The Lumbar Plexus Simplified

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Overview

• The lumbosacral plexus is a network of nerves supplying the lower limb and pelvic region.
• The lumbar plexus forms the upper portion of this network.
• It arises primarily from the anterior rami of spinal nerves L1 to L4, with occasional contribution from T12.
• Part of the L4 nerve root contributes to the lumbosacral trunk, which connects to the sacral plexus.

Lumbar Plexus

Origin

• Formed from the anterior rami of:
  • L1
  • L2
  • L3
  • L4
• May receive a small contribution from T12.

Branches of the Lumbar Plexus

The lumbar plexus gives rise to six main nerves:

• Iliohypogastric nerve
• Ilioinguinal nerve
• Genitofemoral nerve
• Lateral femoral cutaneous nerve
• Obturator nerve
• Femoral nerve

Memory Aid for the Branches

A commonly used sequence to remember the order of branches is:

I Twice Got Lunch On Friday

Meaning:

• I – Iliohypogastric nerve
• I – Ilioinguinal nerve
• G – Genitofemoral nerve
• L – Lateral femoral cutaneous nerve
• O – Obturator nerve
• F – Femoral nerve

Nerve Root Contributions

The six branches arise from different combinations of lumbar nerve roots.

Nerves from One Root

• Iliohypogastric nerve – L1
• Ilioinguinal nerve – L1

The iliohypogastric nerve usually emerges superior to the ilioinguinal nerve.

Nerves from Two Roots

• Genitofemoral nerve – L1 and L2
• Lateral femoral cutaneous nerve – L2 and L3

Nerves from Three Roots

• Obturator nerve – L2, L3, L4
• Femoral nerve – L2, L3, L4

Relationship with the Psoas Major Muscle

The lumbar plexus nerves emerge in relation to the psoas major muscle.

Lateral Border of Psoas Major

Most nerves emerge from the lateral side:

• Iliohypogastric nerve
• Ilioinguinal nerve
• Lateral femoral cutaneous nerve
• Femoral nerve

Anterior Surface of Psoas Major

• Genitofemoral nerve

Medial Border of Psoas Major

• Obturator nerve

Important Lumbar Plexus Nerves

Femoral Nerve

• Nerve roots: L2, L3, L4
• Supplies the anterior compartment of the thigh.

Motor function:

• Innervates quadriceps muscles.
• Responsible for knee extension.

Sensory supply:

• Anterior thigh
• Medial aspect of the leg through the saphenous nerve.

Clinical relevance:

• Injury results in weakness of knee extension and decreased patellar reflex.

Obturator Nerve

• Nerve roots: L2, L3, L4
• Supplies the medial compartment of the thigh.

Motor function:

• Innervates adductor muscles of the thigh.
• Responsible for hip adduction.

Sensory supply:

• Medial aspect of the thigh.

Lateral Femoral Cutaneous Nerve

• Nerve roots: L2, L3
• Purely sensory nerve.

Function:

• Provides sensation to the lateral aspect of the thigh.

Clinical relevance:

• Compression can cause meralgia paresthetica, producing pain or numbness over the lateral thigh.

Genitofemoral Nerve

• Nerve roots: L1, L2.
• Divides into genital and femoral branches.

Functions:

• Genital branch supplies the cremaster muscle and skin of the genital region.
• Femoral branch supplies skin over the upper anterior thigh.

Iliohypogastric Nerve

• Nerve root: L1.

Function:

• Supplies abdominal wall muscles.
• Provides sensory supply to the lower abdominal region and upper lateral gluteal area.

Ilioinguinal Nerve

• Nerve root: L1.

Function:

• Supplies abdominal wall muscles.
• Provides sensory supply to the upper medial thigh and parts of the external genitalia.

Key Summary Points

• The lumbar plexus arises from L1–L4 with occasional contribution from T12.
• It produces six main branches.
• Two nerves arise from one root, two from two roots, and two from three roots.
• The femoral and obturator nerves both arise from L2–L4.
• Most nerves emerge laterally from the psoas major muscle, except:
  • Genitofemoral nerve (anterior)
  • Obturator nerve (medial)