Otolith endorgan input to the Mauthner neuron in the goldfish.

The Mauthner (M-) cell of the goldfish, Carassius auratus, triggers the rapid escape response of the fish in response to various stimuli, including visual and auditory. The large size and accessibility of the M-cell make it an ideal model system for the study of synaptic transmission, membrane properties, and sensory-motor gating. Although physiological recordings have suggested that afferents from all three of the inner ear endorgans (the saccule, lagena, and utricle) synapse directly on the ipsilateral M-cell, the specific contacts and anatomical distributions of these inputs along the M-cell lateral dendrite remain unknown. We traced specific branches of the auditory (VIIIth) nerve from the three otolith organs of the fish inner ear to the M-cell. The goldfish sacculus gives rise to the vast majority of inputs that contact a large portion of the M-cell lateral dendrite, and these inputs vary greatly in size. In contrast to the ubiquitous distribution of saccular inputs, those from the lagena are segregated to distal regions of the M-cell and synapse on the distal dorsal branch of the lateral dendrite. Similarly, inputs from the utricle are also segregated to distal regions, synapsing on the ventral branch of the lateral dendrite. These results demonstrate that nerves from all three endorgans contact the M-cell, with input-specific segregation of synapses along the M-cell lateral dendrite.