Thalamocortical Connections

Such data as are available pertain mainly to thalamo-cortical systems terminating in primary sensory areas since these can often be identified by physiological and/or structural criteria. For example, the two main groups of geniculocortical axons terminating in the primary visual cortex have been physiologically characterized, labeled by intraaxonal injection of tracers, and then morphologically analyzed by histological reconstructions. Such studies demonstrate that parvo-cellular axons have a small diameter, small arbors (~0.2 mm in diameter), and a moderate number of terminal specializations (< 1500), consistent with their functional specialization for form and color vision. Magnocellular axons, which are associated with motion vision (requiring low spatial but high temporal resolution), have a larger diameter, larger arbors, and a large number of terminal specializations (>3000). Since each termination makes on average two synapses, if there is one synapse per postsynaptic neuron, then a reasonable estimate is that a single parvocellular axon diverges to about 3000 postsynaptic elements and a single magnocellular axon to at least 6000. The degree of convergence of parvocellular axons has been estimated to be at least 20-40. These numbers are important for further understanding of neuronal transformations, which are considerably more complicated than simple "relays."

EM studies suggest that both parvo- and magno-cellular pathways have similar postsynaptic pools, which suggests that other aspects of circuitry, such as recombinations with local and other extrinsic connections, contribute to the physiological differences associated with the cortical stages of these two pathways. Further analysis of network properties, at the resolution described previously for dual or triple intracellular recordings, will depend on new technical advances.

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