Acid sensing ion channels (ASIC) are found in a variety of tissues, including the brain. They are responsible for sensing and responding to the presence of extracellular protons (H+). They play a critical role in the regulation of neuronal excitability, synaptic transmission, and the modulation of behaviour. ASICs are classified according to their primary function. ASIC1a and ASIC2a are involved in the detection of H+ and generation of action potentials, respectively. ASIC3 is thought to be involved in the regulation of pain processing. ASIC4 is involved in the regulation of synaptic plasticity and learning.