Cortical microcircuits are defined as stereotyped, repeating patterns of local connectivity in the cortex. Those pathways by which thalamic afferents influence pyramidal neu?rons, through either direct synaptic connections or through interneurons, constitute feedforward pathways in the microcircuit. The anatomical organization of feedforward pathways in turtle visual cortex was studied by comparing the dendritic geometry and spatial distribution of Golgi-impregnated neurons to the spatial distribution of HRP-filled geniculocortical afferents. Seven morphologically distinct cortical neuron types are positioned to receive geniculocortical synapses. The geniculocortical afferents form parallel, horizontal delay lines that run from lateral to medial across the visual cortex in the coronal plane. The horizontal dimensions of dendrite arbors for populations of each neuron type were thus measured and found to vary between extremes of 70 u,m and 1100 urn. The time it takes an afferent volley to cross the arbors of the seven types of neurons should vary and determine the properties of the compound postsynaptic potentials elicited by geniculate inputs to each type of cell.
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