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I am curious about how many different thalamocortical relay cells synapse onto each layer IV spiny stellate cell, on average? The answer is likely to be different per region and species, of course. I understand the topographic mapping that occurs, but how narrow or wide is the final projection?

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From Shepherd's Synaptic Organization of the Brain (5th ed, pg 513):

In the cat visual cortex, the terminal arbors of each individual thalamic afferent may extend over 1-5 mm of the cortical surface (Fig. 12.11) so that each point in layer 4 is covered by the arbors of at least 1000 separate thalamic relay cells. Thus, the dendritic tree of an average layer 4 neuron, which extends for 200-300 micron, could receive input from many more thalamic afferents. However, the connections are not made randomly between the geniculate afferents and the cortical neurons. Selectivity is expressed in several ways. For example, there is a high degree of precision in the visuotopic map recorded in the first-order cortical neurons in the input layer, i.e., those receiving mono-synaptic activation by the thalamic afferents. This clustering is made according to the eye preference of the arbors. The afferents of those thalamic relay neurons that are driven by the right eye cluster together in regions about 0.5 mm in diameter and are partially segregated from the afferents that are driven by the left eye. This segregation forms the basis of ocular dominance columns.

There are, of course, many other factors in the specificity of thalamocortical projections. Point here being that thalamocortical projections are not necessarily limited to cortical neurons with a shared receptive field in visual space, but may be limited in other ways.

Interestingly, the corticothalamic feedback projection is also not limited to shared spatial receptive fields (Shepherd, p. 321ff.):

Thus, for the lateral geniculate nucleus, this cortical pathway comes from visual cortex (mostly areas 17, 18, and 19), and likewise, somatosensory and auditory cortex project back, re- spectively, to the ventral posterior lateral and medial geniculate nuclei. One implication of this reciprocity is that the corticothalamic pathway faithfully ad- heres to the map established in the thalamic nucleus. For instance, the corticogenicu- late pathway conforms to the retinotopic map in the lateral geniculate nucleus. However, there is some question as to the extent to which the maps match at the cellular level. This is based on evidence that, in the cat (Murphy and Sillito, 1996), the spread of an individual corticogeniculate axon arbor can be quite extensive, reaching well beyond the region within which receptive fields that match those of the cortical axon can be recorded. The corticogeniculate terminals have a maximal extent of 1.5 mm compared with the spread of a typical retinogeniculate arbor of only about 0.2-0.4 mm (Bowling and Michael, 1984; Sur et al., 1987). The retinogeniculate arbor's expanse roughly corresponds to the size of a geniculate receptive field, implying that the corticogeniculate axonal arbor can contribute to subtle effects on relay responses beyond the "clas- sic" receptive field. However, the majority of the corticothalamic terminals lie in a central core that roughly corresponds to the classical receptive field.

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