The synaptic glomerulus and the intrinsic neuron in the dorsal lateral geniculate nucleus of the cat.

Abstract

AbstractThe morphological characteristics of Class III, Golgi Type II neurons are analyzed in Golgi impregnations of the lateral geniculate nucleus (LGN) in the cat with special attention given to the complex dendritic appendages of these cells, the multilobed dendritic terminals. In addition, electron microscopic studies demonstrate the presence of presynaptic dendritic (ID) terminals as major components of synaptic glomeruli. Electron microscopic analysis of serial thin sections confirm the dendritic origin and the complex, multilobed nature of these terminals. An hypothesis is proposed identifying the ID terminal with the multilobed dendritic terminals of light microscopically identified Class III cells and the IA terminal, the “peripheral axon” of the glomerulus, with the axonal endings of all Golgi Type II cells.The Class III neuron has a short axon ramifying near the cell of origin. The axon terminates in boutons en passant and boutons terminaux: single, bilobed or “claw‐like” endings. Optic axons terminate not in “claw‐like” fashion, as previously thought, but in club‐shaped endings with scalloped silhouettes. The principal dendrites of Class III cells are characterized by long‐stalked appendages, often multilobed and occurring at the end of preterminal stalks 5 to 100 μ in length. One impregnated multilobed terminal studied in the electron microscope was found to innervate two adjacent glomeruli.The relationship of ID terminals to optic axon terminals and to other processes in synaptic glomeruli is analysed with the aid of the electron microscope. The ID terminal contains vesicles of the same size as those in presynaptic dendrites and makes small, macular synaptic contacts with other pre‐ and postsynaptic dendrites. ID terminals are the only elements in the LGN which synapse on others of their own kind, and are postsynaptic to all types of axon identified. As the only postsynaptic processes containing synaptic vesicles in the dorsal layers of the LGN, they support the conclusion that there are no axo‐axonic synapses in these layers. The ID terminal, the optic terminal and spinous dendrites originating presumably from geniculo‐cortical relay (GCR) cells, are seen to enter into a triadic relationship characteristic of the synaptic glomerulus in which the presynaptic dendrite synapses on the spinous dendrites and the optic terminal synapses on both dendrites. ID terminals reconstructed from serial sections communicate as “internuncial” presynaptic dendrites between optic central axons in adjacent synaptic glomeruli. IA terminals, which like ID terminals contain flattened vesicles, are presynaptic to ID terminals, as well as to postsynaptic dendrites, cell somata and axon initial segments.A model for the internuncial character of multilobed dendritic terminals and the potential for inhibition of Golgi Type II axon terminals is proposed, and theories about the function of Golgi Type II cells in lateral geniculate transmission are briefly considered.