Synaptic Organization of the Lateral Pulvinar Nucleus in the Cat

Abstract

Synaptic organization of the cat lateral pulvinar nucleus was studied electron microscopically in the materials perfused with buffered formalin solution and post-fixed with OsO4. The receptive areas of the pulvinar neuron can be classified into 1) small, direct issues of the dendritic trunk, which make the glomerular synapse (MAJOROSSY et al. 1965), 2) pro imal den ritic branches making complex axo-dendritic synapses and 3) distal fine dendritic branches with rather simple synaptic contacts with nerve terminals. On the other hand, three kinds of axon terminals are identified in the neuropile of the pulvinar: a) most frequently occurring, smaller terminals containing spherical vasicles alone, b) larger terminals with cylindrical vesicles and c) the largest mossy endings of rather rare occurrence. The ratio in number of the terminals of each type in about a thousand of terminals sampled at random from this brain region is considerably constant among materials from different control animals. Ablation of the cortical regions which project to the pulvinar induced the changes in this ratio, thus suggesting that the reduced type of terminals might correspond, at least in part, to the endings of the ablated projection fibers. The utility of this phenomenon in determining the type of axon terminals of a given projection system was discussed.