The lumbar plexus is a complex network of nerves located deep within the lower back. This intricate structure enables movement and sensation throughout the lower body and legs. It distributes nerve signals for actions like walking and feeling the ground beneath your feet.
Understanding Its Structure
The lumbar plexus originates from spinal nerve roots, primarily L1 to L4, with a common contribution from T12 and sometimes L5. These roots emerge from the spinal cord and converge to form this network. The plexus is situated deep within the abdominal region, embedded within the posterior part of the psoas major muscle and anterior to the transverse processes of the lumbar vertebrae. This anatomical positioning provides a protected yet central location for the organization of these nerves before they branch out to the lower limbs and pelvic girdle.
Major Nerves and Their Paths
Several distinct nerves emerge from the lumbar plexus, each following a specific route. The iliohypogastric nerve, typically from T12 and L1, travels across the quadratus lumborum muscle, piercing the transversus abdominis to run between the transversus abdominis and internal oblique muscles. The ilioinguinal nerve, primarily from L1, follows a similar path, emerging from the lateral border of the psoas major to enter the abdominal wall near the anterior superior iliac spine and the inguinal canal.
The genitofemoral nerve, formed by L1 and L2, descends through the psoas major muscle, emerging on its anterior surface before dividing into genital and femoral branches. The lateral femoral cutaneous nerve, from L2 and L3, leaves the psoas major muscle laterally, courses obliquely across the iliacus muscle, passing under or through the inguinal ligament to reach the thigh.
The obturator nerve, from L2, L3, and L4, descends through the psoas major muscle and exits the pelvis through the obturator canal to enter the medial compartment of the thigh, where it divides into anterior and posterior branches. The femoral nerve, the largest branch, arises from L2, L3, and L4. It emerges from the lower lateral border of the psoas major muscle and passes beneath the inguinal ligament, lateral to the femoral vessels, to enter the anterior thigh.
Motor and Sensory Roles
Each nerve from the lumbar plexus contributes to movement and sensation in the lower body. The iliohypogastric nerve provides motor innervation to the internal oblique and transversus abdominis muscles of the abdominal wall. It also supplies sensation to the skin over the lateral gluteal region and the area just above the pubic bone. The ilioinguinal nerve also innervates parts of the transversus abdominis and internal oblique muscles. Its sensory functions include providing sensation to the skin of the upper and medial thigh, as well as the anterior scrotum in males and the mons pubis and labia majora in females.
The genitofemoral nerve has both motor and sensory roles. Its genital branch provides motor control to the cremaster muscle in males and sensory innervation to the skin of the anterior scrotum in males and the mons pubis and labia majora in females. The femoral branch supplies sensation to the skin of the upper anterior thigh. The lateral femoral cutaneous nerve is purely sensory, providing sensation to the skin of the anterior and lateral aspects of the thigh, extending down toward the knee.
The obturator nerve provides motor innervation to most muscles in the medial compartment of the thigh, including the adductor longus, adductor brevis, gracilis, obturator externus, and a portion of the adductor magnus, which enable thigh adduction. It also provides sensory innervation to the skin on the medial side of the upper thigh. The femoral nerve, the largest, has extensive functions. It innervates hip flexors like the iliacus, pectineus, and sartorius, and the quadriceps femoris muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius), which are responsible for knee extension. Sensitively, it provides sensation to the skin of the anterior and medial thigh, and through its saphenous branch, to the medial leg and foot.
When Things Go Wrong
Various factors can affect the lumbar plexus or its individual nerves, leading to symptoms. Nerve compression can occur from tight muscles, scar tissue, or pelvic masses like tumors or abscesses. Direct injuries, such as motor vehicle accidents or falls that fracture the pelvis, can also damage these nerves. Surgical complications after hip replacements or abdominal operations can injure nearby nerves within the plexus.
Inflammatory conditions, including diabetic amyotrophy, can also impact the lumbar plexus. Radiation therapy for pelvic cancers can lead to delayed nerve damage. When affected, symptoms include pain, numbness, tingling, or weakness in the lower back, hip, thigh, or leg, corresponding to the specific nerves involved. For instance, weakness in hip flexion or knee extension, along with numbness in the anterior thigh, might suggest issues with the femoral nerve.