Microcircuitry of cat visual cortex: classification of neurons in layer IV of area 17, and identification of the patterns of lateral geniculate input.

Neurons in the cerebral cortex have been classified primarily by their differences in axonal and dendritic branching patterns observed in material impregnated by the Golgi method. Although these morphological differences are widely belieyed to reflect differences in connectivity, very little is actually known about the patterns of synaptic input to different cell types. We have obtained such information for 32 adjacent neurons in layer IVab of cat cortical area 17 by reconstructing them from electron micrographs of 150 serial sections. Synaptic terminals from the lateral geniculate nucleus were labeled in this material by anterograde degeneration and their distribution, as well as that of normal terminals containing flat or round vesicles, was recorded. The neurons were divided into seven classes based on differences in size, shape, dendritic branching pattern and synaptic input. Class I cells were pyramidal with apical and basilar dendrites, dendritic spines, exclusively flat-vesicle terminals on the somas (11/100 μm2), and geniculate terminals on the basilar dendrites. Class II cells were large stellates (20 μm diameter) with dark cytoplasm and numerous flat-vesicle and round-vesicle terminals on the somas (48/100 μm2). Geniculate terminals contacted the cell bodies and primary, secondary, and tertiary dendrites. The Class III cell was stellate with varicose dendrites, a sparse distribution of flat-vesicle terminals (8/100 μm2) on the soma, and both geniculate and round-vesicle terminals on the dendrites. Class IV cells had radially elongated somas with sharply tapered apical and basilar dendrites bearing spines. There was a meduim distribution of flat-vesicles terminals (17/100 μ2), to the somas while geniculate terminals were restricted to the secondary dendrites. Class V cells were multipolar with flat-vesicle terminals on the somas (11/100 μm2) and a few geniculate terminals on the dendrites. Class VI cells were mostly small (as small as 7 μm diameter), with a sparse distribution on the somas of both flat-vesicle and round-vesicle terminals (7/100 μm2). Two cells had geniculate terminals on their somas. Class VII cells had sharply tapered apical and basilar dendrites, both flat-vesicle and round-vesicle terminals on the somas (14/100 μm2), and no geniculate input. The results make clear that the neurons in layer IVab are quite heterogeneous, not merely in their intrinsic morphology, but also in their patterns of connectivity. The geniculate input is not funneled to a single type of neuron but diverges widely, contacting at least six different cell types, and may form on each a pattern that is characteristic for the type. The reconstruction approach, in providing a detailed identification of the synaptic patterns on substantial numbers of adjacent cells, should make it possible to address directly certain unanswered questions about cortical circuitry. It should be possible to determine, for example, whether the synaptic patterns described here are constant enough to allow identification of the same cell class in different animals as is the case for neurons of invertebrates. It might also be possible to determine for specific cell classes how the synaptic patterns change during development and under unusual rearing conditions.