The femoral nerve is the largest nerve arising from the lumbar plexus. It functions as a mixed nerve, carrying both motor signals for muscle movement and sensory information related to feeling. Understanding its anatomy is foundational to comprehending the mechanics of leg movement and the sensation felt across the thigh and lower leg.
The Lumbar Plexus and Spinal Roots
The femoral nerve originates in the lower back within the lumbar plexus, an intricate network of nerve fibers. This plexus is formed by the ventral rami, or anterior divisions, of several lumbar spinal nerves. Specifically, the femoral nerve is constructed from contributions of the second, third, and fourth lumbar spinal nerves (L2, L3, and L4).
The lumbar plexus resides within the psoas major muscle, a large muscle of the posterior abdominal wall. As the femoral nerve forms, it emerges from this muscular structure as a single trunk.
The Nerve’s Path Through the Body
After forming in the lumbar plexus, the femoral nerve descends deep within the abdominal cavity. It travels downward in the groove between the psoas major muscle and the adjacent iliacus muscle. While deep, the nerve supplies a branch to the iliacus muscle, which is a hip flexor.
The nerve progresses toward the leg by passing beneath the inguinal ligament. This passage marks its transition into the upper thigh, where it enters the femoral triangle. Inside the triangle, the nerve is positioned laterally to the main artery and vein of the leg.
Shortly after entering the thigh, the main trunk divides into several branches. These are categorized into an anterior and a posterior division, which fan out to supply the structures of the anterior thigh.
What the Femoral Nerve Controls
The femoral nerve provides both motor control over the muscles of the thigh and sensory input from the skin. Its motor branches are responsible for flexing the hip and extending the knee.
The ability to straighten the leg is controlled by its innervation of the quadriceps femoris muscle group, which includes:
- Rectus femoris
- Vastus lateralis
- Vastus medialis
- Vastus intermedius
The nerve also supplies the iliacus muscle, which flexes the hip joint, and the sartorius muscle, which assists in both hip and knee flexion. Damage to the motor components of this nerve results in an inability to perform knee extension, affecting walking and standing.
The sensory role is carried out by its cutaneous branches. The anterior cutaneous nerves of the thigh provide sensation to the skin on the front and inner portion of the thigh. The largest sensory branch is the saphenous nerve, a continuation of the posterior division.
The saphenous nerve travels down the leg, providing sensation to the skin over the inner side of the knee, the lower leg, and the medial side of the foot.
Common Causes of Femoral Nerve Damage
Damage to the femoral nerve can arise from trauma or compression along its anatomical course. Direct physical injury, such as a fractured pelvis or hip surgery, is a frequent cause where the nerve may be stretched or cut. Bleeding into the pelvic or abdominal space (hematoma) can also cause compression on the nerve trunk.
Prolonged pressure during medical procedures is another recognized cause of injury. For instance, patients maintained in the lithotomy position (lying on the back with hips and knees flexed) for an extended period can experience nerve compression. Tumors or masses growing near the nerve’s path may also impinge on its function.
Systemic conditions like diabetes can also affect the femoral nerve, leading to widespread nerve damage. When the nerve is injured, symptoms reflect the loss of motor and sensory functions. Patients often experience quadriceps weakness, making it difficult to straighten the knee or climb stairs.
Sensory changes manifest as numbness, tingling, or a burning sensation across the anterior thigh and inner leg. Early recognition of these symptoms is important for identifying the cause and initiating treatment to prevent permanent nerve dysfunction.