There are three types of  ion channels in cells of human body. They are as follows: 1. Ungated Ion Channels or leak channels     These channels are always open and there is free diffusion of ions through these channels along the concentration gradient of the ions. Leak channels are responsible for maintenance of Resting membrane potential of a cell. 2. Voltage gated Ion Channels Voltage gated ion channels respond to the change in resting membrane potential of the cell. These channels either open or close based on the voltage of the resting membrane potential. 3. Ligand gated ion channels These ion channels have a additional binding site for ligands and open or close based on weather the ligand is attached or not. Ligands act as keys and the channels act as locks.   A receptor can act as one of the types. NMDA receptor is an exception. NMDA receptor can act both as voltage gated and ligand gated channel.