There are two different types of fluid that surround the hair cells of the inner ear. The endolymph is the fluid that surrounds the apical surfaces of hair cells. Potassium is the major cation in the endolymph and is thought to be responsible for carrying the receptor currents in the cochlea. Perilymph is found surrounding the sides and the bases of the hair cells. Perilymph is low in potassium and high in sodium. The different ionic makeups of the surrounding fluid in addition to the resting potential of the hair cell creates a potential difference across the apical membrane of the hair cell, so potassium enters when transduction channels open. An influx of potassium ions depolarizes the cell and causes the release of a neurotransmitter that can initiate nerve impulses in the sensory neurons that synapse on the base of the hair cell.