Macrophages reside in the muscle spindle to control sensorimotor function at millisecond timescale

The stretch reflex is a fundamental component of the motor system to orchestrate the coordinated muscle contractions underlying movement. At the heart of this process lie the muscle spindles (MS), specialized receptors finely attuned to fluctuations in tension within intrafusal muscle fibers. The tension variation in the MS triggers a series of neuronal events: an initial depolarization of sensory type Ia afferents that subsequently causes the activation of  motoneurons within the spinal cord. This neuronal cascade culminates in the execution of muscle contraction, underscoring a presumed closed-loop mechanism between the muscular and nervous systems. In contrast, surprisingly here we report the discovery of a new population of macrophages with exclusive molecular and functional signatures within the muscle spindle that express the machinery for synthesizing and releasing glutamate. Using mouse intersectional genetics with optogenetics and electrophysiology, we show that muscle spindle macrophages (MSMP) activation drives proprioceptive sensory neurons firing at millisecond timescale with subsequent activation of spinal circuitries, motor neurons, and muscles by a glutamate-dependent mechanism. Additionally, we found that MSMP can also respond to the activation of the stretch reflex circuitry by increasing the expression of glutaminase, allowing them to convert the glutamine released by myocytes during muscle contraction into glutamate. Our results identified a new cellular component, the MSMP, that directly regulates neural activity and muscle contraction. The glutamate-mediated signalling of MSMP and their dynamic response to sensory cues introduces a novel dimension to our understanding of sensation and motor action, potentially offering innovative therapeutic approaches in conditions that affect sensory-motor function.