[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/) › S1 nerve root [Nervous system](https://durgical.com/en/anatomy/system/nervous-system/) · [Spine](https://durgical.com/en/anatomy/region/spine/) # S1 nerve root The S1 nerve root passes behind the L5-S1 disc and leaves through the [sacrum](https://durgical.com/en/anatomy/sacrum/); it pushes the foot down and gives the ankle-jerk reflex. Official name: S1 [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=spine&select=s1_root&side=r) Opens in the Durgical app, no sign-up needed. ## What it is The S1 nerve root: it runs past the back of the L5-S1 disc and leaves through the first opening in the sacrum. ## What it does Pushes the foot down (calf), gives the ankle-jerk reflex, and feeling in the sole and outer edge of the foot. ## Key points - ISNCSCI key muscle: ankle plantar flexors (gastrocnemius, [soleus](https://durgical.com/en/anatomy/soleus-muscle/)); key sensory point: lateral heel - Compressed by a posterolateral L5–S1 disc; L4–L5 and L5–S1 together account for most lumbar herniations - A lost ankle reflex is the most reliable single sign of S1 radiculopathy, although it also fades with age and in peripheral neuropathy ## Key facts **Roots** S1 dorsal and ventral roots; exits the first sacral foramina; compressed by L5–S1 posterolateral herniation **Moves** Gastrocnemius and soleus (plantarflexion), [fibularis longus](https://durgical.com/en/anatomy/fibularis-longus-muscle/) and brevis (eversion), [gluteus maximus](https://durgical.com/en/anatomy/gluteus-maximus-muscle/); Achilles reflex (S1) **Feeling from** Lateral border of foot, [little toe](https://durgical.com/en/anatomy/little-toe/), sole and heel ## Relations **Course** Leaves the dural sac at the level of the [L5–S1 disc](https://durgical.com/en/anatomy/l5-s1/) (where herniations catch it), descends in the sacral canal and exits the first anterior sacral foramen to join the sacral plexus on the [piriformis](https://durgical.com/en/anatomy/piriformis-muscle/) **In front** The L5–S1 disc and the S1 vertebral body **Behind** The L5–S1 facet joint and [ligamentum flavum](https://durgical.com/en/anatomy/ligamenta-flava/), then the sacral lamina **Medial** The S2–S5 roots and the lower dural sac ## How to find it It cannot be felt. It crosses the back of the L5–S1 disc and enters the sacral canal; its ventral ramus leaves through the first anterior sacral foramen, and S1 transforaminal injections aim for the first posterior sacral foramen, just below the S1 pedicle on fluoroscopy. ## Examination - Ankle reflex, tested kneeling on a chair if it is hard to obtain - Repeated single-leg heel raises: fewer on one side reveals mild calf weakness that manual testing misses - Straight-leg raise and slump test reproducing pain down the back of the leg - Pinprick at the lateral heel (S1 key point) ## On imaging MRI is the test of choice: axial T2 at L5–S1 shows a paracentral herniation displacing the S1 root backwards as it leaves the dural sac. Sacral MRI also often shows perineural (Tarlov) cysts on the S1–S2 roots, usually incidental. A transitional vertebra can change the numbering, so the level is confirmed by counting from C2 on a whole- [spine](https://durgical.com/en/anatomy/region/spine/) scout. ## Common problems - Sciatica from an L5-S1 herniation: pain down the back of the leg into the sole, weak tiptoe, reduced ankle reflex. ## How it is diagnosed Exam (straight-leg raise, [big toe](https://durgical.com/en/anatomy/great-toe/) and foot lift strength, feeling); MRI. ## Treatment - Most sciatica improves within weeks to months with activity, physical therapy, medicine or an epidural injection. - Severe, lasting pain or weakness: microdiscectomy. ## Clinical notes - S1 vs [tibial nerve](https://durgical.com/en/anatomy/tibial-nerve/) or tarsal tunnel: S1 also weakens the gluteus maximus and foot eversion and reduces the ankle reflex; tarsal tunnel syndrome numbs only the sole - S1 sciatica vs piriformis syndrome or [sacroiliac joint](https://durgical.com/en/anatomy/sacroiliac-joint/) pain: the latter two do not change the reflex or cause dermatomal numbness - The H-reflex, the electrical equivalent of the ankle jerk, is delayed or absent in S1 radiculopathy ## Related conditions - [Lumbar Disc Herniation](https://durgical.com/en/conditions/lumbar-disc-herniation/) - [Sciatica (Lumbar Radiculopathy)](https://durgical.com/en/conditions/sciatica/) ## Test yourself Think of the answer, then tap the question to see it. ## Which disc herniation compresses the S1 nerve root? A posterolateral L5–S1 disc herniation; the S1 root leaves the dural sac at that level. ## What are the ISNCSCI key muscle and key sensory point for the S1 nerve root? Key muscle: ankle plantar flexors (gastrocnemius, soleus). Key sensory point: lateral heel. ## What is the most reliable single sign of S1 radiculopathy? A lost ankle (Achilles) reflex, although it also fades with age and in peripheral neuropathy. ## How is S1 sciatica told apart from piriformis syndrome or sacroiliac joint pain? Piriformis syndrome and sacroiliac joint pain do not change the ankle reflex or cause dermatomal numbness. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anatomy/s1/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** radix nervi spinalis S1 TA2 6507 **Official Spanish (DeCS)** Raíz S1 DeCS D013126 **English (TA2)** S1 **Also called** spinal nerves S1-S5 ## Related - [L5–S1 disc](https://durgical.com/en/anatomy/l5-s1/) - [Cauda equina](https://durgical.com/en/anatomy/cauda-equina/) - [Sciatic nerve](https://durgical.com/en/anatomy/sciatic-nerve/) Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=13510&filter=ths_termall&q=Ra%C3%ADces%20Nerviosas%20Espinales); [Anatomy, Skin, Dermatomes](https://www.ncbi.nlm.nih.gov/books/NBK535401/); [Neuroanatomy, Spinal Nerves](https://www.ncbi.nlm.nih.gov/books/NBK542218/). ## **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/s1/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/s1/