Synaptic organization of individual neurons in the macaque lateral geniculate nucleus

Parvocellular and magnocellular neurons in the dorsal lateral geniculate nucleus of macaque monkeys were recorded electrophysiologically and then injected with HRP. The injected neurons were examined with the electron microscope. Synaptic terminals contacting the dendrites of individual neurons were classified and the synapses counted to estimate the number and distribution of each type over the entire dendritic tree. Seven parvocellular and 2 magnocellular neurons were analyzed. Two of the parvocellular neurons had presynaptic dendrites and no axons. These interneurons had electro-physiological characteristics much like those of relay neurons with the exception that their receptive field center responses had the opposite sign; i.e., they had OFF centers, while most neurons around them had ON centers. All of the relay neurons had similar types and distributions of terminal contacts. However, the distribution of each synaptic type along the dendrites of an individual neuron was not homogeneous. Retinal and F terminals were located predominantly on proximal dendrites whereas RSD terminals, either from the cortex and/or brain stem, predominated on the intermediate and distal dendrites. Parvocellular neurons were estimated to have about 500 total synapses on their dendritic trees, while magnocellular neurons had about 3000 total synapses on their dendritic trees. The retinal terminals making synaptic contacts with magnocellular neurons were also presynaptic to terminals containing flattened vesicles; these latter terminals also had synapses onto the magnocellular neuron's dendrites. Such a synaptic arrangement is called a triadic arrangement, or triad. Parvocellular neurons rarely had such triadic arrangements. In comparing these data with those of the cat, it was concluded that the major synaptic difference between relay cell types in both species (Class 1/Class 2 cells for the cat and parvo/magno cells for the monkey) was the frequent occurrence of triads for Class 2 cells and magnocellular cells versus the infrequent occurrence of triads for Class 1 cells and parvocellular cells. Although these triadic arrangements have been studied for over 2 decades, their function has yet to be determined, but probably relates to inhibition of retina signals at dendrites of magnocellular neurons in the monkey and Class 2 cells in the cat.