Action potential

• ACTION POTENTIAL

  • key to communication in the nervous system
  • excitable cells allow sensing of the environment and responds to it --> uses AP
  • AP --> transient reversal of membrane potential

  1. duration --> few ms in nerve and skeletal muscle + few hunder ms in the heart

  
• ALL OR NOTHING

  • small stimulus causes a brief stimulus but nothing actually happens --> graded potential (not a fixed size)
  • if it goes about -55mn then action potentials occur
  • rising phase dependent on activity of voltage-gated sodium channels (open)
  • when peak is reached --> voltage-gated sodium channels close and voltage-gated potassium channels open
  • falling phase --> when potassium channels open + will overshoot
  • depolarisation --> increasing chances of sodium channels to be open
  • cardiac muscle --> 200nm
  • skeletal muscle --> 5nm
  • neuronal --> 2ms

  
• ION CHANNELS IN THE ACTION POTENTIAL

  • rapid opening of sodium channels when stimulus is applied until the peak

  1. sodium channels are open
  2. most sodium channels are open at the peak
  3. when peak starts to fall --> sodium channels close (inactivate)

  • rapid opening of potassium channels a little after the peak
  • potassium channels close at relative refractory period
  • sodium channels have an activation gets and inactivation gate

  1. activate because voltage sensors are moving and opens the gate
  2. at positive potentials --> the inactivation gates swing up and close the gate
• ION CHANNELS IN THE ACTION POTENTIAL

  • potassium channels activate more slowly and also inactivate
  • positive feedback loop in the AP

  1. depolarisation depolarise the membrane --> open sodium channels
  2. sodium channels inactivation help control the process

  • negative feedback loop with potassium channels

  1. membrane repolarises --> were and fewer potassium channels are opening

  
• REFRACTORY PERIOD

  • absolute refractory period --> between the beginning and end of the peak --> can't produce another AP
  • relative refractory period --> cell is less excitable and requires a larger stimulus
  • due to inactivation of sodium current and activation of potassium current
• DRAVET SYNDROME

  • rare inheritable childhood epilepsy disease
  • produces serious seizures
  • difficult to treat with conventional anticonvulsant drugs
  • caused by mutation of nav1.1 sodium channels --> prevents action potentials in inhibitory neurons --> nervous system have too little inhibitory transmission
  • cannabinoids help in dravet syndrome

  1. cod-based drugs have works
  2. the works better
• PROPOGATION OF THE ACTION POTENTIAL

  • spread of charge allows movement of charge to the next segment

  1. can't move back as the ones before and in refractory period
  2. directional movement

  • long piece of cell --> axon
  • in heart --> can spread through via the conduction system in an coordinated way

  1. different to neuronal action potential