T reticular interneurons: a class of serially repeating cells in the zebrafish hindbrain.

We describe a class of reticular neurons, named T interneurons after the branching pattern of their axons, in young larvae of the zebrafish Brachydanio rerio. The cells were identified by filling them with HRP from application sites within the CNS. A serially repeating set of about ten such neurons is present in a longitudinal column on each side of the caudal hindbrain. The T axons project across the midline, and branches course both rostrally and caudally within the medial longitudinal fascicle (mlf). The cells receive synaptic input from the Mauthner neurons, from unidentified axons in the mlf, and perhaps from trigeminal sensory fibers. They project to cranial and pectoral motor nuclei. T interneurons appear to be homologous to giant fiber neurons in the hatchetfish and to some of the cranial relay neurons in the goldfish. We discuss a possible functional role and comparative implications of their distribution in the hindbrain.