S1 nerve root !

The S1 nerve root passes behind the L5-S1 disc and leaves through the sacrum; it pushes the foot down and gives the ankle-jerk reflex.

Official name: S1

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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 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 and brevis (eversion), gluteus maximus; Achilles reflex (S1)
Feeling from: Lateral border of foot, little toe, sole and heel

Relations

Course: Leaves the dural sac at the level of the L5–S1 disc (where herniations catch it), descends in the sacral canal and exits the first anterior sacral foramen to join the sacral plexus on the piriformis
In front: The L5–S1 disc and the S1 vertebral body
Behind: The L5–S1 facet joint and ligamentum flavum, 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 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 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 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 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

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.

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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

Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Skin, Dermatomes; Neuroanatomy, Spinal Nerves.

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Representative model, for educational purposes only.

S1 nerve root highlighted on the Durgical 3D model