And these membrane channels basically respond to chemical or sometimes physical changes in the environment around that particular channel. And that's exactly what stimulates the opening or the closing of that channel as we'll discuss in just a moment. So we're going to focus on three different types of membrane channels. We're going to look at ligand gated ion channels, we're going to look at voltage gated ion channels and also discuss gap junction. So in this lecture, I'd like to introduce these three different types of channels. And in the following lectures, we're actually going to focus on the details of each one of these channels.