01. Voltage Gated Ion Channels

• voltage gated ion channels open in response to a change in plasma membrane voltage
• sodium channels are formed from a single polypeptide with four domains

  • each domain has 6 alpha helices
  • helix 4 senses the voltage
  • pore forms between helix 5 and 6
• VG sodium channel activation:

  1. S4 region positively charged
  2. depolarisation causes S4 to move outwards
  3. conformational change allows channel to open
  4. hydrophobic inactivating segment enters channel and blocks it
• potassium channels are made of multiple polypeptides

  • each unit has inactivating segment (not always present in all channels)
• TTX blocks Na+ channels
• TEA blocks K+ channels
• Na+ current voltage plots are bi-phasic as ENa is 60mVs, so current flips where it crosses x axis
• channel conductance = number of channels that are open

  • calculated using Hodgkin and Hutchleys equation

  cannot have current without conductance but can have conductance without current
• inactivating K+ channels generate an A-type current which delays AP initiation