Quantitative ultrastructural study of boutons of ascending afferents to the feline lateral cervical nucleus.

A quantitative ultrastructural study has been performed of 300 labelled and the same number of unlabelled boutons in the feline lateral cervical nucleus (LCN) of 5 adult cats after cervical or lumbar injections of lectin-conjugated horseradish peroxidase and histochemical reaction with tetramethylbenzidine. On an average the labelled boutons were slightly elongated, had areas of 2.25 micron 2, mitochondrial volume fractions of 11.2% and densities of synaptic vesicles of 35.4.micron-2. The synaptic vesicles were round to flattened with a mean length of 60 nm and length/width ratios of 1.21. The labelled boutons were significantly larger and had significantly lower densities of synaptic vesicles than the unlabelled boutons. The labelled boutons constituted about 14% of the bouton volume of the investigated areas of the LCN. Most of them were axo-dendritic and about 16% synapsed with cell bodies with ultrastructural characteristics similar to the cervico-thalamic projection neurones of the LCN. The number of boutons labelled after the injections comprising 3-5 adjacent segments of the spinal cord was calculated to 6.8 X 10(6). Based on the assumption that they represented terminals of spinocervical tract cells, it was calculated that each of these cells in the lumbar cord gives rise to an average of 4400 boutons in the LCN.