Ion channels play a fundamental role in the way cells communicate. They generate the electrical signals that make hearts beat and muscles contract, and allow brains to receive and process information. This communication between cells allows for the orchestration of physical and mental activities in humans. Many diseases result from ion channels that do not function properly.
Ion channels are transmembrane proteins that span the cell membrane and are formed from one or more protein subunits. The channels are shaped like tunnels, which form pores through the plasma membrane. The pores have gates that open and close to allow ions to diffuse down their chemical gradient and move in or out of a cell. Ion channels are specific for certain types (and combinations of types) of ions, such as chloride, sodium, potassium, and calcium.
Ions are atoms or molecules that have gained or lost one or more electrons to give them either a net positive or negative electrical charge. They are unequally distributed, creating a separation of charge across a membrane, resulting in an electrical potential. When an ion channel is open, a million ions can flow in or out of the cell per second. This causes an electrical signal or current, which allows cells to communicate very quickly.
There are two major classes of ion channels defined by the way the channel opens: ligand-gated and voltage-gated. Ligand-gated ion channels open when a specific chemical signal called a neurotransmitter is released from one cell, diffuses through a gap known as a synapse, and binds to receptors on ion channels of a second cell. The binding of the neurotransmitter causes the ion channel gate to open. Voltage-gated ion channels have sensors for the electrical potential across the membrane. They open when the cell is at a specific membrane potential. Some other channels can open due to mechanical stress or the levels of signaling molecules inside the cell.
An important function of ion channels is to regulate when cells are at rest and when they are communicating. When a neuron is at its resting potential, it is not sending a signal to any other cells. The inside of a neuron at rest is more negative than the outside. If a neuron is stimulated to communicate with other cells, ion channels open to make the inside of the cell more positive than the resting potential. The neuron will reach a threshold and fire an action potential, where voltage-gated ions channels open, allowing sodium ions to rush in and potassium ions to rush out of the cell. Action potentials propagate and repeat many times to carry a signal the length of a nerve cell. This cell can then continue to communicate by releasing neurotransmitter to bind ligand-gated ion channels on another cell.
A number of diseases occur when ion channels do not function properly. Some examples are epilepsy, cystic fibrosis, heart arrhythmia, and high blood pressure. Ion channels are also the target of many types of drugs and toxins, which can alter the fundamental communication between cells.
Ingrid A. Lobo
R. Adron Harris
Changeux, Jean-Pierre (1993). "Chemical Signaling in the Brain." Scientific American 269(5):58–62.
Keynes, Richard D. (1989). "Synapse Formation in the Developing Brain." Scientific American 240(3):126–132, 134–135.