Freeze-fracture of neurons in nucleus magnocellularis of the chick.

4- to 8-day-old chicks were used to examine the ultrastructure of end bulbs and neurons of nucleus magnocellularis by freeze-fracture and thin section techniques. Both normal animals and animals deafferented by removal of the basilar papilla were used to describe the pre- and postsynaptic membranes of these neurons. Postsynaptic membrane specializations similar to those described in rodents were found in normal and deafferented chicks. In the presynaptic end bulb there was an immediate cessation of endocytotic vesicle formation after deafferentation. There was also a shift in the preferential fracture plane from the presynaptic to postsynaptic membranes 48-72 h after basilar papilla removal. These results indicate that the ultrastructure of this synaptic complex is comparable to that seen in mammals.