The femoral nerve is one of the largest nerves originating from the lumbar plexus, a network of nerves in the lower spine. It is a mixed nerve, containing both motor and sensory fibers responsible for movement and sensation in the lower limb. The nerve plays a substantial role in thigh function, controlling muscle groups essential for standing, walking, and maintaining balance. Understanding its anatomical course and the muscles it supplies is foundational to comprehending its function.
Anatomical Origin and Path
The femoral nerve arises from the posterior divisions of the ventral rami of the L2, L3, and L4 spinal nerves, making it the largest branch of the lumbar plexus. It descends deep within the abdominal cavity, traveling through the fibers of the psoas major muscle.
The nerve emerges from the lateral border of the psoas major and descends between the psoas muscle and the iliacus muscle. It provides branches to the iliacus and the pectineus muscle before leaving the pelvis. The nerve then passes under the inguinal ligament to enter the upper thigh, known as the femoral triangle.
Inside the femoral triangle, the nerve is located lateral to the femoral artery and vein. Here, it quickly divides into its terminal branches, typically a few centimeters below the inguinal ligament. This division creates a superficial anterior division and a deep posterior division. The anterior division primarily gives rise to cutaneous nerves, while the posterior division supplies the major motor innervation to the thigh musculature.
Motor Innervation: Muscles of the Anterior Thigh
The primary function of the femoral nerve’s motor branches is to innervate the muscles of the anterior compartment of the thigh, responsible for powerful movements like hip flexion and knee extension. The most prominent targets are the four muscles that collectively form the quadriceps femoris group, which are the main extensors of the leg at the knee joint.
The quadriceps femoris group includes:
- Rectus femoris
- Vastus lateralis
- Vastus medialis
- Vastus intermedius
The rectus femoris is unique because it crosses the hip joint, allowing it to assist in hip flexion alongside knee extension. The three vasti muscles originate directly from the femur and are dedicated solely to knee extension.
The femoral nerve also supplies the sartorius muscle, which contributes to hip flexion, abduction, and lateral rotation. Additionally, the iliacus muscle, which works with the psoas major to form the iliopsoas group, is innervated by the femoral nerve, making it a powerful hip flexor. The nerve typically provides a branch to the pectineus muscle, which adducts the thigh.
Sensory Innervation and Distribution
The femoral nerve collects sensory information from the lower limb. This sensory function is carried out by several cutaneous branches that supply the skin over the front and inner parts of the thigh, collectively known as the anterior cutaneous nerves of the thigh.
The largest sensory branch is the saphenous nerve, which is a continuation of the nerve’s posterior division. The saphenous nerve travels down the thigh, continuing past the knee to supply sensation to the skin of the medial side of the leg. This coverage extends down to the ankle and the medial border of the foot.
The saphenous nerve also provides an infrapatellar branch, which supplies the skin over the front of the knee. This sensory distribution allows the brain to receive feedback regarding touch, pain, temperature, and pressure across these regions.
Impact of Femoral Nerve Damage
Damage to the femoral nerve, known as femoral neuropathy or palsy, results in specific functional deficits. The most significant motor consequence is severe weakness or complete inability to extend the knee joint due to paralysis of the quadriceps femoris group. This loss of knee extension makes actions like walking or standing challenging, often causing the knee to buckle unexpectedly.
Damage also impairs hip flexion due to the loss of function in the iliacus and sartorius. A person with femoral nerve damage will struggle with lifting the thigh forward and may experience wasting of the anterior thigh muscles. Sensory symptoms involve numbness, tingling, or a burning sensation distributed over the anterior thigh and along the medial aspect of the leg and foot, following the saphenous nerve path.
Common causes of injury include direct trauma to the pelvis or thigh, compression from a hematoma, or injury sustained during abdominal or hip surgery. Generalized nerve damage from conditions like diabetes mellitus can also affect the femoral nerve. Clinicians diagnose the location and extent of the injury by understanding the precise pattern of motor weakness and sensory loss.