The termination of geniculocortical fibres in area 17 of the visual cortex in the macaque monkey.

The termination of geniculocortical fibres within the different subdivisions of lamina IV in area 17 of the visual cortex of the monkey has been studied quantitatively with the electron microscope. In lamina IVC alpha the axon terminals of fibres coming from the magnocellular layers of the lateral geniculate nucleus (LGN) make significantly more synapses per bouton than those of fibres arising from the parvocellular layers and terminating in laminae IVA and IVC beta. In all parts of area 17 examined there was a clear difference in the relative proportions of multisynaptic geniculocortical boutons between the alpha and beta divisions of lamina IVC. Calculations have shown that a single cell in the magnocellular laminae of the LGN may make about 6 times as many synaptic contacts within lamina IV of the visual cortex than one in the parvocellular laminae. It has also been estimated that there are at least 500 million geniculocortical boutons, or 1200 million synapses, in lamina IVC alpha and 100 million boutons, 1200 million synapses, in lamina IVC beta for one hemisphere, giving an approximate total number of 1500 million boutons and 2400 million synapses.