Functional and molecular insights into muscle proprioceptors

Proprioception, the innate ability to perceive body positions and movements, enables us to perform daily activities without thinking about it. In mammals, this process primarily involves the activation of three types of proprioceptive neuron (PN) endings in muscles (Ia and II-PNs) or tendons (Ib-PNs). However, recent research indicates that these cardinal classes exhibit molecular diversity that likely reflects differences in connectivity, morphology, and activity patterns, contributing to the detection of various kinematic parameters. In this review, we summarize the properties and functions of PNs and propose a comprehensive cell-type classification. By systematically mapping functionally relevant molecular markers to specific PN subtypes, we establish a tentative, yet insightful taxonomy based on their functional characteristics. This foundational work lays the groundwork for future research aimed at elucidating the distinct physiological properties of each PN subtype and their interactions within central motor circuits. Understanding these nuances will be critical for advancing our knowledge of sensorimotor circuitry and its role in movement control.