Today we are not here to talk about Bruce Willis, nor is it necessary to be Peter Parker’s secret identity to have a sixth sense. Sight, hearing, smell, taste, touch: we are all familiar with the five senses that allow us to experience our surroundings. But, iEqually important, but much less well known, is the sixth sense: “Its job is to collect information from the muscles and joints about our movements, our posture, and our position in space, and then transmit it to our central nervous system,” as as explained by Dr. Niccolò Zampieri, head of the Laboratory for the Development and Function of Neural Circuits at the Max Delbrück Center in Berlin, who has discovered one of the hitherto unknown keys to this sense, also known as proprioception, “it is what allows the central nervous system sends the correct signals through the motor neurons to the muscles so that we can perform a specific movement,” Zampieri said.
To carry out coordinated movements, we rely on special sensory neurons in our muscles and joints. Without them, the brain would not know what the rest of our body is doing. Now a team led by Niccolò Zampieri has studied their molecular markers to better understand how they work and has reported their results in Nature Communications.
“Proprioception allows us to perceive the location, movement and action of body parts. It constantly informs us through different receptors, in order to allow us to be more efficient in our movement, which entails less energy expenditure and, of course, avoiding any type of injury in an unexpected way”, pointed out in The Conversation the professor of Physiotherapy at the University of Las Palmas de Gran Canaria AnÃbal Báez Suárez.
This sixth sense, which can be trained and which, unlike the other five, is completely unconscious, is what prevents us from falling into the dark and what allows us to lift a cup of coffee to our mouths with our eyes closed for a moment. the morning. But that’s not all: “People without proprioception can’t really make coordinated movements,” says Zampieri. He and his team have now published an article in which they describe the molecular markers of the cells involved in this sixth sense. The findings should help researchers better understand how proprioceptive sensory neurons (pSNs) work.
Cell bodies of pSN are found in the dorsal root ganglia of the spinal cord. They are connected via long nerve fibers to muscle spindles and Golgi tendon organs that constantly record the stretch and tension in every muscle in the body. The pSNs send this information to the central nervous system, where it is used to control the activity of motor neurons so that we can make movements.
“A prerequisite for this is that pSN connects precisely to different muscles in our body,” says Dr. Stephan Dietrich, a member of Zampieri’s lab. However, almost nothing was known about the molecular programs that allow for these precise connections and give the muscle-specific pSN its unique identity. “This is why we used our study to look for molecular markers that differentiate pSN for abdominal, back, and limb muscles in mice,” says Dietrich, lead author of the study, which was conducted at the Max Delbrück Center. .
Guidance for nascent nerve fibers
Using single cell sequencing, the team investigated which genes in the pSN of abdominal, back and leg muscles are read and translated into RNA. “And we found characteristic genes for the pSN connected to each muscle group,” says Dietrich. “We also show that these genes are already active at the embryonic stage and remain active for at least some time after birth.” Dietrich explains that this means there are fixed genetic programs that decide whether a proprioceptor will innervate abdominal, back, or limb muscles.
Among their findings, the Berlin researchers identified several genes for ephrins and their receptors. “We know that these proteins are involved in guiding nascent nerve fibers to their target during nervous system development,” says Dietrich. The team found that connections between proprioceptors and hindlimb muscles were impaired in mice that cannot produce ephrin-A5.
A goal to improve neuroprosthetics
“The markers we now identify should help us further investigate the development and function of individual muscle-specific sensory networks,” says Dietrich. “With optogenetics, for example, we can use light to turn proprioceptors on and off, either individually or in groups. This will allow us to reveal their specific role in our sixth sense,” adds Zampieri.
This knowledge should eventually benefit patients, such as those with spinal cord injuries. “Once we better understand the details of proprioception, we will be able to optimize the design of neuroprostheses, which take over motor or sensory abilities that have been affected by injury,” says Zampieri.
“If we can better understand our sixth sense, it will be possible to develop new therapies that counteract spinal cord injuries, scoliosis, hip dysplasia and other types of skeletal damage”
He adds that researchers in Israel have recently discovered that properly functioning proprioception is also important for a healthy skeleton. Scoliosis, for example, is a condition that sometimes develops during childhood growth and causes the spine to twist and twist. “We suspect this is due to dysfunctional proprioception, which alters muscle tension in the back and distorts the spine,” says Zampieri.
Hip dysplasia, an abnormality of the hip joint, can also be caused by faulty proprioception. This has led Zampieri to imagine another result of the research: “If we can better understand our sixth sense, it will be possible to develop new therapies that effectively counteract these and other types of skeletal damage.”
Source: Max Delbrück Center
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