4 min readNew DelhiDec 14, 2022 11:30 AM IST
First published on: Dec 12, 2022 at 04:24 PM IST
Ask someone how many senses humans have and they are likely to say five—sight, hearing, smell, taste and touch. But depending on who you ask, that number can be much higher. For example, if you close your eye and touch your left knee with your right hand, you have already used a sixth sense—proprioception. If you actually did that, you will notice that you did not use your sense of sight to locate your knee, your eyes were likely closed. You didn’t smell your knee, or hear it, or taste it. You already knew where it was before you touched it. This sense of your own body, its position and its movement is called “proprioception.”
“This sense is what allows the central nervous system to send the right signals through motor neurons to muscles so that we can perform a specific movement. Its job is to collect information from the muscles and joints about our movements, our posture and our position in space, and then pass that on to our central nervous system,” Niccolò Zampieri, corresponding author of a study on proprioception published in the journal Nature said in a press statement.
Zampieri is head of the Development and Function of Neural Circuits Lab at the Max Delbrück Center in Berlin. He led the team that described the molecular markers of of the cells involved in this “sixth sense” in the research article. According to the Max Delbruck Center, this research will help scientists better understand how the proprioceptive sensory neurons (pSN) work.
Crucially precise connections
These pSN cell bodies are located near the dorsal root of the spinal cord. They are connected to muscle spindles and “Golgi tendon organs” via long nerve fibers. These muscle spindles and Golgi tendon organs constantly register stretch and tension in every muscle body. This information is then sent to the central nervous system by the pSN cell bodies, where it can be used to control neuron activity to perform movements.
“One prerequisite for this is that pSN precisely connect to different muscles in our bodies,” said Stephan Dietrich, the lead author of the study, in a press statement. Unfortunately, scientists knew next to nothing about the “molecular programs” that allow these precise connections. “That’s why we used our study to look for molecular markers that differentiate the pSN for the abdominal, back and limb muscles in mice,” added Dietrich.
Investigating the pSN genes
The researchers used single-cell sequencing to investigate which genes in the pSN cell bodies of the abdominal, back and leg muscles are read and translated into RNA. The researchers found the characteristic genes for the pSN bodies connected to each muscle group and were also able to show that these genes are already active during the embryonic stage of a human’s growth. They remain active for a while after birth. According to Dietrich, this means that there are fixed “genetic programs” that decide whether a proprioceptor will connect with the abdominal, back or limb muscles.
Understanding pSN to develop neuroprostheses
Understanding the genetic markers and the working of pSN cell bodies is about more than just scientific curiosity. Knowledge of the sensory network and future research could potentially be used to help patients, like those who have spinal cord injuries. “Once we better understand the details of proprioception, we’ll be able to optimize the design of neuroprostheses, which take over motor or sensory abilities that have been impaired by an injury,” added Zampieri.
The researchers will now use techniques like optogenetics—where they use light to turn proprioceptors on or off—to understand the exact specific role played by different pSN cell bodies in the sensory network.
The cells in green represent "proprioceptive" information while the cells in red represent thermal and tactile information. (Image credit: Stephan Dietrich, AG Zampieri, Max Delbrück Center) 
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