Morphophysiological and ultrastructural studies in the mammalian cristae ampullares

There are three kinds of afferent terminations in the cristae ampullares. Calyx units innervate a few neighboring type I hair cells. Bouton units contact several type II hair cells. Dimorphic units innervate both kinds of receptors. Axon diameters are largest for calyx fibers and smallest for bouton fibers. Dimorphic units supply all parts of the sensory epithelium. Calyx units are confined to the central zone of the crista and bouton units to its peripheral zone. Intra-axonal labeling was used to determine the innervation patterns of physiologically characterized afferents. Calyx units are irregularly discharging. Dimorphic units in the central zone have a more irregular discharge than those in the peripheral zone. Bouton units, which have also been identified by their slow conduction velocities, are regularly discharging. An afferent's discharge regularity, sensitivity to externally applied galvanic currents and response dynamics are more closely related to its epithelial location than to its branching pattern or to the types and number of hair cells it contacts. Of the various discharge properties studied, only the rotational gains seemed closely related to terminal morphology. Afferents innervating the central and peripheral zones differ in their innervation patterns and discharge properties. A preliminary ultrastructural study indicates that there also are regional variations in synaptic organization. Type II hair cells in the peripheral zone are contacted by many more afferent boutons than those in the central zone. Individual central boutons have multiple ribbon synapses with type II hair cells, whereas each peripheral bouton usually has a single synaptic contact. Synapses between type II hair cells and calyx endings are common centrally, but not peripherally. Two synaptic features did not vary regionally: 1) type I hair cells usually make 10-20 ribbon synapses with their calyx endings; and 2) each type II hair cell is contacted by 2-6 efferent endings. The number of efferent boutons in contact with each calyx ending declines slightly from the peripheral zone to the central zone. Reciprocal synapses were rare.