In the vestibular organs of birds and mammals, two types of hair cells are usually distinguished although they may in fact represent the extremes of a spectrum of morphological types. Along with the most common type, called type II, are found flask-shaped cells, called type I enclosed up to their neck by a large nerve chalice which may enclose more than one cell. Between the hair cell and membrane of the chalice is a complicated pattern of contacts where the synaptic space is reduced to less than 100A. Synaptic terminals densely packed with vesicles are regularly observed in contact with the base of the chalice. These are interpreted as being presynaptic terminals. Type II hair cells are innervated by several thin nerve branches forming synaptic contact with the bottom of the cell.
Efferent endings are presynaptic to the hair cell and are filled with vesicles.
Terminals may be transitional between chalice and bouton-type endings. Information is transmitted between hair cell and eighth nerve terminal by normal chemical transmission. The vestibular hair cells, like those in the cochlea, are directionally sensitive displacement detector. As in all hair cells, regardless of their differences in morphology, the resultant displacement of the stereocilia opens ion channels resulting in inward ionic current. This leads to a receptor potential, release of neurotransmitter and the generation of action potentials in the distal processes of afferent nerve fibers.