Factors Involved in the Establishment of Specific Interconnections between Thalamus and Cerebral Cortex

How, during development, do nerve fibers from one part of the brain find their way to the appropriate target cells? This problem seems particularly acute for the cerebral cortex, which is characterized by the remarkable diversity but exquisite specificity of its afferent inputs and efferent outputs. In humans, this vast sheet of tissue is about 2000 cm2 in area, is 3–4 mm thick, and contains perhaps 20 billion neurons. Although there is morphological similarity across the entire neocortex (everywhere it has a six-layered structure with the same basic types of differentiated neurons), there are distinctive regional variations in the proportions of different cell types and the relative thickness of layers.