 Dear students, in this module we shall discuss the role of ion channels in electrical responses of excitable membranes. You know that plasma membrane has many integral proteins that function as ion channels. These ion channels have very particular role in the electrical responses of electrically excitable cells. There are many types of ion channels. These include the resting potassium selective channels, voltage-gated ion selective channels, ligand-gated ion selective channels, and stimulus-specific ion channels. We shall discuss the resting potassium selective channels first. The resting potassium selective channels are uniformly distributed over the entire surface of the plasma membrane of excitable cells. They always remain open. They are mainly responsible for the maintenance of resting membrane potential. They are also responsible for the passive changes that take place during the action potentials. Voltage-gated ion selective channels are the channels which make the cell membrane excitable. They are responsible for nearly all the electrical signals produced in the living tissues. These channels are not distributed on the entire surface but they are localized to particular regions of the plasma membrane, for example, axonal membrane of neurons. Active changes in membrane potential in response to depolarizing ionic current depend on opening of these channels. These voltage-gated ion channels exhibit ion selectivity, allowing only one or a few species of ions to pass through them. There are many types of voltage-gated ion channels which are named for the ionic species they allow to pass through them. In these, voltage-gated sodium channels, voltage-gated calcium channels, voltage-gated potassium channels and calcium-activated potassium channels are included. Voltage-gated sodium channels are called fast-acting channels because they activate in depolarization conditions and produce a rising phase of action potential. Their activation with depolarization does not cause any delay. Voltage-gated calcium channels are also activated by depolarization but they open more slowly as compared to sodium channels. Their opening allows calcium ions to enter the cell where they have to act as second messenger. The voltage-gated potassium channels are also activated by depolarization but very slowly, so they are called delayed rectifiers. In contrast to the previous two channels, that is calcium and sodium channels, when potassium channels open, they allow the outflow of potassium ions. As a result, they repolarize the cell and terminate an action potential. Many cell membranes also have calcium-activated potassium channels. They are also activated by depolarization but only when there is an elevated calcium ion concentration in the cytosol. When there is more calcium ion concentration in cytosol, then these calcium-activated potassium channels open and they remain open as long as potassium ion concentration remains high in the cell. These are the three types of ion channels, ligand-gated ion channels. They are activated when specific ligand molecules bind to receptor sites. They will not be opened by depolarization but when they bind to a ligand molecule, which ligand are they? They are second messenger molecules and neurotransmitters. When they bind to a ligand molecule, then these channels are open. As a result of ligand binding, the conformational changes in these channels as a result, their channel gates are open and the influx of ions start. In addition to these channels, there are stimulus-specific ion channels present in certain membranes of sensory organs. These ion channels are activated by stimulus synergies. They are found in sensory receptor cells. For example, specific ion channels are found in photoreceptors, which are activated by light. Another type of ion channels are found in the taste buds and olfactory organs, olfactory neurons. They are activated by chemicals. Similarly, ion channels in mechanoreceptors are activated by mechanical strain.