Ion channels have pores that act as conduits for ionic currents to pass through the membrane. The state of conduction of an ion channel can be modulated by external stimuli. For example, some ion channels have chemical binding sites, and their state of ionic conduction depends on whether the binding site is occupied. Such ion channels act as chemical sensors. They underlie sensory functions ranging from olfaction to the detection of neurotransmitters.
Many ion channels are sensitive to other stimuli, such as voltage.
Mechanical stress is transmitted to ion channels in hair cells via tiny filaments that connect neighboring hairs in a hair bundle. The hairs are about 500 nm in diameter and tip links are on the order of 2 nm in diameter. Displacements of the hair bundle modulate the mechanical stress in the tip links and that stress is transmitted to ion channels, as illustrated in the attached animation . These trap doors represent highly simplified models of ion channels, which are actually complex molecules built out of thousands of amino acids. Nevertheless, these complex molecules can exist in a number of stable states, and this cartoon accurately depicts the fact that mechanical stress modulates the state of conduction of ion channels.