Anatomy chapters
The lumbar and sacral plexuses
Map the roots, branches and relations of the two plexuses, then use patterns of weakness to locate injuries.
- The lumbar plexus is mainly L1–L4; the sacral plexus is L4–S4.
- The L4–L5 lumbosacral trunk connects the two regions.
- Use psoas and piriformis to organise the branches and their exits.
On this page
Overview
The lumbar and sacral plexuses mix fibres from the ventral rami before distributing them to the lower limb. The lumbar plexus chiefly serves the anterior and medial thigh.
The sacral plexus serves the buttock, posterior thigh and most structures below the knee.
Each named nerve therefore contains fibres from several segments. A plexus injury can affect several terminal nerves together, while a single peripheral lesion has a more restricted distribution.
Lumbar plexus formation and relations
The lumbar plexus is formed mainly by L1–L4, sometimes with a T12 contribution. It lies within the posterior part of psoas major against the posterior abdominal wall.
Branches emerge from different aspects of psoas. Femoral, lateral femoral cutaneous, iliohypogastric and ilioinguinal nerves emerge laterally. The obturator nerve emerges medially, and the genitofemoral nerve appears on its anterior surface.
Part of L4 joins L5 to form the lumbosacral trunk. This descends over the pelvic brim to join the sacral plexus. It explains the shared L4 contribution to femoral, obturator and sciatic pathways.
Femoral and obturator nerves both contain L2–L4, but serve different thigh compartments.
Lumbar branches
| Name | Origin/roots | Supply/action | Clinical note |
|---|---|---|---|
| Iliohypogastric | T12–L1 | Lower abdominal muscles; suprapubic and lateral gluteal skin | May be affected in lower abdominal surgery |
| Ilioinguinal | L1 | Lower abdominal muscles; groin and upper medial thigh skin | Not a nerve to the whole medial thigh |
| Genitofemoral | L1–L2 | Genital and proximal anterior thigh territories; cremaster in males | Runs on anterior psoas |
| Lateral femoral cutaneous | L2–L3 | Anterolateral thigh skin | Pure sensory compression causes meralgia paraesthetica |
| Femoral | Posterior divisions L2–L4 | Anterior thigh muscles and skin; medial leg through saphenous | Quadriceps palsy impairs knee extension |
| Obturator | Anterior divisions L2–L4 | Most thigh adductors; small medial thigh skin area | Adduction becomes weak |
The division labels refer to subdivisions of ventral rami. They do not mean that the femoral nerve arises from dorsal spinal rami.
Posterior divisions of ventral rami are not posterior rami; both plexuses are formed from ventral rami.
The femoral nerve passes deep to the inguinal ligament, lateral to the femoral sheath. The obturator nerve reaches the thigh through the obturator canal and divides around adductor brevis.
The cremasteric reflex uses ilioinguinal sensory input and the genital branch of genitofemoral for its motor response.
Sacral plexus formation
The sacral plexus combines the lumbosacral trunk with ventral rami from S1–S4. It lies on the anterior surface of piriformis, against the posterior pelvic wall.
Most major lower-limb branches leave through the greater sciatic foramen. Piriformis divides that exit into an upper and a lower passage. Superior gluteal nerve and vessels use the upper passage; the sciatic and inferior gluteal bundles pass below.
Superior gluteal nerve passes above piriformis; the sciatic nerve normally passes below it.
Sacral branches
| Name | Origin/roots | Supply/action | Clinical note |
|---|---|---|---|
| Superior gluteal | L4–S1 | Gluteus medius, minimus and tensor fasciae latae | Loss impairs pelvic stability in single-leg stance |
| Inferior gluteal | L5–S2 | Gluteus maximus | Weak rising from sitting and forceful hip extension |
| Sciatic | L4–S3 | Hamstrings and, through terminal branches, most structures below knee | Posterior hip injury can damage it |
| Posterior femoral cutaneous | S1–S3 | Posterior thigh, lower buttock and adjoining perineal skin | Separate from the sciatic nerve |
| Nerve to piriformis | S1–S2 | Piriformis | Short branch within the pelvis |
| Nerve to obturator internus | L5–S2 | Obturator internus and superior gemellus | Also traverses lesser sciatic foramen |
| Nerve to quadratus femoris | L4–S1 | Quadratus femoris and inferior gemellus; hip branch | Deep to the short lateral rotators |
| Pudendal | S2–S4 | Perineal muscles and sensation | Not the motor supply of the lower limb |
The pudendal nerve exits below piriformis, curves around the ischial spine/sacrospinous ligament region and enters the perineum through the lesser sciatic foramen. Internal pudendal vessels accompany it.
