Lumbosacral Plexus

The lumbosacral plexus is divided anatomically into 3 main components

  1. Lumbar plexus (L 1,2,3,4)
  2. Lumbosacral trunk (L4,5)
  3. Sacral plexus (LS trunk + S 1,2,3,4)
  • The lumbar plexus is formed by the ventral rami of L1,2,3, and most of the L4 roots). These rami divide near the vertebral column into ventral and dorsal branches:
    • Ventral branches from the obturator nerve
    • Dorsal branches form the femoral nerve
  • The lumbosacral trunk is formed primarily by the L5 root, with a contributing branch from the L4 root. It then travels a relatively long distance before at the pelvic rim it where it is joined first by the S1 root
  • The sacral plexus is formed by the fusion of the LS trunk with the ventral rami of S1, S2, S3, and S4. Its main branches are:
    • Sciatic Nerve (L4-S3)
    • Superior Gluteal Nerve (L4-S1)
    • Inferior Gluteal Nerve (L5-S2)

Lumbosacral plexus image

LS Plexus 2

Note:

The lumbar plexus gives rise to direct motor branches to the psoas and iliacus,  ilioinguinal nerve (L1 – sensory) , iliohypogastric nerve (L1 – sensory) , genitofemoral nerve (L1. L2   – sensory/motor cremasteric reflex), and the lateral cutaneous nerve of the thigh L2, L3).

The main branches of the sacral plexus include the sciatic nerve (L4 – S3), the superior gluteal nerve (which innervates the gluteus medius and minimus) and the inferior gluteal nerve (which innervates the gluteus maximus). Other nerves that arise from the sacral plexus are the posterior cutaneous nerve of the thigh and the pudendal nerves.

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