Although when the “sixth sense” is mentioned it is often in a context about supposed supernatural powers of perception or about true senses that some animals do have but not humans, or simply about the power of human intuition, the truth is that We do have a sensory ability that could be classified as our sixth sense. It is about proprioception. This faculty allows us to know the position of the parts of our body without having to look at or touch them. Thanks to proprioception, we can touch our nose with a finger and bring a cup of coffee to our lips in total darkness that allows us to see nothing. Proprioception also allows us to put one foot in front of the other to walk without having to look at them.
Proprioception is made possible by special sensory neurons in our muscles and joints. Without them, the brain would not know what the rest of our body is doing. People without proprioception cannot execute coordinated movements.
A team including Niccolò Zampieri and Stephan Dietrich, from the Helmholtz Association’s Max Delbrück Center for Molecular Medicine in Germany, have studied the molecular markers of cells involved in this sixth sense, in order to better understand how they work. And he has managed to describe them in quite detail.
The findings should help researchers better understand how proprioceptive sensory neurons work.
The cell bodies of proprioceptive sensory neurons are located in the dorsal root ganglia of the spinal cord. They are connected by long nerve fibers to structures attached to the muscles and tendons that constantly record the stretch and tension in all the muscles of the body. Proprioceptive sensory neurons send this information to the central nervous system, where it is used to control the activity of motor neurons so that we can perform movements in a controlled manner.
Different cell body populations of sensory neurons in a dorsal root ganglion (right) and their axons in the spinal cord (left): Cells in green detect proprioceptive information, while cells in red take in thermal and tactile information. (Photo: Stephan Dietrich, Zampieri Lab, Max Delbrück Center)
Precise connections are crucial. A prerequisite for the sixth sense to function well is that the proprioceptive sensory neurons connect precisely to the different muscles in our body.
However, almost nothing was known about the molecular programs that allow these precise connections and make these muscle-specific connections possible in proprioceptive sensory neurons.
This is the main reason why Zampieri and his colleagues set out to search for molecular markers capable of differentiating proprioceptive sensory neurons depending on whether they are linked to abdominal, back or limb muscles in rats.
The team investigated which genes in the proprioceptive sensory neurons of the abdominal, back and limb muscles are read and translated into RNA. And they found genes characteristic of proprioceptive sensory neurons connected to each muscle group.
The findings will surely help make better neuroprostheses that allow people with spinal cord injury to perform movements of parts of their body at will.
The study is titled “Molecular identity of proprioceptor subtypes innervating different muscle groups in mice”. And it has been published in the academic journal Nature Communications. (Fountain: NCYT de Amazings)