An antibody to microtubule-associated protein 2 (MAP2) has been used to examine the arrangements of neurons in striate cortex of the cat. It is found that the apical dendrites of medium-sized and large pyramidal cells in layer V group together to form clusters that have a center-to-center spacing of about 56 μm. As these clusters ascend, the apical dendrites of pyramidal cells in layer ll/lll are added to them. The thinner apical dendrites of the smaller pyramidal cells in layer VIa also form groups that are referred to as bundles. These bundles ascend into layer IV independent of the clusters, and their arrangement suggests that the bundles are formed so that the apical dendrites of the layer VIa pyramids can pass between the groups of cell bodies of the layer V neurons.
It is proposed that the clusters formed from the apical dendrites of the layer V and layer ll/lll pyramidal cells represent the axes of vertical modules of pyramidal cells, which represent the basic neuronal aggregates within area 17 of cat visual cortex. And it is suggested that these modules can be recruited in various combinations, most obviously by the excitation provided by the thalamic inputs, to form the functional columns, such as the ones that are concerned with eye preference and orientation. Based upon the distribution of the dendritic clusters, the pyramidal cell modules would have diameters of 56 μm, and since they are considered to extend through the depth of the cortex, each one would contain some 203 neurons. The striate cortex of one hemisphere contains 160, 000 of these modules, which is about the same as the number of X-cells projecting to one hemisphere from the dorsal lateral geniculate nucleus and twice the number of X- or β-ganglion cells in the retina. The form of the pyramidal cell modules in cat striate cortex is compared to those present in monkey striate cortex, in which the similar modules are about 10 times more numerous, but only 31 μm in diameter (Peters and Sethares, 1991a).