Lumbar Plexus
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Direct answer
Buried in the substance of psoas major, the lumbar plexus is built by the ventral rami of L1 to L4, often with a twig from T12. Its two great branches leave the muscle in opposite directions — the femoral nerve from the lateral border between psoas and iliacus, the obturator nerve from the medial border into the pelvis — to supply the anterior and medial compartments of the thigh. A fourth-year favourite because one abscess, one haematoma or one nerve block in the psoas sheath explains everything the plexus does.
What you must remember
- Roots and position: ventral rami of L1–L4 (often T12, occasionally L5), uniting within psoas major in front of the transverse processes, behind the psoas fascia; the rami receive grey rami communicantes from the sympathetic chain.
- Superficial branches: iliohypogastric and ilioinguinal (L1) to the lower abdominal wall and groin; genitofemoral (L1–L2), the only branch to pierce psoas and run on its front; lateral cutaneous nerve of the thigh (L2–L3), crossing the iliac fossa to the anterior superior iliac spine.
- Deep branches: femoral nerve (L2–L4) in the groove between psoas and iliacus; obturator nerve (L2–L4) along psoas' medial border into the lesser pelvis; an accessory obturator nerve (L2–L4) occasionally.
- Lumbosacral trunk: L4 (with all of L5) descends over the pelvic brim to join the sacral plexus — the bridge that carries L5 into the sciatic nerve.
- Muscular twigs: segmental branches to psoas major and minor and quadratus lumborum; iliacus is supplied by the femoral nerve in the iliac fossa.
- Clinical anchors: a psoas abscess from spinal tuberculosis tracks down the psoas sheath to point below the inguinal ligament; retroperitoneal haemorrhage or haemophilia can compress the plexus, weakening knee extension; the psoas compartment (lumbar plexus) block anaesthetises hip and knee surgery.
Reading a psoas abscess through the plexus
A young woman with night-time back pain walks with the hip held flexed; a fullness appears above the inguinal ligament. The spine film shows tuberculosis of L2, and the anatomy writes the rest: pus tracks into the psoas sheath, the fascial envelope around psoas major, follows the muscle across the pelvic brim and presents as a cold abscess below the inguinal ligament or in the thigh. Because the plexus lies within the same sheath, the patient flexes the hip to relax the muscle — the psoas sign — and quadriceps weakness with anterior thigh numbness appears when the abscess compresses the femoral roots. The same geometry serves the anaesthetist: a psoas compartment block at L3–L4 captures femoral, obturator and lateral cutaneous nerves together, the effect a groin-level femoral block cannot reproduce because the obturator nerve has already diverged into the pelvis.
Where students slip
The plexus is drawn on the surface of psoas; it is inside the muscle, which is why a psoas abscess or haematoma involves it and why the block works where it does. The lumbar sympathetic trunk is confused with the plexus — the trunk lies on the vertebral bodies medial to psoas and supplies no limb. The branches' emergence pattern is muddled: only genitofemoral emerges on the anterior surface, the femoral laterally, the obturator medially — the examiner's question "which branch pierces psoas?" expects genitofemoral alone. And the lumbosacral trunk is forgotten, although it is the reason an L4–L5 disc prolapse produces foot drop: those roots continue through the trunk into the sacral plexus and the sciatic and common peroneal nerves.
Frequently asked questions
Where is the lumbar plexus formed, and what are its roots?
In the substance of psoas major, by the ventral rami of L1–L4 with a frequent contribution from T12, lying behind the psoas fascia in front of the transverse processes.
Name the branches in order, with root values.
Iliohypogastric and ilioinguinal (L1), genitofemoral (L1–L2), lateral cutaneous nerve of the thigh (L2–L3), femoral nerve (L2–L4), obturator nerve (L2–L4), occasional accessory obturator, muscular twigs to psoas and quadratus lumborum, and the lumbosacral trunk.
What is the lumbosacral trunk?
L4 (with L5) descending over the pelvic brim in front of the sacroiliac joint to join the sacral plexus — the bridge by which L5 reaches the sciatic and common peroneal nerves.
Why is a psoas abscess relevant to the lumbar plexus?
The abscess tracks within the psoas sheath, which encloses muscle and plexus together, pointing below the inguinal ligament and compressing the roots — pain, hip flexion and quadriceps weakness from one tuberculous vertebra.
Which branch supplies iliacus?
The femoral nerve, while still in the iliac fossa in the groove between psoas and iliacus — before entering the thigh lateral to the femoral artery.
Which branch pierces psoas major, and what does it supply?
The genitofemoral nerve (L1–L2), emerging on the anterior surface and splitting into a genital branch to the cremaster and scrotum or labium, and a femoral branch to the skin over the femoral triangle.