The fine morphology and synaptic organization of the accessory optic tract nucleus was investigated in monkeys (Macaca mulatta) by light and electron microscopic methods. Nissl stains delineated the wedge-shaped nucleus between the brachium of the inferior colliculus and the medial lemniscus. Most of the neurons are of medium size with dark coarse Nissl bodies. Smaller and paler cells are also present. Golgi material revealed the two classic type I and II neurons, the former with early bifurcating dendrites exhibiting numerous spines. Electron microscopy showed mostly medium size neurons with relatively frequent occurrence of somatic spines. The dendritic profiles have a thick smooth initial protion followed by the appearance of increasing number of spines. The dendrites bifurcate close to the soma and finally end in brushlike fashion. Axon terminals make synaptic contacts with the soma, dendrites (spines and trunk) and dendritic endings. The latter articulations appear as "glomeruli". At this level, profiles of ambiguous nature, possibly dendrites of Golgi type II cells, containing synaptic vesicles are frequently seen. Following eye enucleation, both the contralateral and ipsilateral nuclei showed terminal boutons in different stages of degeneration depending on the survival time. The optic fibers terminate on the soma as well as on the proximal spineless part of the dendrites and at the dendritic bifurcations. There are no optic terminals participating in the "glomeruli". This nucleus receives direct retinal fibers and the system is both crossed and uncrossed. The presence of abundant axosomatic contacts of optic terminals as well as the absence of such endings in the glomeruli suggest a very different functional mechanism than that of the geniculate pathway. The nucleus receives many more afferents of unknown origin.
ASJC Scopus subject areas
- Developmental Neuroscience