A new neural pathway has been identified by a group of neuroscientists that could underlie our ability to make the coordinated hand movements needed to reach out and manipulate objects within immediate extrapersonal space. The discovery, described in the Proceedings of the National Academy of Sciences, was made in non-human primates, but the team believes that a similar pathway is likely to be present in humans as well.
Lobes of the brain. Parietal lobe is yellow, and the posterior portion is near the red region. Image credit: Henry Vandyke Carter / Henry Gray.
Researchers long believed that the brain signals for those and related movements originated from motor areas in the frontal lobe of brain, which control voluntary movement. But that may not always be true.
The results of the new study show that the newly-discovered neural pathway originates not from the frontal lobe, but from the posterior parietal cortex (PPC), a brain region that neuroscientists previously thought was involved only in associating sensory inputs and building a representation of extrapersonal space.
“The findings break the hard and fast rule that a furrow in the brain called the central sulcus splits up the areas controlling sensory and motor function,” said senior author Peter Strick, professor of neuroscience and chair of neurobiology at the University of Pittsburgh School of Medicine and scientific director of the University of Pittsburgh Brain Institute.
“This has implications for how we understand hand movement and may help us develop better treatments for patients in whom motor function is affected, such as those who have had a stroke.”
“Our study also will have a direct impact on the efforts of researchers studying neural prosthetics and brain computer interfaces.”
In 1975, renowned neuroscientist Vernon Mountcastle and co-authors proposed the presence of a movement control center in the PPC and termed it a ‘command apparatus’ for operation of the limbs, hands and eyes within immediate extrapersonal space.
In the current study, Prof. Strick and colleagues confirm that such a command apparatus exists and demonstrate a new pathway that connects the PPC directly to neurons in the spinal cord that control hand movement.
The authors conducted three separate experiments in a non-human primate model to make the discovery.
They first showed that electrical stimulation in a region of the PPC called ‘lateral area 5’ evoked finger and wrist movements in the animal.
When they injected a protein marker into lateral area 5, they found that the marker made its way to the spinal cord and ended in the same location where the neurons controlling hand muscles are known to be present, suggesting a connection.
“The wiring and the connections from the PPC to the spinal cord and the hand look extremely similar to those from the frontal lobe that have been extensively studied,” said lead author Dr. Jean-Alban Rathelot, from the University of Pittsburgh School of Medicine.
“Similar form suggests similar function in controlling movement.”
For their final experiment, the researchers used a strain of rabies virus as a ‘tracker’ since it has the ability to jump across connected neurons. They found that when they injected the virus into a hand muscle, it was indeed transported back to neurons in the same region of PPC where stimulation evoked hand movements.
This result demonstrated the existence of a direct pathway from lateral area 5 to spinal cord regions that control hand muscles.
“We know from previous research that individuals who have suffered brain injuries in this area have trouble with dexterous finger movements like finding keys in a bag containing many other things, which strongly supports our findings,” said co-author Dr. Richard Dum, also from the University of Pittsburgh School of Medicine.
The neuroscientists believe that the multiple pathways for controlling hand movement from the frontal lobe and the PPC could work together to execute one complex hand task or could work in parallel to speed up movement, much like multiple processors in a computer can enhance efficacy.
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Jean-Alban Rathelot et al. Posterior parietal cortex contains a command apparatus for hand movements. PNAS, published online April 3, 2017; doi: 10.1073/pnas.1608132114
This article is based on text provided by the University of Pittsburgh School of Medicine.
