What would happen if a voltage gated sodium channel was without an inactivation gate?

At the peak of the action potential, when enough Na+ has entered the neuron and the membrane's potential has become high enough, the Na+ channels inactivate themselves by closing their inactivation gates. The inactivation gate can be thought of as a "plug" tethered to domains III and IV of the channel's intracellular alpha subunit. Closure of the inactivation gate causes Na+ flow through the channel to stop, which in turn causes the membrane potential to stop rising. With its inactivation gate closed, the channel is said to be inactivated. With the Na+ channel no longer contributing to the membrane potential, the potential decreases back to its resting potential as the neuron repolarizes and subsequently hyperpolarizes itself. This decrease in voltage constitutes the falling phase of the action potential.

When the membrane's voltage becomes low enough, the inactivation gate reopens and the activation gate closes in a process called deinactivation, or removal of inactivation. With the activation gate closed and the inactivation gate open, the Na+ channel is once again in its deactivated state, and is ready to participate in another action potential.

When any kind of ion channel does not inactivate itself, it is said to be persistently (or tonically) active. Some kinds of ion channels are naturally persistently active. However, genetic mutations that cause persistent activity in other channels can cause disease by creating excessive activity of certain kinds of neurons. Mutations that interfere with Na+ channel inactivation can contribute to cardiovascular diseases or epileptic seizures by window currents, which can cause muscle and/or nerve cells to become over-excited.