Section_2_Notes

Section_2_Notes - Cognitive Neuroscience Lecture 2...

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Cognitive Neuroscience Lecture 2 Phylogeny of the cortex The neocortex of mammals developed out of the primordial neopallium , which, like that of certain present-day amphibians, consisted of one layer of pyramidal cells. Evolution of the neocortex led to the present-day layered neocortex, which is similar across all mammalian species. The volume of tissue represented by the neocortex has also grown with evolution in comparison to the rest of the brain. The process by which the neocortex becomes larger and heavier with evolution is called neocorticalization . Evolutionary development of the neocortex with respect to total brain size is greatest in primates. Growth takes place in both thickness and surface area, but more so in surface area. The expansion in surface area is 1000-fold from mouse to man. Achieved by increase in number of radial columnar units. New areas developed from older ones in this process.
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Each mammalian species has cortical representations for the same 5 basic sensory modalities, but the number of specialized areas for each modality varies greatly across species. The apical dendrites of cortical pyramidal cells lengthen with neocorticalization. The pyramidal cells with soma in the deepest layers (V & VI) have the longest apical dendrites, and are the largest and oldest. Their greater dendritic surface area allows greater numbers of spines and synapses. Consequently, they have greater network-forming capacity. The subcortical white matter also greatly increases in volume along with neocorticalization. This indicates an increasing number of cortical areas and a greater number of pathways between them. Conclusion : 3 factors cause capacity of neocortical neurons to form synaptic connections with one another to increase in phylogeny: 1) the addition of areas and columns 2) the elongation of apical dendrites 3) the expansion of the white matter Because this capacity is greatest in the human brain, the ability to form complex networks that support cognition is greatest.
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Ontogeny of the cortex In ontogeny, the neocortex develops to a greater degree in size and volume than other parts of the brain. Also, the relative growth of the white matter beneath the cortex is far greater than elsewhere in the CNS. The neocortex is largest relative to the rest of the brain in humans. During embryogenesis, neuroblasts migrate along glial fibers from the ventricular wall of the neural tube to the cortical plate . Ontogenetic sequence: 1) formation of a primordial plexiform layer under the cortical surface 2) formation of the cortical plate in it as the base for the layered adult neocortex 3) outward migration of neuroblasts from the underlying ventricular zone to the upper layer of the cortical plate 4) relative descent of cell bodies to deeper cortical layers 5) proliferation of layers II to VI (and subdivisions), as descending cells settle at various distances from the surface The cortical plate thickens as cells stack up: newer cells migrate past older ones,
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Section_2_Notes - Cognitive Neuroscience Lecture 2...

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