Month: September 2022

CORTEX AND SIZE. “The increase in relative size of the cerebral cortex with increasing brain size simultaneously with no systematic change in the relative size of the cerebellum has been used as evidence that these structures are functionally independent and have been evolving separately.” https://lnkd.in/d2aUKat View in LinkedIn

FRESH EYES. “Such discrepancy would support the popular notion that brain evolution equates with development of the cerebral cortex, which comes to predominate over the other brain structures. However, analysis of absolute, rather than relative, cerebral cortical and cerebellar volumes in the same dataset leads to the opposite conclusion: the surface areas of these structures, would be evidence that the cerebral cortex and cerebellum are functionally related and have been evolving coordinately.” https://lnkd.in/d2aUKat View in LinkedIn

THE CLANGER. “As it turns out, however, the underlying assumption that the relative size of a brain structure reflects the relative number of brain neurons that it contains is flawed.” https://lnkd.in/d2aUKat View in LinkedIn

THE DATA. “Now that numbers of neurons are available across rodents, primates and insectivores, we find that the cerebral cortex, despite varying in relative size from 42% (in the mouse) to 82% of brain mass (in the human), contains between 13 and 28% of all brain neurons in 15 of 18 species studied, ranging between 13% (in moles) and 41% (in the squirrel monkey.” https://lnkd.in/d2aUKat View in LinkedIn

THE RATIO. “Instead, the number of neurons in the cerebral cortex increases coordinately with the number of neurons in the cerebellum.” https://lnkd.in/d2aUKat View in LinkedIn

THE DISCONNECT. “Most importantly, this fractional number of neurons in the cerebral cortex relative to the whole brain is not correlated with the relative size of the cerebral cortex.” https://lnkd.in/d2aUKat View in LinkedIn

THE SCALING RULES. “Through the estimation of absolute numbers of neuronal and non-neuronal cells in the brains of different mammalian species and their comparison within individual orders, we have been able to determine the scaling rules that apply to the brains of species spanning a wide range of body and brain masses in rodents, primates, and more recently in insectivores.” https://lnkd.in/d2aUKat View in LinkedIn

SCALING BY LINEAGE. “In contrast to rodent brains, which scale hypermetrically in size with their numbers of neurons, primate brain size increases approximately isometrically as a function of neuron number, with no systematic change in neuronal density or in the non-neuronal/neuronal ratio with increasing brain size.” https://lnkd.in/d2aUKat View in LinkedIn

SCALING DETAILS. “Across insectivore species, on the other hand, the cerebellum increases linearly in size as a function of its number of neurons (as in primates), while the cerebral cortex increases in size hypermetrically as it gains neurons (as in rodents).” https://lnkd.in/d2aUKat View in LinkedIn

IMPLICATIONS. “In view of the similar non-neuronal cell densities across species, hypermetric scaling of brain structure mass as a function of its number of neurons implies a concurrent increase in the average neuronal size (which, in the method's definition, includes not only the cell soma but also the entire dendritic and axonal arborizations as well as synapses.” https://lnkd.in/d2aUKat View in LinkedIn