Touching your nose with your eyes closed relies on proprioception, the sense of body position, movement and force that neurophysiologist Charles Scott Sherrington named in 1906. This episode explains the hardware: muscle spindles that track muscle length and stretch speed, Golgi tendon organs that measure tension, and joint receptors that fire near a joint’s limit. Insects and crabs use chordotonal organs and campaniform sensilla for the same job. Most of this data never reaches awareness, handled by spinal reflex loops and the cerebellum, and the PIEZO2 gene encodes the stretch activated ion channel that turns mechanical pull into nerve signals.

The episode then covers what happens when the map fails. Growth spurts, gymnasts’ twisties, alcohol and Ehlers-Danlos syndromes all disrupt it, and a vibrating device on the biceps tendon can make people feel their nose growing. Ian Waterman, Charles Freed and a patient described by Oliver Sacks lost proprioception from the neck down after rare autoimmune reactions and had to watch their limbs to move. Finally, plants turn out to sense their own curvature through actin and myosin to straighten without overshooting.

• The stretch reflex that steadies you when a bus lurches
• People with PIEZO2 mutations and profound deficits in joint position sense
• Sacks’ patient gripping cups until her knuckles turned white
• Phantom limbs and the brain’s hyperexcitable proprioceptive memory
• Why gravity sensing alone would leave a bent plant stem swinging back and forth