A light and electron microscopic study of the nucleus isthmo-opticus in the pigeon

A morphological study of the nucleus isthmo-opticus (NIO) has been carried out using several histological approaches as well as electron microscopic analysis. The nucleus is composed of bipolar cells of relatively constant size. The neurones within a given nuclear region are oriented in the same plane and direction. EM observations show that neighbouring cells often give off complex spinous extensions that come in close contact with each other. The apposed membranes of these extensions display large attachment plaques. The neuropil contains relatively abundantly myelinated axons, thin dendrites, and various types of terminals. A majority of the terminals contain rounded vesicles. The tecto-isthmic projection has been studied electronmicroscopically, by using the degeneration technique. The tecto-isthmic terminals contain predominantly rounded vesicles. They almost always make axo-dendritic contacts. Degeneration and autoradiographic techniques at the light microscope level fully confirm previous data by others on the topologic organization of the projection. The above two techniques were applied to the study of NIO-retinal interrelationships. No retinal projection to NIO could be confirmed. On the other hand, enucleation induces a terminal degeneration of slow time course. This finding suggests the presence of intrinsic fibre collaterals of isthmo-retinal axons. The existence of fibre projections from the oculomotor nuclei is shown by a light microscope degeneration study. Fibres coming from the trochlear nucleus might correspond to collaterals of motor axons.