[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Spine](https://durgical.com/en/anatomy/region/spine/) › L5–S1 nucleus [Joints and ligaments](https://durgical.com/en/anatomy/system/joints-and-ligaments/) · [Spine](https://durgical.com/en/anatomy/region/spine/) # L5–S1 nucleus The nucleus pulposus is the soft, gel-like centre of the [L5-S1](https://durgical.com/en/anatomy/l5-s1/) disc in the lower back; it is the part that bulges out in a disc herniation. Official name: L5-S1 [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=spine&select=nucleus_l5_s1&side=r) Opens in the Durgical app, no sign-up needed. ## What it is The soft, gel-like centre (nucleus pulposus) of the L5–S1 disc. ## What it does Works like a water cushion: it turns pressure from above into outward push on the fibre ring, spreading the load. It is under the most pressure when you bend forward and lift. ## Key points - A central L5–S1 herniation can cause [cauda equina](https://durgical.com/en/anatomy/cauda-equina/) syndrome with less leg pain than expected: the lower sacral roots for bladder, bowel and saddle lie centrally in the dural sac - A sequestered fragment can migrate up behind L5 or down behind S1; the root it lands on depends on where it ends up ## Key facts **Where** Centre of the L5–S1 disc, slightly toward the back, inside the annulus fibrosus **Function** Notochord-derived gel (about 70–90% water, proteoglycans); turns axial load into annulus tension; herniates posterolaterally toward the S1 root ## Relations **In front** Below the aortic bifurcation: between the common iliac vessels lie the median sacral vessels and the [superior hypogastric plexus](https://durgical.com/en/anatomy/superior-hypogastric-plexus/), whose injury in anterior approaches can cause retrograde ejaculation **Behind** [Posterior longitudinal ligament](https://durgical.com/en/anatomy/posterior-longitudinal-ligament/) and the dural sac; the S1 root runs in the lateral recess **Lateral** The L5 root in its foramen, which is often narrowed at this level ## How to find it From behind, the L5–S1 interlaminar window lies between the L5 spinous process and the back of the [sacrum](https://durgical.com/en/anatomy/sacrum/) and is the widest in the lumbar [spine](https://durgical.com/en/anatomy/region/spine/). From the front, the disc sits just below the sacral promontory, between the common iliac vessels. ## Examination - S1 root: weak toe walking and calf raises, numb outer foot and sole, reduced or absent ankle jerk ## On imaging Sagittal T2 MRI follows a migrated fragment up or down; a dark, thin L5–S1 disc with a «vacuum» (gas) on CT or X-ray marks advanced degeneration. ## Common problems - Squeezes out through a tear in the ring (herniation), usually backward and to one side, toward a nerve root. - Loses water with age, so the disc gets thinner and stiffer. ## How it is diagnosed On MRI a healthy nucleus is bright on T2 images; a worn one turns dark ('black disc'). ## Treatment - In a microdiscectomy only the loose piece that has pushed out is removed; most of the disc stays. ## Clinical notes - L4–L5 and L5–S1 together account for most lumbar herniations; a posterolateral L5–S1 fragment usually takes the S1 root, a far-lateral one the L5 root - In isthmic spondylolisthesis of L5 on S1 the disc wears and the slip pinches the L5 root in its foramen, rather than the S1 root a herniation usually hits - Because the interlaminar window is widest here, an L5–S1 fragment can often be removed with little or no bone taken from the lamina ## Anatomical variants - Lumbosacral transitional vertebrae (a sacralised L5 or a lumbarised S1) are common and change the numbering, so the disc called L5–S1 on one report may be named differently on another; levels are checked against a whole-spine image ## Development The nucleus pulposus comes from the notochord; notochordal cells give way to cartilage-like cells during childhood. Notochord remnants in the sacrum are the usual source of sacral chordoma. ## Related conditions - [Lumbar Disc Herniation](https://durgical.com/en/conditions/lumbar-disc-herniation/) ## Test yourself Think of the answer, then tap the question to see it. ## Toward which root does a posterolateral herniation of the L5–S1 nucleus usually go? The S1 root, from a fragment escaping backward and to one side. ## Why can a central L5–S1 nucleus herniation cause cauda equina syndrome with less leg pain than expected? The lower sacral roots for bladder, bowel and saddle lie centrally in the dural sac, so a central fragment hits them first. ## What decides which root a sequestered L5–S1 nucleus fragment compresses? Where it ends up: it can migrate up behind L5 or down behind S1. ## Which tumour arises from notochord remnants in the sacrum, near the L5–S1 nucleus? Sacral chordoma; the nucleus pulposus itself comes from the notochord. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anatomy/l5-s1-spine-and-pelvis/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** nucleus pulposus L5-S1 TA2 1686 **Official Spanish (DeCS)** Núcleo pulposo L5-S1 DeCS D000070614 **English (TA2)** L5-S1 ## Related - [L5–S1 disc](https://durgical.com/en/anatomy/l5-s1/) - [Sacrum](https://durgical.com/en/anatomy/sacrum/) Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=56248&filter=ths_termall&q=N%C3%BAcleo%20Pulposo); [Anatomy, Back, Nucleus Pulposus](https://www.ncbi.nlm.nih.gov/books/NBK535373/); [Anatomy, Back, Intervertebral Discs](https://www.ncbi.nlm.nih.gov/books/NBK470583/); [7.3 The Vertebral Column](https://openstax.org/books/anatomy-and-physiology-2e/pages/7-3-the-vertebral-column) (OpenStax (Rice University), 2022) · adapted, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/l5-s1-spine-and-pelvis/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/l5-s1-spine-and-pelvis/