Swelling of dendritic spines in the fascia dentata after stimulation of the perforant fibers as a mechanism of post-tetanic potentiation

Abstract Perforant fibers taking their origin in the entorhinal region of the pyri-form cortex of the mouse make synaptic connections with dendritic spines in the distal (with respect to the granular cells) two-thirds of the molecular layer of the fascia dentata. In electron micrograhs, prepared by freeze-substitution, the mean area of the spines in the proximal one-third of this layer, where no endings of perforant fibers are present, was compared with the mean area of the spines in the distal one-third. Such measurements were made in preparations in which the perforant fibers had been stimulated (at 30/sec, for 30 sec) and in unstimulated controls. In 11 control preparations, the mean area of the distal spines was 13% larger than that of the proximal spines. However, in 14 preparations in which the perforant fibers had been stimulated, the mean area of the distal spines was 53% larger than that of the proximal ones. This difference is statistically highly significant. The spine swelling was present 2 min after the end of stimulation and had not noticeably decreased after 60 min (the longest time investigated). The swelling of spines, resulting in a decrease of their length resistance and an increase of their length constant, is proposed as a mechanism of the long lasting potentiation observed in this monosynaptic pathway.

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