Are there structural alterations in synapses related to functioning?

Abstract

One would like to know whether synapses change structurally during learning or as a result of increased use, or disuse. The difficulties lie in deciding where learning takes place, how to produce a considerable increase in the amount of learning, or of use, or disuse, and how to produce such a change uniformly throughout a piece of tissue which can be examined anatomically. In recent years it has been found that exposure of light-reared animals to darkness, or of dark-reared animals to light, has effect on the cellular structure of the visual system that can be measured by light microscopy (see review by Riesen 1966). I have tried to extend this work by looking at synapses in the visual system with electron microscopy. This technique allows one to estimate the area of the synaptic profiles, to calculate the density of the axon terminals in the tissue, to measure the length of thickened apposition with the post-synaptic process and to look at the density and size of the synaptic vesicles within the synaptic profiles. Local variations in these parameters necessitate random sampling of the tissue, and this is provided by examination of several small blocks obtained by chopping the formalin-perfused tissue in osmium tetroxide solution. At all stages of the preparation from the living animal to the measurement of synapses it is essential to maintain strict pairing and identical treatment of the tissues to be compared. Sections from the light- and dark-exposed animals were mounted on opposite sides of finder grids, and batches of six plates were made from each kind of tissue alternately. The material was photographed and measured without knowing at the time whether it came from a light- or a dark-exposed animal.