TENNESSE Tennesse: Muscle movement is enabled by signals sent from the brain to motor neurons; However, these impulses often pass through spinal interneurons before reaching their targets. How the brain and this highly diverse group of “switchboard operator” cells interact is not well understood.
To address this, researchers at St. Jude Children's Research Hospital have developed a whole-brain atlas that shows the areas of the brain that transmit input directly to the V1 interneuron, a type of cell essential for movement. The resulting atlas and associated three-dimensional interactive website provide a foundation for learning more about the physical landscape of the nervous system and how the brain communicates with the spine. Results reported today in Neuron
“We have known for decades that the motor system is a distributed network, but the final output is through the spinal cord,” said corresponding author Jay Bikoff, PhD, of the St. Jude Department of Developmental Neurobiology. “There, you have motor neurons that cause muscle contraction, but motor neurons do not work in isolation. Their activity is orchestrated by a network of molecularly and functionally diverse interneurons.”
While huge leaps have been made in understanding how different areas of the brain relate to different aspects of motor control, precisely how this connects to specific neurons in the spinal cord has been a blind spot in the field. Interneurons are difficult to study, primarily because they come in hundreds of different, intermingled varieties. “It's like unwrapping a ball of Christmas lights, except it's more challenging because of what we're trying to figure out.” That's the result of more than 3 billion years of evolution”, said co-first author Anand Kulkarni, PhD.
Recent advances have demonstrated the existence of molecularly and evolutionarily distinct interneuron subclasses, but much is still unknown about their place within neural communication. “Defining the cellular targets of descending motor systems is fundamental to understanding the neural control of movement and behavior,” Bikoff said. “We need to know how the brain is communicating these signals.”
To dissect the circuits connecting the brain to the spinal cord, the researchers used a genetically modified version of the rabies virus that is missing a key protein, the glycoprotein, from its surface. This disrupted the virus's ability to spread between neurons.
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