<P> Ion channels are integral membrane proteins, typically formed as assemblies of several individual proteins . Such "multi-subunit" assemblies usually involve a circular arrangement of identical or homologous proteins closely packed around a water - filled pore through the plane of the membrane or lipid bilayer . For most voltage - gated ion channels, the pore - forming subunit (s) are called the α subunit, while the auxiliary subunits are denoted β, γ, and so on . </P> <P> Because channels underlie the nerve impulse and because "transmitter - activated" channels mediate conduction across the synapses, channels are especially prominent components of the nervous system . Indeed, numerous toxins that organisms have evolved for shutting down the nervous systems of predators and prey (e.g., the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails, and others) work by modulating ion channel conductance and / or kinetics . In addition, ion channels are key components in a wide variety of biological processes that involve rapid changes in cells, such as cardiac, skeletal, and smooth muscle contraction, epithelial transport of nutrients and ions, T - cell activation and pancreatic beta - cell insulin release . In the search for new drugs, ion channels are a frequent target . </P> <P> There are over 300 types of ion channels just in the cells of the inner ear . Ion channels may be classified by the nature of their gating, the species of ions passing through those gates, the number of gates (pores) and localization of proteins . </P> <P> Further heterogeneity of ion channels arises when channels with different constitutive subunits give rise to a specific kind of current . Absence or mutation of one or more of the contributing types of channel subunits can result in loss of function and, potentially, underlie neurologic diseases . </P>

How many types of ion channels are there