Journal of Neuroscience, Vol 9, 2931-2953, Copyright © 1989 by Society for Neuroscience
Synaptic organization of individual neurons in the macaque lateral geniculate nucleus
JR Wilson
Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia 30322.
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.
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