Organization of the nucleus magnocellularis and the nucleus laminaris in the barn owl: encoding and measuring interaural time differences.

The circuit from the cochlear nucleus magnocellularis to the nucleus laminaris supports the encoding and measurement of interaural time differences in the auditory brainstem. Specializations for the encoding of temporal information include the few and/or short dendrites and thick axons of the magnocellular and laminaris neurons, and the high degree of convergence in the circuit. Magnocellular cells have large cell bodies covered with somatic spines. The cells have few dendrites, and the number of dendrites decreases from low to high best frequency regions of the nucleus. Magnocellular neurons receive both auditory nerve terminals and GABAergic terminals with symmetric synapses and terminals filled with pleomorphic vesicles. The axonal projections of magnocellular neurons to the nucleus laminaris form maps of interaural time difference. About 100 magnocellular afferents from each side converge on each laminaris neuron, and the terminals from each side do not occupy separate domains on the cell. These terminals form punctate asymmetric synapses on both the dendrites and the cell bodies of laminaris neurons. Laminaris neurons also receive GABAergic terminals which form symmetric synapses. Laminaris neurons have oval cell bodies covered with very short dendrites. The cells in the low best frequency region of the nucleus laminaris have longer dendrites.