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The Sciatic Nerve — Complete Notes

Overview

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Origin & Root Value

• Largest branch of the sacral plexus
• Root value: L4, L5, S1, S2, S3
• It is actually two nerves wrapped in a common sheath:

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Course

In the Pelvis

• Lies in front of the piriformis, under cover of its fascia
• It is part of the network of nerves of the posterior pelvis (sometimes noted as the network of the "Posterior Pelvis")

In the Gluteal Region

• Enters through the greater sciatic foramen, below the piriformis
• Runs downwards with a slight lateral convexity
• Passes between the ischial tuberosity and the greater trochanter

Relations in the Gluteal Region:

In the Thigh

• Enters the back of the thigh at the lower border of gluteus maximus
• Runs vertically downwards up to the superior angle of the popliteal fossa
• Terminates at the junction of the upper 2/3 and lower 1/3 of the thigh by dividing into:
  • Tibial nerve
  • Common peroneal nerve

Relations in the Thigh:

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Branches

1. Articular Branches

• To the hip joint — arise in the gluteal region

2. Muscular Branches

• Semitendinosus
• Semimembranosus
• Long head of biceps femoris
• Ischial head of adductor magnus (medial side)

• Short head of biceps femoris only

Memory Aid: Medial = Semimembranosus & Semitendinosus | Lateral = Biceps femoris

3. Terminal Branches

• Tibial nerve (L4, L5, S1, S2, S3)
• Common peroneal nerve (L4, L5, S1, S2)

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Accompanying Artery

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What the Sciatic Nerve Ultimately Innervates

• All muscles in the posterior compartment of the thigh (hamstrings + ischial head of adductor magnus)
• All muscles of the leg and foot (via its terminal branches)

• Lateral side of the leg
• Lateral side and sole of the foot

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Clinical Anatomy

1. Sleeping Foot

2. Sciatica

• Shooting pain along the cutaneous distribution of the sciatic nerve and its terminal branches, chiefly the common peroneal
• Pain begins in the gluteal region and radiates along the back of the thigh → lateral side of the leg → dorsum of the foot
• Cause: compression of one or more nerve roots forming the sciatic nerve
  • Disc prolapse (most common — nucleus pulposus herniates through annulus fibrosus, compressing the spinal nerve root)
  • Neuritis, etc.

3. Sciatic Nerve Injury

• Hamstrings
• Calf muscles
• Peroneal muscles
• Dorsiflexors
• Muscles of the sole

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Key Diagrams from Your Pages

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Cadaveric Image — Sciatic Nerve in Posterior Thigh

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Quick Summary Table

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Relations of the Sciatic Nerve

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1. In the Pelvis

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2. In the Gluteal Region

Superficial (Posterior) Relations

Deep (Anterior) Relations

Medial Relations

Lateral Relations

Key point: At the midpoint between the ischial tuberosity and the greater trochanter, the nerve lies just deep to gluteus maximus — this is the surface marking used clinically.

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3. In the Thigh (Back of Thigh)

Superficial (Posterior) Relations

Deep (Anterior) Relations

Medial Relations

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Summary Diagram (Text Format)

GLUTEAL REGION — Cross-sectional view (posterior → anterior)

POSTERIOR (Superficial)
        ↓
  Gluteus maximus
        ↓
  SCIATIC NERVE  ←── Inferior gluteal nerve & vessels (medial)
        ↓
  Obturator internus + Gemelli
        ↓
  Quadratus femoris
        ↓
  Hip joint capsule / Adductor magnus
        ↓
ANTERIOR (Deep)

THIGH — Cross-sectional view (posterior → anterior)

POSTERIOR (Superficial)
        ↓
  Long head of Biceps femoris
        ↓
  SCIATIC NERVE
        ↓
  Adductor magnus
        ↓
ANTERIOR (Deep)

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Quick Reference Table

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Clinical Relevance of Relations

• The sciatic nerve's deep relation to the hip joint capsule explains why hip dislocation can injure it.
• Its relation to quadratus femoris and obturator internus means deep gluteal injections can damage it if given incorrectly — injections must be given in the upper outer quadrant of the buttock.
• The long head of biceps femoris crossing it in the thigh acts as a surgical landmark during posterior thigh dissections.