Numerical relationships between geniculocortical afferents and pyramidal cell modules in cat primary visual cortex.

An analysis has been made of the quantitative data available on the number of pyramidal cell modules of layer IV neurons, and of geniculocortical axons and their synapses in cat striate cortex. It is found that the convergence of geniculocortical afferents upon any one pyramidal cell module is enormous, since in any one location there is overlap between 360-540 X-axons and 300-540 Y-axons. In total, the X- and Y-axonal arbors provide some 1640 x 10(6) synapses to area 17, which is equivalent to a ratio of 160-200 synapses per layer IV neuron. These values assume that geniculocortical terminals synapse only with the spiny stellate cells of layer IV. The values are reduced to 100-125 per spiny stellate cell when account is taken of the synapses that involve the dendrites that enter layer IV from neurons with cell bodies in other layers. Since each layer IV neuron receives some 2500 asymmetric synapses, this means that only 5% of the total excitatory input to a layer IV neuron seems to be provided by the geniculocortical afferents. Further, if the boutons in the geniculocortical axonal arbors are distributed homogeneously across layer IV, each axon could only provide one synapse to about one in four of the layer IV neurons encompassed by its plexus. It may be, however, that instead of being spread evenly, boutons in individual arbors converge upon individual neurons to supply a number of synapses to them. But even so, it seems unlikely that any individual geniculate axon could dominate the activity of a particular cortical neuron.