Not Just an Anchor: The Human Filum Terminale Contains Stretch Sensitive and Nociceptive Nerve Endings and Responds to Electrical Stimulation With Paraspinal Muscle Activation

Abstract

BACKGROUND:Neural components of the fibrous filum terminale (FT) are well known but are considered as embryonic remnants without functionality.OBJECTIVE:To investigate the ultrastructure of human FT specimens for sensory nerve endings and record paraspinal muscle activity on electrostimulation of the FT.METHODS:We prospectively investigated a cohort of 53 patients who underwent excision of the FT for the treatment of tethered cord syndrome. Surgical FT specimens were investigated by light and transmission electron microscopy. Intraoperative electrophysiological routine monitoring was extended by recording paraspinal muscles above and below the laminotomy level.RESULTS:Light microscopy revealed tiny peripheral nerves piercing the pia mater of the FT and entering its fibrous core. Transmission electron microscopy unveiled within the fibrous core of the FT myelinated nerve structures in 8 of the 53 patients and unmyelinated ones in 10 of the 53 patients. Both nerve endings encapsulated in fibrous tissue or unencapsulated nonmyelinated Schwann cell nerve bundles, that is, Remak cells, were found. Those nerve endings resembled mechanoreceptor and nociceptive receptor structures found in human skin, muscle tendons, and skeletal ligaments. Specifically, we found Ruffini mechanoreceptors and in addition nerve endings which resembled nociceptive glioneural structures of the skin. Bipolar electrostimulation of the FT was associated with paraspinal muscle activity above and below the spinal segment at which the FT was stimulated.CONCLUSION:Morphological and electrophysiological results indicate the presence of functional sensory nerve endings in the FT. Like other spine ligaments, the FT may serve as a proprioceptive element but may also contribute to back pain in spine disorders.

The Surgical Histopathology of the Filum Terminale: Findings from a Large Series of Patients with Tethered Cord Syndrome

“…arachnoid remnants, consistent with its embryonic origin [4,[6][7][8], as it is formed during the secondary neurulation stage of development-the so-called "retrogressive differentiation" [9]. It is thus recognized that the FT is the remnant of an embryonic spinal cord [10].…”

“…The FT is a fibrovascular structure, anatomically “anchoring” the spinal cord to the distal thecal sac at the sacral 1–2 level ( Figure 1 ). In addition to connective tissue, the FT histology has arbitrarily revealed both ependymal, ganglion, and glial cells, as well as arachnoid remnants, consistent with its embryonic origin [ 4 , 6 , 7 , 8 ], as it is formed during the secondary neurulation stage of development—the so-called “retrogressive differentiation” [ 9 ]. It is thus recognized that the FT is the remnant of an embryonic spinal cord [ 10 ].…”