Types of degenerating geniculocortical axon terminals and their contribution to layer IV of area 17 in the squirrel monkey (Saimiri).

Radiofrequency lesions were made in the lateral geniculate nuclei of six squirrel monkeys. The resulting degenerating terminals and their postsynaptic structures in layer IV of area 17 were quantitatively categorized on photomontages covering large areas of neuropil. Two to five days after the lesion, numerous axon terminals were affected by a variety of degenerative changes, i.e., enlargement and distortion of synaptic vesicles, neurofilamentous hyperplasia, electron-lucent and electron-dense reactions. Based on the aggregation of electron-dense material beneath the postsynaptic membrane, the degenerating terminals were considered to be of the asymmetric type. Among the degenerating boutons were the largest axon endings that occur in layer IV. Three days postoperatively, degenerating boutons contributed an average of 16.2% to the total synapse population; five days postoperatively, the average had increased to 19.3%. The percentage of degenerating boutons on individual montages, however, amounted to as much as 29%. This amount probably reflects more closely the actual contribution of the geniculocortical fiber system to layer IV of striate cortex. The postsynaptic structure most frequently contacted by degenerating axon endings was the dendritic spine, followed by dendrites of small diameter. To account for the diversity of degenerative changes in the same fiber system, we offer the tentative suggestion that heterogeneously degenerating axon terminals arise from a heterogeneous population of neurons in the lateral geniculate nucleus, i.e., from magnocellular versus parvocellular laminae.