Morphology and origin of axonal endings in nucleus laminaris of the chicken.

The axonal endings on the somata and dendrites of third-order auditory neurons in nucleus laminaris (NL) were measured and classified in thin-sectioned material from adult chickens. Two methods were used to determine which ending types arise from second-order auditory neurons in nucleus magnocellularis (NM): 1) degeneration of axonal endings in NL after transection of the crossed dorsal cochlear tract (CTrX) carrying the axons of each NM to the contralateral NL, and 2) injection of horseradish peroxidase (HRP) into NM or the CTrX to label endings in NL by anterograde transport. About 42% of the perikaryl surface of NL neurons and 63% of the dendritic surface are apposed by axon terminals arising from NM; these endings are also prevalent on the axon hillock and initial segment. The NM endings are characterized by round, clear synaptic vesicles distributed at an average density of 76/microns2 behind small punctate synaptic junctions with thick synaptic densities. These endings degenerate preferentially after transection of the CTrX and are the only endings consistently labeled after HRP injections. About 31% of the perikaryl surface of NL and 10% of the distal dendritic surface are apposed by a prominent non-NM axonal ending type. This ending is characterized by a synaptic vesicle density of 135/microns2 and a single large area of synaptic contact which bears very slight densities on both pre- and postsynaptic membranes. On the perikaryon, about 80% of these non-NM terminals end on the cell body proper, with the rest found on proximal dendrites or capping the short somatic processes or spines occasionally seen in NL. The distribution of the two ending types does not differ significantly along the posterior-to-anterior axis of NL.