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Changes in some parameters of the presynaptic bag in synapses of the somatosensory cortex of the rat brain after repetitive electrical stimulation prolonging self-sustained after-discharges. An electron microscopic morphometric study.

The sensorimotor area of the rat cerebral cortex was subjected to repeated electrical stimulation at 10 min intervals; this progressively prolonged the duration of self-sustained after-discharges (SSAD). Ten minutes after the third SSAD ended, the animals' brains were perfused with fixing fluid. The homotopic area of the contralateral hemisphere was examined electron microscopically. In type I synapses in the second cortical layer we determined A) the mean areas of sections of the presynaptic bag, B) the length of the active zone of the presynaptic bag, C) the mean total number of agranular synaptic vesicles per 0.01 micron2 area of the presynaptic bag in the experimental and control rats. In the experimental animals value A) rose by 34%, value B) by 14%, value C) by 95% and value D) by 52%. The differences are all statistically significant. Findings A) and B) are most likely evidence of ionic shifts, while findings C) and D) testify to the increase in the number of synaptic vesicles in the whole presynaptic bag and also to their redistribution towards the synaptic cleft. The possibility that findings C) and D) are the morphological correlate of increased synaptic transmission during experimental treatment leading to the kindling phenomenon cannot be ruled out.