| Mind
& Maze |
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From the Research Front
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| Did
you know there are cells in your body that can grow up to be neurons?
Stem cells are undeveloped or “baby” cells. Typically stem cells
are found in your bone marrow and grow up to be red blood cells.
But, recently scientists have found that these same stem cells can
grow up to be neurons if they are transplanted to the brain. This
is exciting, because it may help doctors to treat people with brain
injuries. |
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| Mind
& Muscle Maze Module |
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Authors:
Lynne
E. Houtz, Ph.D., Mindy Oxenford, Erinn Hoagland, Lisa Eaton,
Andrea Zardetto-Smith, Ph.D.
Developers: David Crotts,
Lisa Eaton, Marcie Frazier, Erinn Hoagland, Mindy Oxenford |
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The
future peripheral nervous system begins as a population of cells
along the edges of the neural tube known as the neural crest.
The output processes of these cells – the axons of developing
neurons in the spinal cord- must find their way to a particular
muscle in the body in order to make precise connections with
those muscle cells for motor movement to be possible. This process,
called axon migration, has been an area of intense research
in the last 25 years, and neuroscientists have discovered much
about how this amazing process occurs.
The
first axons to develop and migrate in the embryo, whose tracts
are used by later developing axons to migrate to their target
(although not all late-growing axons project along such a
pre-established pathway) are called pioneer axons. The tips
of the growing axons form structures called growth cones.
This structure moves in an ameboid-like fashion, with spikes
or “toes” called filopodia extending in and out
from the growth cone. They extend from the cone to explore
the environment and may attach so the growth cone advances.
The path of the growth cone is determined by the presence
or absence of growth cues. These growth cues are, in actuality,
adhesion molecules (proteins) in the extracellular matrix.
Cues can be short-range (local) or long-range, and each can
be positive (and attracting) or negative (and repelling).
The cues are recognized by specific receptors on the growth
cones. By following the layout of adhesion molecules in the
embryonic environment, axons can be directed along a particular
pathway to its ultimate destination. Guidepost cells assist
by marking off the end of one pathway and the beginning of
the next that developing axons use to migrate to their target.
Developing
axons also undergo chemotropic guidance by responding to chemoattractants
and chemorepellants. Chemoattractive factors are secreted
by the targets of developing axons; the factors are diffusible
and can attract the axons from a distance to help guide them
to the correct site where synaptic contact should be made.
Chemorepellants
are the opposite of attractants. Cells located near neurons
may secrete a repellant to make sure the axons are “pushed”
away in the right direction of their target organ. Another
type of chemorepulsion occurs when factors deflect axons away
from a particular target. Also, chemorepellant factors may
assist in keeping the axon in the vicinity of the target organ
and not grow beyond (or “overshoot”) the correct
destination.
Once the axon has reached the target organ, and the appropriate
cell selected with which to make a connection, the synapse
must be established. Initially more synapses than needed will
be produced. Later, a refinement process (in part depending
on the activity of the target cell) to eliminate the excess
synapses. How synaptic targets of regulate the development
of the motor and sensory cells that innervate them is one
of the most unique chapters in neuroscience history, and spotlights
the work of Nobel prize winner Dr. Rita Levi-Montalcini.
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