Week 2: Action Potentials

Week 2: Action Potentials

1. If the membrane becomes freely permeable to either K+ or Na+, the membrane potential quickly moves close to equilibrium potential for ion

2. Variable Permeability

   • Membrane potential of a nerve cell can range from potassium equilibrium potential to the sodium equilibrium potential just by varying its conductance to K+ and Na+ ions

3. Voltage-Sensitive Ion Channels

   • Action Potential is generated and regenerated on basis of opening and closing of specialsied Na+ and K+ ion channels
   • Na+ and K+ channels responsible for it are called 'voltage sensitive' - open depending on value of Vm
   • Channels for each ion species have different voltage-sensitivities
     • I.e. they are open and close at voltage specific for each channel and with different timing

4. Action Potential

   • Action potential is shown on fast-time base
   • Resting potential is about -66mV
   • At its peak, action potential over-shoots zero getting closer to Ena
   • Membrane potential then repolarises due to K+ channels opening
   • It then undershoots its normal resting potential for a period of several milliseconds (after-hyperpolarisation)
   • Membrane potential then returns to normal resting potential
   • Action potential has a 'rising phase' followed by 'overshoot' followed by 'falling phase' and 'undershoot'

5. Ionic events during action potential

   • At normal resting potential, pNa+ is small but pK+ is larger
   • If a cell depolarises to its 'threshold voltage activated' Na channels open, they inactivate spontaneously about 0.5 ms later
   • During depolarisation, voltage-sensitive K+ channels also open but with a delay, hyperpolarising the membrane toward Ek
   • Membrane potential then undershoots due to continuing K+ Current
   • Voltage-sensitive K+ channels close and membrane potential returns to the original resting membrane potential

6. Voltage-activated sodium channels

   • Normal resting potential: channel closed
   • Depolarised channel opens for about 0.5-1ms
   • Depolarised >1ms channel stays open but it is inactivated by tethered particle - no longer permeable to Na+
   • Depolarisation removed: channel closed and inactivation particle is removed

7. Refractory Period

   • Time when a nerve cell is inexcitable after an action
   • Absolute refractory period: as long as Na+ channels are open or inactivated, no stimulus will trigger another action potential
   • Relative refractory period: as long as K+ channels are open, only especially strong depolarising signal will trigger a new action potential

8. Threshold potential

   • Threshold Potential is the potential at which the opening of voltage-sensitive Na+ channels is initiated leading the generation of action potential

9. Nerve cell firing action potentials

   • Nerve cells fire action potentials at a frequency that reflects the strength of stimulus/depolarisation