Ask anyone how many senses humans have and they’ll probably tell you 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 this, you’ll notice that you didn’t use your sense of sight to locate your knee, your eyes were probably closed. You didn’t feel your knee, or hear it, or taste it. You already knew where it was before you touched it. This perception 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 the motor neurons to the 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, then transmitting them to our central nervous system”, Niccolò Zampieri, corresponding author of a study on proprioception published in the journal Nature said in a press release.
Zampieri heads the Neural Circuit Development and Functioning Laboratory at the Max Delbrück Center in Berlin. He led the team that described the molecular markers of cells involved in this “sixth sense” in the research paper. According to the Max Delbruck Center, this research will help scientists better understand how proprioceptive sensory neurons (pSN) work.
Connections with crucial precision
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 Golgi muscle spindles and tendon organs constantly register stretch and tension in each muscle body. This information is then sent to the central nervous system by the cell bodies of the pSN, where it can be used to control the activity of neurons to perform movements.
“A prerequisite for this is that the pSN connects precisely to different muscles in our body,” said Stephan Dietrich, the study’s lead author, in a press release. Unfortunately, scientists knew next to nothing about the “molecular programs” that make these precise connections possible. “That’s why we used our study to search for molecular markers that differentiate pSN from abdominal, back and limb muscles in mice,” Dietrich added.
Investigation of pSN genes
The researchers used single-cell sequencing to determine which genes from the pSN cell bodies of the abdominal, back and leg muscles are read and translated into RNA. The researchers found the characteristic genes of the pSN bodies connected to each muscle group and were also able to show that these genes are already active in the embryonic stage of human growth. They remain active for some time after birth. According to Dietrich, this means that there are fixed “genetic programs” that decide whether a proprioceptor will connect to abdominal, back or limb muscles.
Understanding pSN to develop neuroprostheses
Understanding the genetic markers and the functioning of pSN cell bodies is much more than a simple scientific curiosity. Knowledge of the sensory network and future research could potentially be used to help patients, such as those with spinal cord injuries. “Once we better understand the details of proprioception, we can optimize the design of neuroprostheses, which support motor or sensory abilities that have been impaired by injury,” Zampieri added.
Researchers will now use techniques like optogenetics – where they use light to switch proprioceptors on or off – to understand the exact specific role that different pSN cell bodies play in the sensory network.