So if true, then I should be able to walk on ice even with my damage to my pre-motor and motor cortex. Not completely true, the last time I walked on ice I fell twice.
http://www.salk.edu/news/pressrelease_details.php?press_id=2070
Walking across an icy parking lot in winter–and remaining
upright–takes intense concentration. But a new discovery suggests that
much of the balancing act that our bodies perform when faced with such a
task happens unconsciously, thanks to a cluster of neurons in our
spinal cord that function as a “mini-brain” to integrate sensory
information and make the necessary adjustments to our muscles so that we
don’t slip and fall.
In a paper published January 29, 2015 in the journal
Cell,
Salk Institute scientists map the neural circuitry of the spinal cord
that processes the sense of light touch. This circuit allows the body to
reflexively make small adjustments to foot position and balance using
light touch sensors in the feet. The study, conducted in mice, provides
the first detailed blueprint for a spinal circuit that serves as control
center for integrating motor commands from the brain with sensory
information from the limbs.
A better understanding of these circuits
should eventually aid in developing therapies for spinal cord injury and
diseases that affect motor skills and balance, as well as the means to
prevent falls for the elderly.
Salk Institute researchers mapped neural circuits in the spinal cord
that process light touch signals from the feet, a critical function for
fine motor tasks, such as walking on ice. This image shows this neural
circuitry in a mouse spinal cord. The red cells are RORα neurons, which
merge signals coming from neural fibers coming from the brain and limbs
(both colored blue).
“When we stand and walk, touch sensors on the soles of our feet detect
subtle changes in pressure and movement. These sensors send signals to
our spinal cord and then to the brain,” says
Martyn Goulding,
a Salk professor and senior author on the paper. “Our study opens what
was essentially a black box, as up until now we didn’t know how these
signals are encoded or processed in the spinal cord. Moreover, it was
unclear how this touch information was merged with other sensory
information to control movement and posture.”
While the brain’s role in cerebral achievements such as philosophy,
mathematics and art often take center stage, much of what the nervous
system does is to use information gathered from our environment to guide
our movements. Walking across that icy parking lot, for instance,
engages a number of our senses to prevent us from falling. Our eyes tell
us whether we’re on shiny black ice or damp asphalt. Balance sensors in
our inner ear keep our heads level with the ground. And sensors in our
muscles and joints track the changing positions of our arms and legs.
Every millisecond, multiple streams of information, including signals
from the light touch transmission pathway that Goulding’s team has
identified, flow into the brain. One way the brain handles this data is
by preprocessing it in sensory way stations such as the eye or spinal
cord. The eye, for instance, has a layer of neurons and light sensors at
its back that performs visual calculations–a process known as
“encoding”–before the information goes on to the visual centers in the
brain. In the case of touch, scientists have long thought that the
neurological choreography of movement relies on data-crunching circuits
in the spinal cord. But until now, it has been exceedingly difficult to
precisely identify the types of neurons involved and chart how they are
wired together.
In their study, the Salk scientists demystified this fine-tuned,
sensory-motor control system. Using cutting-edge imaging techniques that
rely on a reengineered rabies virus, they traced nerve fibers that
carry signals from the touch sensors in the feet to their connections in
the spinal cord. They found that these sensory fibers connect in the
spinal cord with a group of neurons known as RORα neurons, named for a
specific type of molecular receptor found in the nucleus of these cells.
The RORα neurons in turn are connected by neurons in the motor region
of brain, suggesting they might serve as a critical link between the
brain and the feet.
When Goulding’s team disabled the RORα neurons in the spinal cord using
genetically modified mice developed at Salk, they found that these mice
were substantially less sensitive to movement across the surface of the
skin or to a sticky piece of tape placed on their feet. Despite this,
the animals were still able to walk and stand normally on flat ground.
However, when the researchers had the animals walk across a narrow,
elevated beam, a task that required more effort and skill, the animals
struggled, performing more clumsily than animals with intact RORα
neurons. The scientists attribute this to the animals’ reduced ability
to sense skin deformation when a foot was slipping off the edge and
respond accordingly with small adjustments in foot position and
balance–motor skills similar to those necessary for balancing on ice or
other slippery surfaces.
Another important characteristic of the RORα neurons is that they don’t
just receive signals from the brain and the light touch sensors, but
also directly connect with neurons in the ventral spinal cord that
control movement. Thus, they are at the center of a “mini-brain” in the
spinal cord that integrates signals from the brain with sensory signals
to make sure the limbs move correctly.
“We think these neurons are responsible for combining all of this
information to tell the feet how to move,” says Steeve Bourane, a
postdoctoral researcher in Goulding’s lab and first author on the new
paper. “If you stand on a slippery surface for a long time, you’ll
notice your calf muscles get stiff, but you may not have noticed you
were using them. Your body is on autopilot, constantly making subtle
corrections while freeing you to attend to other higher-level tasks.”
The team’s study represents the beginning of a new wave of research that
promises to provide precise and comprehensive explanations for how the
nervous system encodes and integrates sensory information to generate
both conscious and unconscious movement.
“How the brain creates a sensory percept and turns it into an action is
one of the central questions in neuroscience,” adds Goulding. “Our work
is offering a really robust view of neural pathways and processes that
underlie the control of movement and how the body senses its
environment. We’re at the beginning of a real sea change in the field,
which is tremendously exciting.”
Other authors on the paper were Katja S. Grossmann, Olivier Britz,
Antoine Dalet, Marta Garcia Del Barrio, Floor J. Stam, Lidia
Garcia-Campmany and Stephanie Koch, all of the Salk Institute.
The research was funding by
National Institutes of Health (Grants NS080586, NS086372 and NS072031), the
Catharina Foundation, the
Humboldt
Foundation and Joan and Irwin Jacobs, through Salk’s Innovation Grants Program.
