5.3.3

• Neurone at rest

  resting neurone - when it does not transmit an action potential however it is actually pumping ions across its plasma membrane

  • Na/K pump use ATP to pump 3 Na+ out of cell + 2 K+ into cell

  → gated ions are kept closed but some of the K + channels are open and therefore the plasma membrane is more permeable to K+ than Na+

  →K+ tends to diffuse out of cell
• Neurone at rest 2

  • cell cytoplasm also contains large organic anions + interior of cell is maintained at a negative potential compared with outside
  • cell membrane is said to be polarised

  → potential difference across cell membrane is about -60 mV (resting potential)
• Generating an action potential

  neurone is at rest + maintaining concentration of Na+ so concentration is higher on outside

  → equally the concentration of K+ is higher inside than outside

  • if some of the Na+ channels are open, they will quickly diffuse down concentration gradient towards surrounding tissue fluid (depolarisation)
  • in the generator region, the gated channels are opened by the action of synapse

  → when a few gated channels are open allow a few Na+ and causing a small depolarisation
• Generating an action potential 2

  • however when more gated channels open they also cause more small depolarisation + when combined it becomes a large depolarisation

  → if the depolarisation reaches a particular magnitude it passes a threshold + will cause an action potential

  • most of Na+ channels in a neurone are opened by changes in potential difference across the membrane = voltage-gated channels

  → when there are sufficient generator potentials to reach the threshold potential they cause the voltage-gated channels to open (example of positive feedback)
• Generating an action potential 3

  • the opening of voltage-gated Na+ channels allow a large influx of Na+ and depolarisation reaches +40mV inside cell

  → once reached, the neurone will transmit action potential; action potential is self-perpetuating because once it starts at one particular point in neurone it will continue along the end of neurone

  • ‘all or nothing’ action potentials because they all have the same magnitude (+40 mV )
• Stages of an action potential

  1. membrane starts in resting state - polarised compared to outside

  • high concentration of Na+ outside whilst K+ is inside

  1. Na+ channels open ~+ some Na+ diffuse into cell
  2. the membrane depolarises - it becomes less negative with respect to outside + reaches the threshold value of - 50mV
  3. positive feedback causes many voltage-based Na+ channels to open + many Na+ flood in making cell more positive
  4. potential difference across the plasma membrane reaches + 40 mV

  → inside of cell is positive compared to outside
• Stages of an action potential

  1. Na+ channels close + K+ channels open
  2. K+ diffuses out of cell bringing the potential difference + ,K+ diffuses out of cell bringing the potential difference potential difference to negative = repolarisation
  3. potential difference overshoot slightly to outside, making cell hyperpolarised
  4. original potential difference is restored so that cell returns to its resting state
• Refractory Period

  after an action potential the Na+ and K+ ions are in the wrong places

  • concentration of these ions inside + outside the cell must be restored by the action of Na/K pumps

  → for a short time after each action potential it is impossible to stimulate the plasma membrane to reach another action potential (refractory period)

  • allows cell to recover after an action potential

  → ensures that action potential are to be transmitted in only one direction
• neurone at rest

  
• ionic movements during an action potential