How Your Brain Knows Where Your Hand Is

Proprioceptors provide the central nervous system (CNS) with the feedback information required to compute body posture and to regulate muscle activation accurately during ongoing movement. A large amount of proprioceptive information arrives at the CNS from diverse receptors distributed among muscles, tendons and joints, raising the question of how such information is encoded, combined and used. We here consider and contrast the two most important proprioceptors, muscle spindles and Golgi tendon organs, from the perspectives of psychophysics, neurophysiology and mathematical modeling. The muscle spindle is a particularly complex transducer of both muscle length and velocity whose properties appear to be dynamically optimized via its fusimotor innervation; its sensorimotor role and signal encoding are starting to be wellunderstood through mathematical modeling but decoding in the CNS appears to be a very difficult computational problem. The Golgi tendon organ is a much simpler transducer whose ensemble properties may provide a linear indication of muscle force; paradoxically, its role in proprioception and sensorimotor regulation remains enigmatic.