Ion channel

a specific macromolecular protein pathway, with an aqueous "pore," that traverses the lipid bilayer of a cell's plasma membrane and maintains or modulates the electrical potential across this barrier by allowing controlled influx or exit of small inorganic ions such as Na+, K+, Cl-, and Ca2+

Description
It plays an important role in propagation of the action potential in neurons, but also may control transduction of extracellular signals and contraction in muscle cells. In general, ion channels are characterized by their selectivity for certain ions, their specific regulation or gating of these ions, and their specific sensitivity to toxins.