Fig. 2. Schematic showing classification of vestibular afferent neurons by terminal morphology as pure-calyx (green, C), dimorphic (orange, D) and pure-bouton (red, B). Grey fibres are efferents, arising from neurons in the brainstem. Pure-calyx afferents exclusively innervate the centre and striola and often form complex calyces around multiple type I hair cells, as illustrated. Pure-bouton afferents exclusively innervate the peripheral zone and extrastriola and can innervate tens of type II hair cells. Dimorphic afferents innervate both zones, but have more compact dendritic trees in the centre and striola than in the periphery and extrastriola (not shown). Pure-calyx afferents express calretinin, calbindin and parvalbumin; dimorphic afferents are thought to express calbindin and parvalbumin; and pure-bouton afferents, which are the thinnest, express only parvalbumin. Some differences in ion channel expression have been noted between large and small isolated ganglion somata and are indicated here on the pure-calyx and pure-bouton somata. Whether dimorphic somata, which are likely to be mid-sized, have intermediate expression is not known. Met, mechanoelectrical transduction channels; see Table 2 and text for details on other ion channels. Asterisks indicate possible sites of spike initiation on each afferent.