Sacral Plexus Anatomy Quiz

Sacral Plexus Anatomy

• The sacral plexus is a network of nerve fibers supplying the skin and muscles of the pelvis and lower limb.
• Located on the posterior pelvic wall, anterior to the piriformis muscle.
• Formed from the anterior rami of sacral spinal nerves S1, S2, S3, and S4.
• Receives contributions from lumbar spinal nerves L4 and L5, forming the lumbosacral trunk.
• Spinal nerves S1 to S4 form the sacral plexus's foundation.
• Each nerve branches into anterior and posterior rami.
• Anterior fibers of S1, S2, S3, and S4 emerge through anterior sacral foramina.
• Joining the fourth and fifth lumbar roots forms the lumbosacral trunk, descending into the pelvis.
• The sacral plexus is located on the anterior surface of the piriformis muscle.

Major Nerves of the Sacral Plexus

• Superior Gluteal Nerve: originates from L4, L5, and S1; innervates gluteus minimus, gluteus medius, and tensor fasciae latae.
• Inferior Gluteal Nerve: originates from L5, S1, and S2; innervates gluteus maximus.
• Sciatic Nerve: the largest nerve; derived from L4 to S3; composed of common fibular and tibial components.
  • Common Fibular Component: formed from dorsal divisions of L4 to S2; innervates the short head of biceps femoris, anterior and lateral leg muscles, and extensor digitorum brevis.
  • Tibial Component: formed from ventral divisions of L4 to S3; innervates all posterior thigh muscles except the short head of biceps femoris, hamstring portion of adductor magnus, and posterior leg and foot muscles.
• Posterior Femoral Cutaneous Nerve: originates from S1, S2, and S3; innervates skin of the posterior thigh and leg, and perineum.
• Pudendal Nerve: originates from S2, S3, and S4; innervates perineal skeletal muscles (external urethral sphincter, external anal sphincter, and levator ani), and provides sensory innervation to the penis/clitoris and perineal skin.

Smaller Branches of the Sacral Plexus

• Perforating Cutaneous Nerve: originates from S2 and S3; innervates inferior gluteal skin; pierces the sacro-tuberous ligament.
• Nerve to Piriformis: formed from S2 (sometimes S1); innervates the piriformis muscle directly.
• Nerve to Obturator Internus: originates from L5 to S2; exits the pelvis via greater sciatic foramen, loops around sacrospinous ligament, re-enters via the lesser sciatic foramen to innervate obturator internus and superior gemellus.
• Nerve to Quadratus Femoris: originates from L4 to S1; exits the pelvis via greater sciatic foramen inferior to the piriformis; innervates quadratus femoris and inferior gemellus.

Nerve Courses

• Nerves from the sacral plexus follow three courses:
  • First Course: leave the pelvis through the greater sciatic foramen, entering the gluteal region of the lower limb.
  • Second Course: remain within the pelvis, innervating pelvic muscles, organs, and perineum.
  • Third Course: leave the greater sciatic foramen, loop around the sacrospinous ligament, re-entering via the lesser sciatic foramen to supply lateral pelvic and perineal structures.

Lumbar Plexus

• The lumbar plexus is formed from the anterior rami of L1 to L4 and receives fibers from T12.
• Formed within the psoas major muscle.
• Mnemonic "I twice get laid on Fridays" helps remember six main nerves:
  • Iliohypogastric
  • Ilioinguinal
  • Genitofemoral
  • Lateral Femoral Cutaneous
  • Obturator
  • Femoral
• All nerves except genitofemoral and obturator emerge from the lateral aspect of the psoas major muscle.
• The genitofemoral nerve emerges anteriorly from the psoas major muscle, splitting into genital and femoral branches.
• The obturator nerve emerges medial to the psoas major muscle.

Nerve Roots

• Mnemonic "2 from 1, 2 from 2, 2 from 3" helps determine nerve root origins:
• "2 from 1": Iliohypogastric nerve originates from L1, Ilioinguinal nerve originates from L1.
• "2 from 2": Genitofemoral nerve originates from L1 and L2, Lateral femoral cutaneous nerve originates from L2 and L3.
• "2 from 3": Femoral nerve originates from L2, L3, and L4, Obturator nerve originates from L2, L3, and L4.

Other Points:

• The subcostal nerve (branch of T12) contributes to the lumbar plexus.
• The accessory obturator nerve is not always present, originating from L3 and L4.
• Muscular branches of the femoral nerve innervate the psoas and iliacus muscles.

Innervation

• Obturator nerve: Innervates the medial compartment muscles of the thigh.
• Femoral nerve: Innervates the anterior compartment muscles of the thigh.