Otolith end organ projections to auditory neurons in the descending octaval nucleus of the goldfish, Carassius auratus: a confocal analysis.

The distribution of axons from the saccule, lagena, and utricle to descending octaval nucleus neurons that project to the auditory midbrain in the goldfish is reported. We have divided these auditory projection neurons, located in the dorsal portion of the descending octaval nucleus (dDO), into two groups, medial and lateral, each of which contains several neuronal populations based on morphology and location. At most levels of the dDO, there are three medial and three lateral populations; the rostral dDO contains an additional lateral population. The saccule provides input to each of the seven medial and lateral populations but appears to be the exclusive/nearly exclusive source of primary input to the most dorsal cell group of the medial population. Along with the saccule, the lagena and utricle each supply the remaining six medial and lateral populations. Neurons in each of these populations receive input from more than one end organ. One medial and one lateral population include neurons that receive remarkably large contacts from utricular afferents. Overall, the results reveal a more substantial input from the lagena and utricle to the main first-order auditory nucleus in the goldfish than was previously recognized, suggest this nucleus is composed of functionally distinct populations, and relate to functional and evolutionary issues about hearing in early vertebrates.