CorpusHuman Anatomy
NervousPlexus

Lumbar plexus

Plexus lumbalis

Also known as: L1–L4 plexus, iliohypogastric nerve, ilioinguinal nerve, genitofemoral nerve, lateral femoral cutaneous nerve

A plexus formed within psoas major by the ventral rami of L1–L4, with a small contribution from T12. It supplies the lower abdominal wall, the groin and genital skin, and the anterior and medial thigh through the femoral and obturator nerves.

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Overview

RootsL1–L4 (± T12)
Formed withinPsoas major
Major branchesFemoral and obturator (both L2–L4)

The lumbar plexus forms inside psoas major, in front of the lumbar transverse processes. Its branches leave the muscle in three places:
• the lateral border: iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral nerves;
• the anterior surface: the genitofemoral nerve;
• the medial border: the obturator nerve and the lumbosacral trunk (L4–L5), which descends into the pelvis to join the sacral plexus.

As in the brachial plexus, anterior divisions supply flexor and adductor compartments and posterior divisions supply extensor compartments. The obturator nerve (anterior divisions, L2–L4) supplies the medial (adductor) compartment of the thigh. The femoral nerve (posterior divisions, L2–L4) supplies the anterior (extensor) compartment. The upper branches (L1) are the lumbar equivalents of intercostal nerves: they run around the abdominal wall above the iliac crest to supply its lowest muscles and the skin of the groin.

Location

Posterior abdominal wall, within psoas major, anterior to the transverse processes of L1–L4.

Parts & subdivisions

  1. Lateral femoral cutaneous nerve
    L2–L3. It passes under or through the inguinal ligament about 1 cm medial to the ASIS. Compression there causes meralgia paraesthetica: burning numbness of the anterolateral thigh with no weakness. Triggers include tight belts or jeans, obesity, pregnancy and iliac crest bone graft harvest.
  2. Genitofemoral nerve
    L1–L2. It pierces the front of psoas. Its genital branch is the efferent limb of the cremasteric reflex: stroking the medial thigh, an afferent pathway via the femoral branch and ilioinguinal nerve (L1), makes the ipsilateral testis rise. The reflex is often absent in testicular torsion.
  3. Iliohypogastric and ilioinguinal nerves
    Both L1. They can be injured by groin incisions, inguinal hernia repair or appendicectomy, causing groin pain and numbness. Loss of their supply to the conjoint tendon may predispose to a direct hernia.

Anatomy

Innervation

- Muscles: the lower abdominal wall, psoas, iliacus, the anterior thigh (quadriceps, sartorius, pectineus), the medial thigh (adductors, gracilis, obturator externus) and cremaster.
• Skin: the lower abdomen, the groin and anterior genitalia, and the anterior, lateral and medial thigh, plus the medial leg via the saphenous nerve.

Nerve roots

Ventral rami of L1–L4 (± T12).

Branches

- Iliohypogastric (L1, ± T12): internal oblique and transversus abdominis; skin over the iliac crest and suprapubic region.
• Ilioinguinal (L1): passes through the inguinal canal and superficial ring to the skin of the upper medial thigh, the root of the penis and anterior scrotum or the mons pubis and labium majus.
• Genitofemoral (L1–L2): its genital branch supplies cremaster and the scrotal or labial skin; its femoral branch supplies the skin over the femoral triangle.
• Lateral femoral cutaneous (L2–L3): passes under the inguinal ligament near the ASIS to the skin of the lateral thigh.
• Femoral (L2–L4).
• Obturator (L2–L4).
• Accessory obturator (L3–L4; inconstant).
• Lumbosacral trunk (L4–L5).
• Muscular branches to psoas, iliacus and quadratus lumborum.

Function

  • Motor supply to hip flexors (iliopsoas), knee extensors (quadriceps) and hip adductors
  • Sensation of the lower abdomen, groin, anterior genitalia and most of the anterior and medial thigh, plus the medial leg and foot (saphenous)
  • Afferent and efferent limbs of the patellar (L3–L4) and cremasteric (L1–L2) reflexes

Clinical significance

Psoas haematoma or abscess

Because the plexus lies within psoas, retroperitoneal haemorrhage, for example in patients on anticoagulants, or a psoas abscess (TB, Crohn disease, spinal infection) can compress several branches at once. The result is a femoral neuropathy with groin pain and a flexed hip.

Mnemonics

Branches (superior → inferior)

"I, I Get Lunch On Fridays": Iliohypogastric, Ilioinguinal, Genitofemoral, Lateral femoral cutaneous, Obturator, Femoral.

Roots

"2 from 1, 2 from 2, 2 from 3": iliohypogastric and ilioinguinal from L1; genitofemoral (L1–2) and lateral femoral cutaneous (L2–3) from two segments each; obturator and femoral from three segments (L2–4).

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.