Cortical Synapse Density
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Synapses are structures in neurons that allow for the transmission of electrical and chemical signals to neighboring cells. Plasticity of synapses, their maturation, and the development of their specific morphology has been shown to reflect selective strengthening and weakening of connections between cells thought to underlie processes of learning and attention. Several lines of indirect evidence have suggested that association areas of the human cerebral cortex, particularly in the prefrontal cortex, may be enriched in the number and distribution of synapses due to the elaboration of dendritic branching patterns and the increased density of dendritic spines in individual neurons. These changes likely reflect differences in the expression patterns of certain genes including THBS4 and SRGAP2.
Cortical synapses are found at the heads of dendritic spines, which develop in early gestation and mature postnatally. During early postnatal development, spine number is at its highest, and pruning gradually reduces the total number of dendritic spines to normal adult levels. Total spine numbers decrease with age. Spines grow and develop throughout the lifespan, due to the motility and plasticity in their cytoskeletons that allows for dynamic interactions with neighboring cells. Mature spines have a distinctive morphology, and the thickness of the synaptic end of dendritic spines is generally thought to be a reliable indicator of synaptic strength, with stronger connections reflected in more robust, mushroom-like spines.
Studies that have examined the extent and distribution of dendritic spines have shown increased density of dendritic spines in higher-order association cortex, which (in contrast to primary areas) integrates information from multiple modalities, important for cognitive integration and processing. Recent research has shown a similar pattern of regional variation in spine density relative to cortical area examined, with a substantial increase in spine density noted in the prefrontal cortex (specifically, in Brodmann’s area 10). It is likely that other cortical areas involved in higher-order cognitive processing display a similar pattern of increased spine density in the human brain as compared to the brains of chimpanzees.
An increase in density of cortical synapses has been proposed to relate to differences in the expression of thrombospondin 4, a glycoprotein found outside of cells that encourages the growth of neurites and synapses. Further, the SRGAP2 gene, which has undergone 2 duplication events in recent human evolution, is also implicated in the growth and maturation of spines. Specifically, its human variant encourages spine growth and greater density, as well as neoteny in the development of cortical spines. This likely allows for greater plasticity during development at the synaptic level, leading to more flexible behavior.
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