<P> Ion channels can be classified by how they respond to their environment . For example, the ion channels involved in the action potential are voltage - sensitive channels; they open and close in response to the voltage across the membrane . Ligand - gated channels form another important class; these ion channels open and close in response to the binding of a ligand molecule, such as a neurotransmitter . Other ion channels open and close with mechanical forces . Still other ion channels--such as those of sensory neurons--open and close in response to other stimuli, such as light, temperature or pressure . </P> <P> Leakage channels are the simplest type of ion channel, in that their permeability is more or less constant . The types of leakage channels that have the greatest significance in neurons are potassium and chloride channels . It should be noted that even these are not perfectly constant in their properties: First, most of them are voltage - dependent in the sense that they conduct better in one direction than the other (in other words, they are rectifiers); second, some of them are capable of being shut off by chemical ligands even though they do not require ligands in order to operate . </P> <P> Ligand - gated ion channels are channels whose permeability is greatly increased when some type of chemical ligand binds to the protein structure . Animal cells contain hundreds, if not thousands, of types of these . A large subset function as neurotransmitter receptors--they occur at postsynaptic sites, and the chemical ligand that gates them is released by the presynaptic axon terminal . One example of this type is the AMPA receptor, a receptor for the neurotransmitter glutamate that when activated allows passage of sodium and potassium ions . Another example is the GABA receptor, a receptor for the neurotransmitter GABA that when activated allows passage of chloride ions . </P> <P> Neurotransmitter receptors are activated by ligands that appear in the extracellular area, but there are other types of ligand - gated channels that are controlled by interactions on the intracellular side . </P>

The concentration difference of ions across the membrane is maintained by