Mechanism of the Voltage Gated Cation Channel
The purple depicts the hydrophobic lipid bilayer membrane. The dark blue band depicts the hydrophobic portion of the channel which must interact with the hydrophobic lipid bilayer. The red depicts the parts of the channel which are hydrophilic. Note that there is a hydrophilic channel which passes completely through the hydrophobic membrane. When the membrane is in its resting state there is a membrane potential of about -20 to -200 mV (inside negative) depending on the cell type. This membrane potential is illustrated with the many + charges along the membrane on the outside of the membrane and with the many - charges along the inner surface of the membrane. The membrane's resting potential keeps the voltage gated channel in its closed conformation. When the membrane potential is suddenly reduced (note the sudden decrease in the number of +'s and -'s), the voltage gated channel undergoes an conformational change (red arms change orientation) and the gate opens. The sudden reduction in the membrane potential can be caused by many different events such as those which occur at the synapse, those which occur during the propagation of an action potential or even those which occur during mechanical injury. Once the gate is open, Na+ ions rush through the open channel, satisfying their concentration gradient (high outside the membrane and very low inside the membrane). As the positively charged Na+ comes in, they associate with the negative charges on the inner surface of the membrane and they further neutralize the membrane potential. This further reduction in the membrane potential allows other adjacent voltage gated channels to reach their opening threshold and so they open leading to a cascade of opening channels which moves away from the initial stimulus. When the membrane potential becomes sufficiently positive (indicated by the excess of Na+ inside the membrane), the voltage gated channel undergoes another conformational change to become inactive. In the inactive form the channel collapses so that Na+ can no longer rush in. Now, elsewhere on the membrane, the Na/K pump can begin the job of pumping the Na+ back to the other side of the membrane. This re-establishes the membrane potential and allows the channel to return to its closed configuration. Click on the animation to see a larger view.