Fig. 8.

Afferent and efferent innervation in the vestibular periphery. The peripheral vestibular afferent pathway consists of vestibular nerve fibers that innervate hair cells (HCs) via calyx-type terminals that ensheath type I HCs and bouton endings that innervate type II HCs. Most vestibular afferent fibers receive inputs from both type I and type II HCs (so-called dimorphic afferents). Type II HCs can form synapses with bouton endings or with the outer wall of calyx afferent terminals. Additionally (not shown), some afferent fibers receive input from type I HCs via “calyx-only” synapses, and others receive input from type II HCs via “bouton-only” synapses. Efferent fibers originating in the brain stem form synapses onto type II HCs as well as onto calyx and bouton afferent endings. Both hair cell types use glutamatergic quantal transmission via ribbon synapses, and type I HCs also mediate signals by nonquantal transmission via potassium and glutamate accumulation in the synaptic cleft. Inhibition of type II HCs by efferents provides a mechanism for type I HC inputs and calyx terminal activity to become the dominant afferent signal.