In a groundbreaking collaborative study supported by Rosetrees, scientists from Imperial College London and the University of Copenhagen have unveiled a novel mechanism by which specialised macrophages control muscle contraction and locomotion. Their discovery contrasts with the previous assumption that movement is only controlled by the nervous system.

The researchers identified a macrophage population, known as muscle spindle macrophages (MSMP), residing within the muscle spindles that play a key role in orchestrating the coordinated muscle contractions underlying movement.

Using cutting-edge techniques, including optogenetics and electrophysiology, the team showed that the MSMP activate sensory neurons via glutamate signalling, influencing neural activity, muscle contractions, and movement.

Glutamate signalling, which is a fast-acting communication system often associated with nerve cells, plays a critical role in key brain functions such as memory, learning, and movement.

The researchers showed that in mice muscle spindles, the MSMP uses glutamate to modulate sensation and activate muscles. In turn, the contractions in the muscles produce glutamine, which reactivates the macrophages, creating a continuous feedback loop. This complex mechanism highlights the key role that MSMP plays in integrating the immune, sensory, and metabolic functions in movement control.

If further research confirms the role of MSMP in human muscles, it could revolutionise therapies for conditions affecting proprioception (the body’s ability to sense movement, action, and location), muscle tone, and motor control.

The study’s lead co-author, Professor Simone Di Giovanni, from Imperial College’s Department of Brain Sciences, said: “The discovery, not just of these immune cells in close contact to muscle spindles but also that they play a key role in movement, is very exciting. This knowledge has implications for conditions like chronic fatigue syndrome and other diseases of the immune, muscular, and nervous system that directly affect movement.”