Voltage gated sodium channels
Voltage Gated Sodium channels (Na+ VGC) are found within the plasma membrane of nerve cell axons. Their main role is to induce an action potential (AP). The Na+ VGC can occupy 3 states: Open, inactive and closed. There are 2 gates in Na+ VGC (unlike K+ VGC which only has 1). There is the activation gate and inactivation gate. The activation gate is voltage dependent and the inactivation gate is time dependent.
- Open - When the plasma membrane begins to depolarize, it triggers the opening of a few Na+ VGC which increases the amount of Na+ entering the cell and therefore continues to depolarise the membrane. This in turn triggers the opening of more Na+ VGC. At this point the cell is permiable to Na+. The open phase is said to be voltage dependent, as they open when the plasma membrane is depolarised.
- Inactive - This stage is said to be time dependent as the inactivation gates close a set time after the activation gates open in the open stage. The cell now becomes impermiable to Na+.
- Closed - When the membrane repolarizes, the Na+ VGC takes on it's closed state. The activation gate closes a set time after the inactivation gate opens so it also time dependent.
The inactive state is important as it is responsible for the absolute refractory period. No matter how strong the stimulus applied is, no AP can form because the inactivation gate is time dependent. When the inactivation gate opens and the Na+ VGC is in it's closed state so only the activation gate is closed. As this is voltage dependent an AP can form but only when the stimulus is greater than usual. This is because some of the K+ VGC are still open so the membrane is more negative. This is the relative refractory period .
The voltage-gated sodium channel is thought to have 4 repeating sequences, each one of these containing 6 transmembrane α-helices. 5 of these helices are hydrophobic, with the 4th helix in each sequence being hydrophilic. This 4th helix contains many positive amino acids (arginine/lysine), making it positively charged. It has been suggested that this 4th helix in each sequence acts as a voltage sensor, maing the channel voltage-gated.