Lumbar Plexus Overview

Overview

This lecture covers the anatomy, nerve branches, and key relationships of the lumbar plexus, using mnemonics and tricks for memorization.

Lumbar Plexus Formation and Anatomy

• The lumbar plexus is formed from the anterior rami of spinal nerves L1–L4, plus fibers from T12.
• The plexus is located within the psoas major muscle.
• Main bony landmarks: vertebrae L1–L5, sacrum, pelvis, iliac crest, iliac fossa, obturator foramen, pubic symphysis, inguinal ligament, and femur.

Nerve Branches of the Lumbar Plexus

• There are six main nerves emerging from the lumbar plexus.
• All nerves emerge lateral to the psoas major except the genitofemoral (anterior) and obturator (medial).

Lumbar Plexus Branches Mnemonic

• Mnemonic: "I twice Get Laid On Fridays" for nerve order.
  • I twice: Iliohypogastric and ilioinguinal nerves (both from L1).
  • Get: Genitofemoral nerve (L1, L2).
  • Laid: Lateral femoral cutaneous nerve (L2, L3).
  • On: Obturator nerve (L2, L3, L4).
  • Fridays: Femoral nerve (L2, L3, L4).
• Memory trick: "Two from one, two from two, two from three" represents nerves originating from one, two, or three root levels.

Detailed Nerve-Root Relationships

• Iliohypogastric & ilioinguinal: both from L1 ("two from one").
• Genitofemoral (L1, L2) & lateral femoral cutaneous (L2, L3): "two from two".
• Obturator (L2–L4) & femoral (L2–L4): "two from three"; innervate major thigh muscle groups.

Additional Nerves

• The subcostal nerve from T12 contributes to the plexus.
• Accessory obturator nerve (variable, from L3 and L4).
• Muscular branches supply psoas and iliacus muscles.

Key Terms & Definitions

• Lumbar plexus — network of nerves from L1–L4 (±T12) in the psoas major muscle.
• Anterior rami — branches of spinal nerves forming plexuses.
• Iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, obturator, femoral nerves — primary branches of the lumbar plexus.

Action Items / Next Steps

• Review and redraw the lumbar plexus for memorization.
• Practice applying the mnemonic to recall nerve order and origins.
• Prepare for next lesson: nerves of the sacral plexus.