SHORT FEATURE
JAYASHREE DAS & PRADIPTA BANERJEE
The word hippocampus means “water-horse” in Latin and the
name originates from the strange semi-circular crescent shape of
this part of the brain.
MAGINE a bright day on a winter
morning, walking down the streets of
Bangalore. All of a sudden, a passerby
asks you “How far to the Lalbagh
Gardens?”
I
Questions such as these are quiet
easy to answer if you are aware of the
location. And your brain performs this
function quite well. You stretch your
head to see the destination (if it is near),
point it out with your hand and verbally
announce the distance and direction. The
job done, the questioner offers gratitude
and you both resume your activity.
You thought it was an easy
accomplishment.
But
have
you
ever wondered: how does the brain
comprehend distance and location? That
pink, soft, mass hidden inside the skull
is just a tangle of a million nerve cells
communicating with each other through
electrical signals. How can complex
ideas such as space, time and location be
comprehended by this tangle of electrical
circuits?
Providing a direction and location
you already know to another person can
be done based on your experience about
the location. This is termed as experience-
dependent navigation
and there are three
essential elements to
this: the evaluation
of the information
obtained
visually
and verbally (your
current position), the
integration of this
information
with
past knowledge about the destination,
and a comparison with internal state
information (such as motivation, stress,
and hormone status). A combination
of these three factors determines the
appropriate behavioral output. This bit
explains the “answering” part but it
doesn’t answer the primary question.
How does the brain remember its location
and distance?
The brain forms a map of the areas
and environment that you frequent and it
has specialized neuronal cells to do so. As
you wander around a new environment,
at least f ive sets of neurons initiate
signaling (or fire up) to form a sort of
“map” of the environment. Most of them
are present in a part of the brain called the
“hippocampus” and its adjoining areas.
(left) The eerie
similarity between the
hippocampus and the
seahorse
The
word
hippocampus means
“water-horse” in Latin
and the name originates from the strange
semi-circular crescent shape of this part
of the brain. It is the main part of the
limbic system of the brain, a part that
deals with emotions. For example, if you
are fascinated by this article, that’s your
limbic system at work. If you are not, well
that’s the limbic system too!
A class of neurons called ‘place
neurons’ in the hippocampus exhibits a
high rate of ring whenever you reach a
specif ic location in that environment. For
example, while walking down a simple
road, you see a fountain, or a strange
looking building, or practically anything
to evoke interest, some place neurons
f ire up. The next time you visit that place,
the same neurons will f ire up with equal
intensity.
Parietal Lobe
The hippocampus may play a
role in calculating the present
Occipital
position by keeping track of the
Lobe
previous movements through the
environment, a process called
“path integration”.
Frontal Lobe
Hippocampus
The mammalian brain showing the
location of the hippocampus
The four lobes of the brain, divided based on
position and functionality
Temporal Lobe
31
Science Reporter, APRIL 2016
SHORT FEATURE
Experiments
in rats have
suggested the
presence of
other neurons.
The grid cells
are embedded
in a network
with “head
direction”
cells and
“border”
cells, and in
many cases,
cells with a
combined
function.
The grey square depicts the environment in which a rat
moves. Place cells fire whenever the animal reaches a specific location (top left figure). Different place cells fire for different areas. A single grid cell fires when the animal reaches a
single location (bottom left figure). The locations are arranged
in a hexagonal pattern. Combined together, place cells help
recognize a specific location in a space field and grid cells
create a meta- representation of the 3-D space by creating a
basic coordinate system (bottom right).
Picture courtesy of nobelcomitee.org
Interestingly, the f iring of place cells
is not controlled by the magnetic north
nor does it depend on the time of the day.
Rather, it depends solely on the location of
the observer relative to the arrangement
of objects within an environment.
When you repeat the steps, for example
on the second or third visit, the place
f ield is strengthened, which indicates
that: place f ields are related with memory
formation, something that also occurs in
the hippocampus (coincidence?).
The hippocampus may play a role
in calculating the present position by
keeping track of the previous movements
through the environment, a process
called “path integration”. Imagine your
first visit to the gardens. When you are
walking in the garden, hippocampal
neurons calculate your present position
by comparing with particular place
cell f iring, the velocity and direction
of movement, as well as the distance
traveled from a known start position.
Grid cells are neurons present in
a portion of the brain called medial
entorhinal cortex. Let us understand the
location of this structure in the human
brain. The mammalian brain is divided
into four lobes: frontal, parietal, occipital
and temporal. The entorhinal cortex is
located in the medial temporal lobe and
functions in navigation and memory
formation. The name indicates that it is
interior (ento) to the “rhinal fissure”, a
furrow in the surface of the brain.
The firing pattern of a rat’s grid cell as the
animal moves around in a one-meter square
box. The red colour is indicative of high gridcell activity and dark blue implies no activity.
The neuronal firing is spaced at regular
distances, with the active areas forming a
hexagonal grid pattern. Photo courtesy of www.
Kavlifoundation.org
Science Reporter, APRIL 2016
Grid cells make up a basic coordinate
system, like the X- and Y-values that you
draw in a graph. They f ire when you
traverse small, equidistant areas in a large
region. So, as you walk and twist and
turn and climb up or down in the Lalbagh
garden (or anywhere else for that matter),
one single grid neuron fires up after every
few steps, irrespective of any prominent
landmark. In the brain, these activated
grid cells lie equidistant from each other
and are arranged in hexagonal shapes
with each grid neuron occupying one
corner of the hexagon.
Together, these hexagonal units
create an exact map of a large threedimensional area. The interesting thing
about grid cells is that though they are
anchored to external landmarks, they
can persist in darkness, suggesting that
you can even form a grid coordinate
32
map of an entire area even if you walk
with closed eyes. Grid cells in the same
area of the medial entorhinal cortex f ire
with the same spacing and orientation
of the grid, but different phases, so that
adding all the phases together they cover
every point in the environment. The grid
system provides a solution to measuring
movement distances and adds a reference
to the spatial maps in hippocampus.
Experiments in rats have suggested
the presence of other neurons. The grid
cells are embedded in a network with
“head direction” cells and “border” cells,
and in many cases, cells with a combined
function. Direction cells act like a compass
and are active when the head of an animal
points in a certain direction. Border cells
are active in reference to walls that the
animal encounters when moving in a
closed environment.
To
summarize,
hippocampal
neural activity helps in creating a threedimensional map of any place inside your
head with the help of a large network of
place and grid neurons, helped further by
boundary-, border- and head direction
moving-cells.
So, the next time someone asks for
directions, respond with “Wait, let me ask
the water-horse!”
Mrs. Jayashree Das (jayashree_das671@gmail.
com) and Dr. Pradipta Banerjee (roon17779@
gmail.com) are Assistant Professors in the
Department of Biochemistry-UG, School of
Biological Sciences, Dayananda Sagar Institution,
Shavige Malleshwara Hills, Kumarswamy Layout,
Bangalore–560078