Lumbosacral Plexus Anatomy
The lumbosacral plexus is a complex network of nerve fibers formed by the anterior rami of spinal nerves L1–S4. It provides the primary innervation for the lower limbs, pelvic floor, and perineum.
Lumbar Plexus (L1–L4)
Formed within the psoas major muscle; primarily innervates the anterior/medial thigh and abdominal wall.
- Femoral Nerve (L2–L4): Innervates the anterior thigh (quadriceps) and skin of the medial leg (saphenous nerve).
- Obturator Nerve (L2–L4): Innervates the medial thigh (adductors).
- Lateral Femoral Cutaneous (L2–L3): Sensory to the lateral thigh.
Sacral Plexus (L4–S4)
Located on the posterior pelvic wall; primarily innervates the posterior thigh, leg, and foot.
- Sciatic Nerve (L4–S3): The largest nerve in the body; exits the pelvis via the greater sciatic foramen. Splits into the Tibial and Common Fibular (Peroneal) nerves.
- Superior/Inferior Gluteal Nerves: Innervate the gluteal muscles.
- Pudendal Nerve (S2–S4): Vital for the pelvic floor, external genitalia, and sphincters.
| Key Nerve | Primary Motor Function |
|---|---|
| Femoral | Hip flexion, Knee extension |
| Obturator | Hip adduction |
| Sciatic | Hip extension, Knee flexion |
High-Yield Exam Pearl:
Clinical testing of the femoral nerve involves the patellar reflex (L4). Injury to the common fibular nerve (often at the fibular head) leads to foot drop (inability to dorsiflex the foot), while injury to the tibial nerve leads to inability to plantarflex or flex the toes.