Brain Aging: Difference between revisions

[[File:Brain weight age.gif|thumb|Average brain weight for males and females over lifespan. From the study ''Changes in brain weights during the span of human life.''|360x360px]]A human baby's brain at birth averages 369&nbsp;cm³and increases, during the first year of life, to about 961&nbsp;cm³, after which the growth rate declines. Brain volume peaks at the teenage years,{{pmid|10491603}} and after the age of 40 it begins declining at 5% per decade, speeding up around 70.{{pmid|16461469}} Average adult male brain weight is {{convert|1345|g}}, while an adult female has an average brain weight of {{convert|1222|g}}.<ref>{{Cite book|url = https://books.google.com/books?id=u1bwQj7qdsYC|title = Reader in Gender archaeology|access-date = 2014-09-21|publisher = Routlegde|author1=Kelley Hays |author2=David S. |isbn = 9780415173605|year = 1998}}</ref> (This does not take into account neuron density nor brain-to-body mass ratio; men on average also have larger bodies than women.) Males have been found to have on average greater cerebral, cerebellar and cerebral cortical lobar volumes, except possibly left parietal.{{pmid|16410199}} The gender differences in size vary by more specific brain regions. Studies have tended to indicate that men have a relatively larger amygdala and hypothalamus, while women have a relatively larger caudate and hippocampi. When covaried for intracranial volume, height, and weight, Kelly (2007) indicates women have a higher percentage of gray matter, whereas men have a higher percentage of white matter and cerebrospinal fluid. There is high variability between individuals in these studies, however.{{pmid|17544382}}

However, Yaki (2011) found no statistically significant gender differences in the gray matter ratio for most ages (grouped by decade), except in the 3rd and 6th decades of life in the sample of 758 women and 702 men aged 20–69.{{pmid|21818377}} The average male in their third decade (ages 20–29) had a significantly higher gray matter ratio than the average female of the same age group. In contrast, among subjects in their sixth decade, the average woman had a significantly larger gray matter ratio, though no meaningful difference was found among those in their 7th decade of life.

Total cerebral and gray matter volumes peak during the ages from 10–20 years (earlier in girls than boys), whereas white matter and ventricular volumes increase. There is a general pattern in neural development of childhood peaks followed by adolescent declines (e.g. synaptic pruning). Consistent with adult findings, average cerebral volume is approximately 10% larger in boys than girls. However, such differences should not be interpreted as imparting any sort of functional advantage or disadvantage; gross structural measures may not reflect functionally relevant factors such as neuronal connectivity and receptor density, and of note is the high variability of brain size even in narrowly defined groups, for example children at the same age may have as much as a 50% differences in total brain volume.{{pmid|18346658}} Young girls have on average relative larger hippocampal volume, whereas the amygdalae are larger in boys.{{pmid|17544382}} However, multiple studies{{pmid|11829343}}{{pmid|11829343}} have found a higher synaptic density in males: a 2008 study reported that men had a significantly higher average synaptic density of 12.9 × 108 per cubic millimeter, whereas in women it was 8.6 × 108 per cubic millimeter, a 33% difference. Other studies have found an average of 4 billion more neurons in the male brain,{{pmid|9215725}} corroborating this difference, as each neuron has on average 7,000 synaptic connections to other neurons.

Significant dynamic changes in brain structure take place through adulthood and aging, with substantial variation between individuals. In later decades, men show greater volume loss in whole brain volume and in the [[frontal lobes]], and [[temporal lobes]], whereas in women there is increased volume loss in the hippocampi and parietal lobes.{{pmid|17544382}} Men show a steeper decline in global gray matter volume, although in both sexes it varies by region with some areas exhibiting little or no age effect. Overall white matter volume does not appear to decline with age, although there is variation between brain regions.{{pmid|11525331}}