The ultrastructure of the pretectal nucleus of the optic tract: characterization of the configuration, cell type, and synaptology
Description
The fine structure and the terminations of the retinal and cortical afferents of the pretectal nucleus of the optic tract in the cat were examined. Data concerning cell size reveal a Gaussian distribution of cells and statistical analyses and histograms indicate there are no subpopulations or laminations of neurons. The parenchyma of the nucleus of the optic tract is separated by numerous myelinated fascicles into synaptic islands. These islands vary in size and may contain perikarya. Within some of the larger synaptic islands, somata are frequently found in close apposition. Electron microscopic examination of these paired somata has identified areas that appear to be adhesion placques rather than chemical or electrical synapses The terminals within the nucleus are divided into five classes based upon their synaptic vesicles, size, and mitochondrial characteristics. These terminal types are: (1) the RLP, (2) RLD, (3) RSD, (4) PD(,1), and (5) PD(,2) profiles. Additionally, type I and type II glomeruli are identified. Type I glomeruli are observed in clusters and are composed of central RLP profiles forming synapses with peripherally placed dendrites of small and medium diameter and PD profiles. Type II glomeruli have central dendrites commonly in contact with RLD and RLP profiles. Serial synaptic complexes with PD(,2) profiles as their intermediate components are also observed After placing injections of HRP into one eye or unilaterally transecting the optic nerve, retinal terminals are identified as only RLP profiles and are observed either as central axons within Type I glomeruli or as extraglomerular profiles After placing injections of HRP into the visual cortex, corticopretectal projections are identified as both RSD and RLD terminals. RSD profiles are found terminating on small and medium diameter dendrites whereas RLD terminals form asymmetric synapses on dendrites of medium and large diameter Double marker experiments involving the simultaneous unilateral transection of the optic nerve and placement of HRP within the visual cortex were performed. Only two examples of degenerating retinal profiles and HRP labelled cortical terminals in contact with the same neuron were observed. Nevertheless, on the basis of these data it is suggested that neurons in the retina and visual cortex do synapse on the same neurons in the nucleus of the optic tract. Functional considerations of the microcircuitry are also presented