Anatomy Atlas

Lumbar plexus

Plexus lumbalis
NervedraftNeuro

Nerve network formed within psoas major from the anterior rami of L1–L4. It supplies the lower abdominal wall, groin and anterior and medial thigh, and its branches are injured in hernia repair and by tight clothing.

Core anatomy

Location

Within the substance of psoas major, on the posterior abdominal wall.

Root values

Anterior rami of L1–L4 (often with a contribution from T12).

Origin

Anterior rami of L1–L4 in psoas major.

Course

Branches emerge from the lateral border, medial border or anterior surface of psoas and pass to the abdominal wall, groin and thigh.

Branches
  • Iliohypogastric nerve (L1): skin over the iliac crest and hypogastrium; internal oblique and transversus
  • Ilioinguinal nerve (L1): through the inguinal canal; skin of the medial thigh, root of the penis and anterior scrotum or mons pubis and labia majora
  • Genitofemoral nerve (L1–L2): genital branch (cremaster, scrotal skin) and femoral branch (skin over the femoral triangle)
  • Lateral femoral cutaneous nerve (L2–L3): skin of the lateral thigh; passes under or through the inguinal ligament near the ASIS
  • Femoral nerve (L2–L4, posterior divisions): anterior thigh muscles (quadriceps, sartorius, pectineus, with iliacus); skin of the anterior thigh and medial leg (saphenous nerve)
  • Obturator nerve (L2–L4, anterior divisions): medial thigh adductors; skin of the medial thigh
  • Lumbosacral trunk (L4–L5): joins the sacral plexus
Motor

Lower abdominal wall muscles (iliohypogastric, ilioinguinal), cremaster, hip flexors, knee extensors (femoral), and hip adductors (obturator).

Sensory

Lower abdominal wall, groin, genitalia (anterior), and the anterior, lateral and medial thigh; medial leg (saphenous).

Notes

Mnemonic for the branches from superior to inferior: "I I Get Lunch On Fridays" (Iliohypogastric, Ilioinguinal, Genitofemoral, Lateral femoral cutaneous, Obturator, Femoral).

Muscles innervated

  • Psoas major — Anterior rami of lumbar nerves L1–L3 (from the lumbar plexus)

Muscles innervated via branches

Branches

Other structures innervated

Mentioned in

Generated automatically from links in other entries.

Posterior abdominal wall showing the lumbar plexus branches emerging around psoas major
Branches of the lumbar plexus on the posterior abdominal wall.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 824. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Schematic of the lumbar plexus from T12–L5 roots to its named branches
Plan of the lumbar plexus and its branches.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 822. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Meralgia paresthetica PANCE

Mechanism / lesion

Compression of the lateral femoral cutaneous nerve as it passes under the inguinal ligament near the ASIS (obesity, pregnancy, tight belts or pants, diabetes).

Classic presentation

Burning, tingling or numbness of the anterolateral thigh, no motor weakness, normal reflexes. Usually managed conservatively (loosen clothing, weight loss).

Ilioinguinal and iliohypogastric nerve injury

Mechanism / lesion

Entrapment or injury during inguinal hernia repair, Pfannenstiel (low transverse) incisions, or laparoscopic port placement.

Classic presentation

Chronic groin pain or numbness over the groin, medial thigh, and scrotum or labia.

Femoral and obturator nerve injury PANCE

Mechanism / lesion

Femoral: psoas/retroperitoneal hematoma, pelvic surgery (retractor compression), hip surgery. Obturator: pelvic surgery or tumor, childbirth.

Classic presentation

Femoral: weak knee extension (knee buckling), reduced patellar reflex (L4), numbness of the anterior thigh and medial leg. Obturator: weak thigh adduction and a wide-based gait, numbness of the medial thigh.

High-yield PANCE pearls

  • Meralgia paresthetica = lateral femoral cutaneous nerve; lateral thigh burning, no weakness.
  • Femoral nerve (L2–L4): knee extension + patellar reflex (L4).
  • Obturator nerve: thigh adduction.
  • Cremasteric reflex: L1–L2 (genitofemoral).

Imaging anatomy

CT

CT shows retroperitoneal hematoma or masses compressing plexus branches.

MRI

MR neurography for plexopathy (tumor, inflammation); MRI of the lumbar spine distinguishes radiculopathy from plexopathy.

Ultrasound

Identifies the lateral femoral cutaneous nerve near the ASIS and the femoral nerve lateral to the femoral artery; used for nerve blocks.

Key views & landmarks
  • EMG/nerve conduction studies to localize (root vs plexus vs peripheral nerve)

Physical exam

Surface landmarks

The lateral femoral cutaneous nerve passes about 1 cm medial to the ASIS under the inguinal ligament. The femoral nerve lies lateral to the femoral artery in the femoral triangle ("NAVEL", lateral to medial).

Strength / function testing

Hip flexion (L2–L3), knee extension (L3–L4), hip adduction (obturator), patellar reflex (L4), cremasteric reflex (L1–L2), and sensory mapping of the thigh.

Self-quiz

Q1.

A 32-year-old pregnant woman has burning and numbness of the anterolateral thigh without weakness. Reflexes are normal. Which nerve is affected?

Show answer

C

Meralgia paresthetica is compression of the purely sensory lateral femoral cutaneous nerve under the inguinal ligament, common with pregnancy, obesity and tight clothing.

Q2.

After a pelvic operation, a patient has difficulty climbing stairs, a buckling knee and an absent patellar reflex. Which nerve is injured?

Show answer

B

The femoral nerve (L2–L4) supplies the quadriceps; injury weakens knee extension and reduces the patellar reflex.

Q3.

Which roots form the lumbar plexus?

Show answer

B

The lumbar plexus forms within psoas major from the anterior rami of L1–L4, often with a T12 contribution. L4–S3 form the sacral plexus.

Flashcards

See also