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esearchers have recently identified a neuron in the
brains of mice that is crucial for navigation.
Researchers at the
University of Michigan in Ann Arbor have identified a previously unrecognized
excitatory neuron in mouse brains. This neuron, they say, is key for
navigation.
This finding, which now
appears in the journal Cell
Reports, may aid scientists’ understanding of how the part of
the brain responsible for navigation — the retrosplenial cortex (RSC) — goes
about navigating prolonged distances.
Navigation and neurology
Scientists know that the
RSC is crucial for navigation. In fact, if a person’s
RSC is damaged, not only can they experience memory loss, but their ability to
navigate will also be severely reduced.
For example, a person with a damaged RSC may find it
very difficult to navigate their usual route from work to home.
However, scientists are
less clear on how the parts of this cortex interact so that a person can
navigate successfully.
By identifying this
previously unrecognized excitatory neuron, the researchers gained information
on what this new neuron does, as well as what the other key neuron in the RSC
does.
Two types of neuron
The researchers studied the
RSCs of mice and produced computational models that were able to simulate the
realistic reactions of RSC cells to various types of information.
This allowed them to
determine that there are two distinct neurons that focus on different forms of
navigational information.
One type, called common
regular-spiking (RS) neurons, responded to changes in the direction of the
head. The other type, called low rheobase (LR) neurons, responded to the head
maintaining a persistent direction.
The neurons do this because
of differences in how they receive information.
The RS neurons respond to
significant changes, but after this, they are unable to keep firing to receive
more information. This is because their signals get slower after the initial
significant change in direction.
By contrast, the LR neurons
fire consistently and more rapidly, and they can respond to more subtle
directional changes. This means that they are able to detect very small
variations in the position of the head that suggest a continuous direction of
movement.
‘The Little Neuron That Could’
According to study
co-author Ellen Brennan, who identified the LR neurons, “A simpler name for
this small yet tenacious little neuron, as suggested by my classmate, would be
‘The Little Neuron That Could.'”
“It’s the perfect name
because it highlights the persistence that makes them optimally suited to code
continued direction. In comparison, the other typical excitatory neurons here
are slow and stubborn.”
As lead study author Dr.
Omar Ahmed explains: “Regular neurons in the cortex are good at encoding
directional information only when you are moving your head, but what happens
when your head is still? You still need to know what direction you are facing
so that you can use this information to plan your route.
“You ideally need another
kind of neuron — a neuron that can continuously encode your orientation over
long durations even when your head is not moving.”
The RSC needs both of these
neurons working together to aid navigation. As Brennan notes in a video detailing
the findings, not having LR neurons that detect directional movement is similar
to having a compass but not knowing which way North is.
A link to Alzheimer’s disease?
The researchers are now
investigating whether or not these findings would be helpful in understanding
how Alzheimer’s disease affects these neurons. Alzheimer’s is the most common
type of dementia. Among other things, it affects a person’s spatial
orientation. Currently, there is no cure for this condition.
As Dr. Ahmed notes: “The
retrosplenial cortex is critical for spatial orientation, but is one of the
earliest brain regions to show dysfunctional activity in [people with
Alzheimer’s]. This is probably why the vast majority of [people with
Alzheimer’s experience] spatial disorientation and get lost easily — because
their retrosplenial cells are not working as they should.
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