Triadic synaptic arrangements and their possible significance in the lateral geniculate nucleus of the monkey.

Abstract Electron microscopic examination of the lateral geniculate nucleus in monkeys reveals the consistent occurrence of triadic synaptic arrangements where a retinal axon terminal is presynaptic to both a principal cell (P-cell) and a non-axonal portion of an interneuron (I-cell) containing flattened vesicles, which in turn is presynaptic to the same P-cell element. These ‘triads’ are found within glomeruli where the P-cell participates with a dendritic protrusion and the I-cell with a dendritic appendage (type 1 triad), and also outside the glomerulus where the P-cell is represented by its soma or very proximal dendrite (type 2 triad), or the I-cell profile is the soma which again contains synaptic vesicles (type 3 triad). The most parsimonious interpretation of the functional significance of the triadic synapse is to consider it as a device for the inhibitory phasing of the P-cell discharge resulting from the release of I-cell inhibitory transmitter by excitation of the interneuron through the same retinal afferent which causes the initial tonic activity of the P-cell. Further roles of the I-cell are discussed on the basic of a model which considers a random geometry of dendritic (long) and axonal (short) arborizations. The model suggests 3 possible I-cell actions: (1) local inhibition limited to the triadic arrangement; (2) inhibition at a distance through an axonal mechanism; and (3) inhibition at a distance by way of the presynaptic dendrites.

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