This chapter discusses senses structure and function of the inner ear, where delicate neural structures constituting the vestibule and cochlea are called the membranous labyrinth. The membranous labyrinth is encased in a closely fitting bony shell called the bony labyrinth. Stria vascularis, a highly vascular structure along the outer side of the cochlear duct, seems to play an important role in maintaining the high potassium concentration of endolymph. Isolated OHC do contract if electric fields, ATP and calcium, or acetyl-choline are applied, further confirming the similarity from a functional point of view. The signals generated in hair cells are transmitted to the central nervous system by afferent nerve fibres. The afferent nerve fibres arising from the IHC are far more than from the OHC. Information about the activity of hair cells is conveyed to the auditory nerve fibres innervating these cells. When the hypopolarization of a hair cell reaches a threshold value, it leads to an increase in the frequency of impulses in the auditory nerve fibres innervating it.