient

  • This starts getting the membrane back to its resting potential
• Describe step 4 of an action potential: Hyperpolarisation?


  Potassium ion channels are slow to close so there's a slight 'overshoot' where too many potassium ions diffuse out of the neurone

  The potential difference becomes more -ive than the resting potential (less than -70mV)
• Describe step 5 of an action potential: RESTING POTENTIAL


  The ion channels are reset

  The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane is excited by another stimulus
• What can not happen after an action potential?


  The neurone cell membrane cannot be excited straight away
• Why is it that, after an action potential, The neurone cell membrane cannot be excited straight away?

  The ion channels are recovering & they cannot be made to open, sodium ion channels are closed during REPOLARISATION and potassium ion channels are closed during hyperpolarisation .

  This period of recovery is called the REFRACTORY PERIOD.
• when are sodium ion channels closed?


  during repolarisation
• when are potassium ion channels closed?


  during hyperpolarisation