• Striking difference between neurons of h

    From ScienceDaily@1:317/3 to All on Wed Nov 10 21:30:42 2021
    Striking difference between neurons of humans and other mammals

    Date:
    November 10, 2021
    Source:
    Massachusetts Institute of Technology
    Summary:
    Human neurons have a lower density of ion channels than expected,
    compared to neurons of other mammals, according to a new study. The
    researchers hypothesize that a lower channel density may have
    helped the human brain evolve to operate more efficiently.



    FULL STORY ========================================================================== Neurons communicate with each other via electrical impulses, which are
    produced by ion channels that control the flow of ions such as potassium
    and sodium. In a surprising new finding, MIT neuroscientists have shown
    that human neurons have a much smaller number of these channels than
    expected, compared to the neurons of other mammals.


    ==========================================================================
    The researchers hypothesize that this reduction in channel density
    may have helped the human brain evolve to operate more efficiently,
    allowing it to divert resources to other energy-intensive processes that
    are required to perform complex cognitive tasks.

    "If the brain can save energy by reducing the density of ion channels,
    it can spend that energy on other neuronal or circuit processes," says
    Mark Harnett, an associate professor of brain and cognitive sciences,
    a member of MIT's McGovern Institute for Brain Research, and the senior
    author of the study.

    Harnett and his colleagues analyzed neurons from 10 different mammals,
    the most extensive electrophysiological study of its kind, and identified
    a "building plan" that holds true for every species they looked at --
    except for humans.

    They found that as the size of neurons increases, the density of channels
    found in the neurons also increases.

    However, human neurons proved to be a striking exception to this rule.

    "Previous comparative studies established that the human brain is built
    like other mammalian brains, so we were surprised to find strong evidence
    that human neurons are special," says former MIT graduate student Lou Beaulieu-Laroche.



    ========================================================================== Beaulieu-Laroche is the lead author of the study, which appears today
    in Nature.

    A building plan Neurons in the mammalian brain can receive electrical
    signals from thousands of other cells, and that input determines whether
    or not they will fire an electrical impulse called an action potential. In 2018, Harnett and Beaulieu- Laroche discovered that human and rat neurons differ in some of their electrical properties, primarily in parts of the
    neuron called dendrites - - tree-like antennas that receive and process
    input from other cells.

    One of the findings from that study was that human neurons had a lower
    density of ion channels than neurons in the rat brain. The researchers
    were surprised by this observation, as ion channel density was generally assumed to be constant across species. In their new study, Harnett
    and Beaulieu-Laroche decided to compare neurons from several different mammalian species to see if they could find any patterns that governed
    the expression of ion channels. They studied two types of voltage-gated potassium channels and the HCN channel, which conducts both potassium
    and sodium, in layer 5 pyramidal neurons, a type of excitatory neurons
    found in the brain's cortex.

    They were able to obtain brain tissue from 10 mammalian species: Etruscan shrews (one of the smallest known mammals), gerbils, mice, rats, Guinea
    pigs, ferrets, rabbits, marmosets, and macaques, as well as human tissue removed from patients with epilepsy during brain surgery. This variety
    allowed the researchers to cover a range of cortical thicknesses and
    neuron sizes across the mammalian kingdom.



    ==========================================================================
    The researchers found that in nearly every mammalian species they looked
    at, the density of ion channels increased as the size of the neurons
    went up. The one exception to this pattern was in human neurons, which
    had a much lower density of ion channels than expected.

    The increase in channel density across species was surprising, Harnett
    says, because the more channels there are, the more energy is required
    to pump ions in and out of the cell. However, it started to make sense
    once the researchers began thinking about the number of channels in the
    overall volume of the cortex, he says.

    In the tiny brain of the Etruscan shrew, which is packed with very small neurons, there are more neurons in a given volume of tissue than in
    the same volume of tissue from the rabbit brain, which has much larger
    neurons. But because the rabbit neurons have a higher density of ion
    channels, the density of channels in a given volume of tissue is the same
    in both species, or any of the nonhuman species the researchers analyzed.

    "This building plan is consistent across nine different mammalian
    species," Harnett says. "What it looks like the cortex is trying to do
    is keep the numbers of ion channels per unit volume the same across all
    the species. This means that for a given volume of cortex, the energetic
    cost is the same, at least for ion channels." Energy efficiency The
    human brain represents a striking deviation from this building plan,
    however. Instead of increased density of ion channels, the researchers
    found a dramatic decrease in the expected density of ion channels for
    a given volume of brain tissue.

    The researchers believe this lower density may have evolved as a way
    to expend less energy on pumping ions, which allows the brain to use
    that energy for something else, like creating more complicated synaptic connections between neurons or firing action potentials at a higher rate.

    "We think that humans have evolved out of this building plan that was previously restricting the size of cortex, and they figured out a way
    to become more energetically efficient, so you spend less ATP per volume compared to other species," Harnett says.

    He now hopes to study where that extra energy might be going, and whether
    there are specific gene mutations that help neurons of the human cortex
    achieve this high efficiency. The researchers are also interested in
    exploring whether primate species that are more closely related to humans
    show similar decreases in ion channel density.

    The research was funded by the Natural Sciences and Engineering Research Council of Canada, a Friends of the McGovern Institute Fellowship,
    the National Institute of General Medical Sciences, the Paul and Daisy
    Soros Fellows Program, the Dana Foundation David Mahoney Neuroimaging
    Grant Program, the National Institutes of Health, and the Harvard-MIT
    Joint Research Grants Program in Basic Neuroscience.

    ========================================================================== Story Source: Materials provided by
    Massachusetts_Institute_of_Technology. Original written by Anne
    Trafton. Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Lou Beaulieu-Laroche, Norma J. Brown, Marissa Hansen, Enrique H. S.

    Toloza, Jitendra Sharma, Ziv M. Williams, Matthew P. Frosch, Garth
    Rees Cosgrove, Sydney S. Cash, Mark T. Harnett. Allometric rules
    for mammalian cortical layer 5 neuron biophysics. Nature, 2021;
    DOI: 10.1038/s41586- 021-04072-3 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2021/11/211110131613.htm

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