Sciatic formation and division
The sciatic nerve is a combined tibial and common fibular bundle enclosed within a shared sheath. “Peroneal” is the name used in the supplied Snell chapter; “fibular” is the equivalent modern anatomical term.
The tibial component comes from anterior divisions of L4–S3. The common fibular component comes from posterior divisions of L4–S2. These are divisions within the ventral-ramus plexus.
The nerve descends deep to gluteus maximus and then through the posterior thigh. It commonly separates near the upper popliteal region, but division may occur higher, even within the pelvis.
Its tibial component supplies the long head of biceps, semitendinosus, semimembranosus and the hamstring part of adductor magnus. The common fibular component supplies the short head of biceps.
The short head of biceps is the common fibular exception; the other listed hamstring muscles receive tibial-component supply.
Clinical relevance
Lumbar plexus injury
How it happens: disease, trauma or an expanding collection within the psoas region affects several plexus branches.
What you see: combined weak knee extension and hip adduction, with sensory changes across femoral and obturator territories. This is broader than an isolated femoral neuropathy.
Superior gluteal lesion
How it happens: pelvic injury or surgery damages the nerve supplying the main hip abductors.
What you see: when standing on the affected limb, the unsupported side of the pelvis drops. This is a positive Trendelenburg sign; hip structural failure can produce the same sign.
Sciatic injury
How it happens: posterior hip fracture-dislocation, penetrating trauma or a misplaced gluteal injection injures the nerve.
What you see: knee flexion is weakened and movements below the knee may be widely lost. Medial leg sensation can remain because the saphenous nerve is femoral, not sciatic.
Meralgia paraesthetica
How it happens: the lateral femoral cutaneous nerve is compressed near the inguinal ligament and anterior superior iliac spine.
What you see: burning or altered sensation over the anterolateral thigh without quadriceps paralysis or loss of the knee reflex.
Use roots, compartment supply and the relation to psoas or piriformis to identify a branch. Combined deficits spanning several terminal nerves suggest a plexus lesion; medial leg sparing supports a sciatic rather than total limb sensory lesion. Remember the direction of pelvic drop in Trendelenburg testing.
Quick revision
- Lumbar plexus is mainly L1–L4; sacral plexus is L4–S4.
- L4–L5 form the lumbosacral trunk.
- Femoral and obturator both receive L2–L4.
- Superior gluteal passes above piriformis; sciatic normally below.
- Sciatic contains tibial and common fibular components.
- Posterior thigh skin is supplied separately from sciatic motor branches.
Sources: Snell's Clinical Anatomy by Regions, supplied Lower Limb chapter. Plexus formation and division details supplement the regional chapter using standard plexus anatomy and Nerve Plexus Anatomy 101. UHS PGET Paper I MBBS TOS determines the topic. Picture links are visual study resources; wording and tables are original.
FAQ
Which roots form the sciatic nerve?
L4, L5, S1, S2 and S3. Its tibial and common fibular components carry different subsets of these roots.
Does sciatic injury numb the entire posterior thigh?
Not necessarily. Posterior thigh skin is supplied by the posterior femoral cutaneous nerve, which is a separate sacral plexus branch.
What connects the lumbar and sacral plexuses?
The lumbosacral trunk, formed by part of L4 and the L5 ventral ramus.
Which side drops in a positive Trendelenburg test?
The unsupported side drops when the patient stands on the affected limb.