Degeneration and electron microscope analysis of the synaptic glomeruli in the lateral geniculate body

Summary

Optic fibers of retinal origin terminate in the lateral geniculate body exclusively in the so called glomerular synapses. They can be recognized on the basis of their unusually large irregular mitochondria having very few cristae. In the cat the structure of the optic terminal profiles is rather dense. The majority of terminals in most glomeruli originate from axons of other source. Relatively large axon terminal profiles of unusually light structure cannot be brought to degeneration by any interference with extraneous pathways. From Golgi information it becomes obvious that they originate from local Golgi 2nd type neurons. Small rather dense axonal profiles of the glomeruli can occasionally be traced back by degeneration to the occipital cortex (parastriate), although most of the descending cortical afferents of the lateral geniculate body terminate outside the glomeruli on more proximal parts of the dendrites. — Axo-axonic synapses are very frequent. If an optic terminal is involved, it appears that by structural standards it is “presynaptic” to the non optic. As judged, however, from the numerous axoaxonic contacts persisting after enucleation, many of the contacts are established between non optic axon terminals. — The progress of secondary degeneration and particularly the removal from the glomeruli of degeneration fragments is unexpectedly rapid. — The possible functional significance of these findings, especially also with regards to presynaptic inhibition, is discussed.