Topography and upper branches
Anatomically, the lumbar plexus is located within the substance of the psoas major muscle (m. psoas major). It lies directly anterior to the transverse processes of the lumbar vertebrae. Nerves responsible for innervating the lower trunk and groin area originate from the upper segments of the plexus.
These include the iliohypogastric nerve (n. iliohypogastricus, segments Th12–L1). It divides into two main branches:
- The lateral cutaneous branch (r. cutaneus lateralis) — supplies the skin of the upper lateral gluteal region.
- The anterior cutaneous branch (r. cutaneus anterior) — provides sensation to the skin of the lower anterior abdominal wall, located above the pubis.
Next is the ilioinguinal nerve (n. ilioinguinalis, L1). Its area of responsibility includes the skin of the pubic region and the upper medial thigh. Additionally, it innervates the anterior scrotum in men and the labia majora in women.
Genitofemoral nerve
The genitofemoral nerve (n. genitofemoralis, L1–L2) has an interesting topography: it passes directly within the substance or on the surface of m. psoas major, where it divides into two functional branches:
- The femoral branch (r. femoralis) — descends onto the thigh, giving off branches to the skin of the femoral triangle region.
- The genital branch (r. genitalis) — heads toward the inguinal region. In men, it innervates the skin of the scrotum and spermatic cord, while in women, it provides sensation to the skin in the area of the inguinal canal.
Lower extremity nerves
The lower segments of the plexus form major nerves that descend onto the thigh and leg.
- The lateral femoral cutaneous nerve (n. cutaneus femoris lateralis, L2–L3). This nerve leaves the pelvic cavity in the region of the anterior superior iliac spine (spina iliaca anterior superior) and passes beneath the inguinal ligament. Its task is to innervate the skin of the anterolateral surface of the thigh.
- The femoral nerve (n. femoralis, L2–L4). It gives rise to the anterior cutaneous branches of the thigh (rr. cutanei anteriores), supplying the anterior surface of the thigh. Its longest branch, the saphenous nerve (n. saphenus), descends further, innervating the skin of the medial surface of the leg and the medial margin of the foot.
- The obturator nerve (n. obturatorius, L2–L4). Gives off an inconstant cutaneous branch that travels to the skin of the medial surface of the thigh.
Briefly: answers to topical questions
Which spinal nerve roots form the lumbar plexus?
The sources indicate the following nerves associated with the lumbar plexus: subcostal nerve (Th₁₂), iliohypogastric nerve (Th₁₂–L₁), ilioinguinal nerve (L₁), genitofemoral nerve (L₁–L₂), lateral femoral cutaneous nerve (L₂–L₃), femoral and obturator nerves (L₂–L₄).
Which muscles are innervated by the branches of the lumbar plexus?
Sources indicate that the lumbar plexus is associated with the innervation of the following muscles and muscle groups:
- The psoas minor muscle (m. psoas minor) — innervated from plexus lumbalis (L₁–L₂).
- Abdominal wall muscles — innervated with the participation of the subcostal nerve.
- The vastus lateralis and vastus medialis muscles (m. vastus lateralis, m. vastus medialis) and the tibialis anterior muscle (m. tibialis anterior) — indicated as indicator muscles for the L₄ root.
What symptoms occur with a femoral nerve lesion?
A femoral nerve lesion may cause:
- Motor disturbances: inability to extend the knee joint; in lesions below the inguinal ligament — paresis of the quadriceps femoris. In advanced stages, partial weakening of hip flexion is possible.
- Sensory disturbances: anesthesia of the anterior surface of the thigh and the medial surface of the leg; pronounced sensory disorders in the anteromedial zone of the leg may occur.
- Reflex changes: reduction or absence of the knee-jerk reflex.
- Pain syndrome: in lesions below the inguinal ligament, pain may occur, along with tenderness upon palpation of the midpoint of the inguinal ligament and a positive Tinel's sign.
Class and exam questions
Where is the lumbar plexus located?
The plexus lies within the substance of the psoas major muscle (m. psoas major), anterior to the transverse processes of the lumbar vertebrae.
Which branches originate from the genitofemoral nerve?
It divides into a femoral branch (innervating the skin of the femoral triangle) and a genital branch (innervating the scrotum and spermatic cord, or the skin of the inguinal canal in women).
Which nerve emerges from the pelvis at the spina iliaca anterior superior?
The lateral femoral cutaneous nerve (n. cutaneus femoris lateralis) emerges near the anterior superior iliac spine.
What else to study on this topic
- Topography of the branches of the iliohypogastric nerve
- Zones of innervation of the ilioinguinal nerve
- Division of the genitofemoral nerve within the psoas major muscle
- Course of the lateral femoral cutaneous nerve beneath the inguinal ligament
- Cutaneous branches of the femoral and obturator nerves
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