 Dear students, in this module, we shall discuss the role of voltage-gated sodium channels in detail. The voltage-gated sodium channels are activated by a stimulus-generated depolarization. They have high sensitivity to voltage changes. So, they are called fast-acting channels. Their opening results in an inflow of sodium ions into the cell. As a result, the rising phase of action potential is started. The channel proteins are inserted in the membrane in the lipid by layer. Every channel, every sodium channel has two gates. One is the activation gate that is present on the outside of the membrane. While the second gate is called the inactivation gate that lies on the inside. Dear students, the number of sodium channels present in a membrane is comparatively less as compared to many other channels. So, they are less densely packed. A micrometer square area of the membrane contains about 500 sodium channels. That means that they occupy only 1% of the total area. However, this number is quite enough to produce the required sodium conductance and sodium crunch. Because every sodium channel can pass 1 million that is 10 raised to power 7 sodium ions per second through it. Dear students, the number of sodium channels open at any moment depends on the membrane potential as well as on time. Time means the phase of action potential. In different phases of action potential, different number of or varying number of sodium channels are open. More the number of open sodium channels, more is the sodium conductance GNA. So, the changes in GNA that is sodium conductance occur as a function of membrane potential and time. Dear students, we shall discuss the Hodgkin cycle that explains the positive feedback mechanism occurring during the opening of sodium channels and flow of sodium ions into the cell. When a stimulus is received on the membrane, it causes a local depolarization of the membrane. As a result, some sodium channels open. This increases sodium conductance. In the cell, more sodium ions enter. That results in further depolarization of the membrane. More depolarization results in opening more channels, more sodium channels and decrease in membrane potential. This relationship between the sodium conductance and membrane potential follows a positive feedback system. And this is termed as Hodgkin cycle. Hodgkin cycle and increase in sodium conductance results in triggering of an action potential. Dear students, now we shall discuss the mechanism of opening of sodium channels. The sodium channels, the channel proteins present in the membrane bear a net charge. So, a change in membrane potential produces an electromotive force on the charge. 2K, there is a charge on the membrane, charge on the proteins. Any change in membrane potential causes electromotive force on charge of the protein. As a result, the electromotive force causes the charge to move in space. This charge movement causes conformational change in the molecule resulting in the opening of the channel. Dear students, the movement of charge through the channel results in small gating grunt. This gating grunt due to sodium ions is denoted as INA that is sodium grunt and the gating grunt IG. The gating grunt is associated with the opening and closing of sodium channels.