Regional differences in synaptogenesis in human cerebral cortex

Abstract

The formation of synaptic contacts in human cerebral cortex was compared in two cortical regions: auditory cortex (Heschl's gyrus) and prefrontal cortex (middle frontal gyrus). Synapse formation in both cortical regions begins in the fetus, before conceptual age 27 weeks. Synaptic density increases more rapidly in auditory cortex, where the maximum is reached near postnatal age 3 months. Maximum synaptic density in middle frontal gyrus is not reached until after age 15 months. Synaptogenesis occurs concurrently with dendritic and axonal growth and with myelination of the subcortical white matter. A phase of net synapse elimination occurs late in childhood, earlier in auditory cortex, where it has ended by age 12 years, than in prefrontal cortex, where it extends to midadolescence. Synaptogenesis and synapse elimination in humans appear to be heterochronous in different cortical regions and, in that respect, appears to differ from the rhesus monkey, where they are concurrent. In other respects, including overproduction of synaptic contacts in infancy, persistence of high levels of synaptic density to late childhood or adolescence, the absolute values of maximum and adult synaptic density, and layer specific differences, findings in the human resemble those in rhesus monkeys.

Altered Cortical Activity in Prelingually Deafened Cochlear Implant Users Following Long Periods of Auditory Deprivation

“…In turn, this might lead to activation of a large number of less specific axons, rather than a few specialized neurons (Innocenti and Price 2005). Since synaptic pruning is thought to be completed by around 12 years of age in the auditory cortex (Huttenlocher and Dabholkar 1997), these aberrant cortical connections might last and be stabilized due to the long-term auditory deprivation in our prelingually deaf group (Innocenti and Price 2005;Uhlhaas and Singer 2011). Kral et al (2006) demonstrated that the naïve auditory cortex of congenitally deaf cats has a reduced sensitivity to auditory input, leading to a smeared representation of auditory stimuli with a deficient representation of auditory features including loss of tonotopy (Kral et al 2006).…”

“…We addressed this cascade of cortical developmental events by recording CAEPs in adult cochlear implant users who became deaf before the age of 2 years and received their implants during adulthood and had therefore much longer durations of auditory deprivation than the late implanted, prelingually deaf children in the abovementioned studies. The long period of auditory deprivation will most likely prevent neural maturation processes, which may normally occur throughout adolescence, such as synaptic pruning (Huttenlocher and Dabholkar 1997), to emerge after cochlear implantation. Extrapolating the results of Gordon et al (2008) a (near-) normal N1 would be possible in these subjects; based on the other studies their CAEPs are expected to lack a significant N1.…”

EEG gamma-band synchronization in visual coding from childhood to old age: Evidence from evoked power and inter-trial phase locking

“…Fine-tuning of visual processing may entail the elimination (pruning) of redundant synapses and the facilitation of inter-areal connectivity (Huttenlocher and Dabholkar, 1997;Thatcher, 1992). In this view, neuronal development may progress towards increasingly sparse representations and higher temporal synchronization leading to more efficient stimulus processing.…”

Abstinent adolescent marijuana users show altered fMRI response during spatial working memory

“…During this period of increasing marijuana use, continued neuromaturation includes synaptic refinement, myelination, and improved cognitive and functional efficiency (Huttenlocher and Dabholkar, 1997;Giedd et al, 1999;Casey et al, 2000;Paus et al, 2001;Gogtay et al, 2004). The potential long-term consequences of marijuana use on the developing adolescent brain have not been well delineated, but they could have major implications for academic, occupational and social achievement.…”