Lumbosacral Plexus - Essentials

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lumbosacral plexus and innervation of the thigh

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See: Lumbosacral Plexus

Relevant regional anatomy

• Lower lumbar vertebral column and sacrum
• The L5/S1 junction is a clinically important level for disc herniation – here the vertebral column angles abruptly posteriorly
• The right pelvic bone
• The femur and the superior aspect of the tibia and fibula
• The inguinal ligament

Lumbosacral Plexus: key anatomy

I. Ilioinguinal nerve

• Exits the pelvis over the inguinal ligament to the superior-medial portion of the thigh.

II. Femoral nerve

• Exits the pelvis, passes in between the psoas and iliacus muscles, underneath the inguinal ligament, down the anterior thigh to innervate the anterior compartment muscles.

III. Obturator nerve

• Descends medial to the femoral nerve, anterior to the sacrospinous ligament and exits via the obturator canal, down the medial aspect of the thigh to innervate the medial compartment muscles.

IV. Sciatic nerve

• Exits the pelvis anterior and inferior to the piriformis via the greater sciatic foramen and then descends posterior to the femur.
• At the inferior femur, it divides into:
  • The tibial nerve, which continues down the posterior tibia to innervate the posterior leg and plantar foot.
  • The peroneal nerve, which wraps around the fibular neck and innervates the anterior and lateral leg and dorsal foot.

V. Superior gluteal nerve

• Exits the greater sciatic foramen superior to the piriformis muscle.

Hip and pelvis innervation: essentials

• Spinal nerves L1 – S4 innervate the hip and pelvis.
• The sciatic nerve is derived from L4 – S3.
  • It innervates the hamstrings, which provide hip extension.
• The femoral and obturator nerves are derived from L2 – L4.
  • The femoral innervates iliopsoas, which provides hip flexion; it is key to climbing upstairs or rising from a low chair.

thigh innervation: essentials

• The femoral nerve innervates the anterior compartment.
• The obturator nerve innervates the medial compartment.
• The sciatic nerve innervates the posterior compartment.
  • The "one compartment — one nerve" principle helps us remember each compartments' innervation and function.

Femoral nerve

• The femoral nerve innervates the quadriceps femoris muscles, which are rectus femoris and the vastus muscles: vastus medialis, vastus intermedius, and vastus lateralis.
  • They provide knee extension, key to walking downstairs.
  • Again, It innervates the iliopsoas muscle, which comprises iliacus and the psoas major and minor muscles: the psoas muscles are innervated by direct branches from L1 – L3.
  • They provide hip flexion, which is key to climbing upstairs or rising from a low chair.

Obturator nerve

• The obturator nerve innervates the adductor muscles, which are adductor longus, adductor brevis, and adductor magnus; note that adductor magnus is also supplied by the sciatic nerve, as we will later show.
  • They provide hip adduction and also a variety of actions intrinsic to gait and stability.

Sciatic nerve

• The sciatic nerve innervates the hamstrings muscles, which are semimembranosus, semitendinosus, and the short and long heads of the biceps femoris muscle.
  • The peroneal division of the sciatic nerve innervates the short head of the biceps femoris.
  • The tibial division innervates the other hamstrings muscles: the long head of biceps femoris, semitendinosus, and semimembranosus.
  • The peroneal innervation of the short head of the biceps femoris is especially important in localization because a peroneal neuropathy at the fibular head (the most common peroneal nerve entrapment site) will spare the short head of the biceps femoris.
  • The sciatic nerve's innervation to the adductor magnus muscle is supplied by L4, L5.
  • The hamstrings and adductor magnus provide knee flexion and hip extension.

The Femoral Triangle

• Within the femoral triangle, the nerve lies most laterally; medial to it is the artery; and medial to it is the vein.
• The mnemonic NAVY is helpful because it incorporates the position of the midline genitalia, the "Y," into the acronym.