Nerves of Lower Limb

Lumbar Plexus - Thigh's Front Wires

• Origin: Ventral rami L1-L4 (± T12, L5); forms in psoas major muscle.
• Key Nerves (Anterior/Medial Thigh):
  • Femoral n. (L2,L3,L4): Knee extension (quadriceps), hip flexion. Sensory: anterior thigh, medial leg/foot (saphenous nerve).
  • Obturator n. (L2,L3,L4): Adductor muscles. Sensory: medial thigh.
  • Lateral Femoral Cutaneous n. (L2,L3): Sensory: lateral thigh.
• 📌 Mnemonic (Femoral/Obturator roots): L2,3,4 keeps the leg off the floor!

⭐ Femoral nerve injury presents with weak knee extension (e.g., difficulty climbing stairs), absent patellar reflex, and sensory loss over the anterior thigh and medial leg/foot.

Sacral Plexus - Back & Below's Network

• Roots: Ventral rami of L4, L5, S1, S2, S3, S4.
• Location: Anterior to piriformis muscle, on posterior pelvic wall.
• Key Nerves & Supply (motor focus):
  • Sciatic (L4-S3): Posterior thigh, all leg & foot muscles.
    • Tibial division
    • Common Peroneal (Fibular) division
  • Superior Gluteal (L4-S1): Gluteus medius, minimus, TFL.
  • Inferior Gluteal (L5-S2): Gluteus maximus.
  • Pudendal (S2-S4): Perineum.
  • Posterior Femoral Cutaneous (S1-S3): Skin of posterior thigh & leg.

⭐ Injury to the Superior Gluteal Nerve causes a positive Trendelenburg sign (pelvic tilt to unsupported side).

Femoral & Obturator Nerves - Thigh's Control Combo

• Femoral Nerve (L2, L3, L4):
  • Largest lumbar plexus branch; supplies anterior thigh.
  • Motor: Quadriceps femoris, Sartorius, Pectineus, Iliacus.
  • Action: Hip flexion, Knee extension.
  • Sensory: Anterior thigh, medial leg & foot (Saphenous nerve).
  • Injury: ↓Knee extension, ↓Patellar reflex, sensory loss.
• Obturator Nerve (L2, L3, L4):
  • Through obturator foramen; supplies medial thigh.
  • Motor: Adductor muscles (Longus, Brevis, Magnus part), Gracilis, Obturator externus.
  • Action: Thigh adduction.
  • Sensory: Medial thigh.
  • Injury: ↓Thigh adduction, sensory loss, wide gait.
• 📌 Mnemonic (Roots L2,L3,L4): Femoral "kicks the door" (knee extension); Obturator "adducts some more".

⭐ Obturator nerve injury can cause referred pain to the knee joint due to shared articular branches (Hilton's Law). oka

Sciatic Nerve - Leg's Main Cable

• Origin: Sacral plexus (L4, L5, S1, S2, S3); body's largest nerve.
• Path: Exits pelvis via greater sciatic foramen (inferior to piriformis). Descends posterior thigh.
• Division: Splits into Tibial & Common Peroneal nerves (popliteal fossa apex).
• Motor (pre-division): Hamstrings (semitendinosus, semimembranosus, biceps femoris long head), adductor magnus (hamstring part).
• Clinical: Sciatica, piriformis syndrome.

⭐ The sciatic nerve is functionally two nerves (tibial and common peroneal) bundled in a common sheath.

Tibial & Common Peroneal Nerves - Foot's Fine Tuners

Sciatic N. (L4-S3) divides in popliteal fossa:

• Tibial Nerve (L4-S3):

  • Motor: Post. leg compartment (plantarflexion, inversion); intrinsic foot muscles.
  • Sensory: Sole of foot.
  • Clinical: Tarsal tunnel syndrome; injury → cannot stand on tiptoes.
  • 📌 TIP: Tibial Inverts & Plantarflexes.

• Common Peroneal (Fibular) Nerve (L4-S2):

  • Around fibular neck (vulnerable!).
  • Superficial Peroneal N.:
    • Motor: Lateral leg compartment (eversion).
    • Sensory: Anterolateral leg, foot dorsum.
  • Deep Peroneal N.:
    • Motor: Anterior leg compartment (dorsiflexion).
    • Sensory: 1st dorsal web space.
  • Clinical: Foot drop, steppage gait.
  • 📌 PED: Peroneal Everts & Dorsiflexes.

⭐ The Common Peroneal Nerve is the most commonly injured nerve in the lower limb, often at the neck of the fibula.

High‑Yield Points - ⚡ Biggest Takeaways

• Femoral nerve (L2-L4) injury: weak knee extension, loss of patellar reflex, sensory loss anterior thigh/medial leg.
• Obturator nerve (L2-L4) injury: weak thigh adduction, sensory loss medial thigh.
• Sciatic nerve (L4-S3): supplies posterior thigh; tibial branch for plantarflexion/inversion, common peroneal for dorsiflexion/eversion.
• Common peroneal nerve injury (fibular neck): classic foot drop, sensory loss dorsum of foot.
• Superior gluteal nerve injury: Trendelenburg gait (weak abductors: gluteus medius/minimus).
• Inferior gluteal nerve injury: weak hip extension (gluteus maximus), difficulty climbing stairs.