Lumbosacral Plexus
Essentials
I. Ilioinguinal nerve
- Exits the pelvis over the inguinal ligament to the superior-medial portion of the thigh.
- 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.
- 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.
- 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.
- 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.
Muscle Groups
- Femoral nerve - Quadriceps
- Gluteal nerves - Gluteal muscles
- Obturator nerve - Adductor muscles
- Sciatic nerve - Extrinsic foot muscles
- Sciatic nerve - Hamstrings
- Sciatic nerve - Intrinsic foot muscles
- Short rotators of the hip
Advanced Innervation
Inferior gluteal nerve (L5 – S2)
- Innervates gluteus maximus, which provides provides hip extension.
Superior gluteal nerve (L4 – S1)
- Innervates gluteus medius, gluteus minimus, and tensor fasciae latae; all of which provide hip abduction – tensor fasciae latae does when the hip is in flexion.
Iliohypogastric and ilioinguinal nerves (L1).
- Both nerves provide motor innervation to the internal oblique and transversus abdominis muscles.
- Iliohypogastric nerve provides sensory coverage to the suprapubic and upper-lateral gluteal areas.
- Ilioinguinal nerve provides sensory coverage to the superior-medial portion of the thigh and proximal external genitalia.
Genitofemoral nerve (L1–L2)
- Genital branch innervates the cremaster muscle.
- Provides sensory coverage to the scrotum or labia.
- Femoral branch is purely sensory.
- Provides sensory coverage to the femoral triangle.
Lateral cutaneous nerve of the thigh (aka lateral femoral cutaneous nerve) (L2 and L3)
- Provides sensory coverage to the lateral thigh.
Posterior cutaneous nerve of the thigh (aka the posterior femoral cutaneous nerve) (S1 – S3)
- Provides sensory coverage to the midline back of the thigh.
Pudendal nerve (S4 + S2 and S3)
- Branches into the inferior rectal nerve, perineal nerve, and dorsal nerve to the penis or clitoris.
- Provides motor innervation to the external urethral and anal sphincters and external genitalia and it provides sensory coverage to the anus and external genitalia.
Short rotators of the hip (L5 – S2)
- Provide external rotation of the hip when it is in extension and hip abduction when it is in flexion.
- The nerve to quadratus femoris innervates quadratus femoris and gemellus inferior; the nerve to obturator internus innervates obturator internus and gemellus superior; the nerve to piriformis innervates piriformis; and the obturator nerve innervates obturator externus (note that obturator externus is supplied by L3, L4).