Another type of receptor is an ion channel. Ion channels can be activated by the arrival of a specific molecule or a change in membrane voltage. A ligand is a molecule that binds to a specific receptor protein to initiate signal transduction. A ligand-gated ion channel is a receptor that acts as a gate, allowing ions into or out of the cell when a signaling molecule binds. Each channel is specific to a particular ion. Calcium (Ca2+), sodium (Na+), and potassium (K+) are examples of ions that move through ion channels. This type of receptor is very important to the proper functioning of the nervous system, as nervous signals are electrochemical in nature and the movement of K+ and Na+ helps to propagate action potentials along nerve cells (neurons). Ca2+ is critical in neurotransmitter release and signaling from one neuron to the next across synapses. Neurotransmitters are chemicals that bind to receptors and excite or inhibit postsynaptic cells. Neurotransmitters are then bound as ligands on the receiving cell, which causes ion channels to open, allowing ions to flow in or out. This creates a cascade of signals that move down the receiving cell.Ligand-gated ion channels are either fully open or fully closed. The active state is the open position of the channel, where ions are able to pass through across the membrane. The closed state prevents anything from crossing the membrane. Movement between the active and closed states occurs in response to the presence or absence of the ligand. A channel can also be inactivated, which prevents the channel from being opened by its usual trigger molecule. Channel inactivation can occur when subtle changes to the structure of the open channel occur, when cytoplasmic parts of the protein plug the open channel, or when large organic or inorganic molecules plug the opening. A voltage-gated ion channel is a receptor that acts as a gate allowing ions into or out of the cell in response to a change in electrical charge across the membrane. A shift in the voltage going across the cell membrane of a nerve cell causes its channels to open and close. As with ligand-gated channels, voltage-gated channels are specific to a single type of ion.