The fibrous filum terminale (FT), previously thought to be non-functional, was studied in patients undergoing surgery for tethered cord syndrome. Microscopy revealed sensory nerve endings within the FT’s core resembling mechanoreceptors and pain receptors found in skin and muscles. Electrical stimulation of the FT triggered muscle activity above and below the stimulated area. This suggests that the FT has functional sensory nerves, potentially serving as a proprioceptive element and contributing to back pain in spinal disorders.