Medical School · Year 1 · Anatomy Msk · includes a quiz and discussion video

Lecture 13: Lower Limb - Neurovasculature

Unit 1.3: Human Gross Anatomy I - Musculoskeletal System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the formation and branches of the lumbar and sacral plexuses
2. Trace the major nerves of the lower limb and their distributions
3. Describe the arterial supply from the common iliac to the foot
4. Explain the venous and lymphatic drainage of the lower limb
5. Identify important neurovascular relationships and compression sites
6. Correlate nerve injury patterns with clinical presentations

Lecture Content

I. The Lumbar Plexus

The lumbar plexus provides innervation to the anterior and medial thigh, forming from the ventral rami of L1 through L4 with a contribution from T12. The plexus lies embedded within the substance of the psoas major muscle, positioned anterior to the lumbar transverse processes.

The iliohypogastric nerve arises from T12 and L1, supplying motor fibers to the abdominal wall muscles and providing sensory innervation to the suprapubic region. The ilioinguinal nerve, derived from L1, similarly innervates the abdominal wall and carries sensory information from the inguinal region and external genitalia. The genitofemoral nerve emerges from L1 and L2, dividing into a genital branch that innervates the cremaster muscle and provides sensation to the scrotum or labia majora, and a femoral branch that carries sensation from the skin over the femoral triangle.

The lateral femoral cutaneous nerve arises from L2 and L3 as a purely sensory nerve supplying the lateral thigh. The femoral nerve, the largest branch derived from L2 through L4, innervates the muscles of the anterior thigh compartment and provides sensory coverage to the anterior thigh and, through its saphenous branch, the medial leg. The obturator nerve also originates from L2 through L4 and serves the medial thigh adductor muscles with variable sensory contribution to the medial thigh.

<image>Panel A: Lumbar plexus formation within psoas major muscle with nerve roots L1-L4 in yellow emerging from intervertebral foramina. Panel B: Iliohypogastric and ilioinguinal nerves (light green) and genitofemoral nerve (orange) dividing into genital and femoral branches with exit points labeled. Panel C: Lateral femoral cutaneous nerve (blue) as purely sensory to lateral thigh, and femoral nerve (red, largest branch) to anterior compartment. Panel D: Obturator nerve (purple) to medial compartment with directional arrows indicating course toward respective target regions.</image>

II. Major Lumbar Plexus Nerves

The Femoral Nerve

The femoral nerve, derived from L2 through L4, emerges from the lateral border of the psoas major and descends between the psoas and iliacus muscles. It enters the thigh by passing deep to the inguinal ligament, lying lateral to the femoral artery within the femoral triangle. Immediately upon entering the thigh, the nerve divides into its multiple branches.

Motor innervation from the femoral nerve reaches the iliacus before the nerve enters the thigh. In the thigh, it supplies pectineus, sartorius, and all four heads of the quadriceps femoris including rectus femoris and the three vastus muscles. These muscles provide hip flexion and knee extension.

Sensory distribution includes the anterior cutaneous branches covering the anterior thigh and the saphenous nerve, which represents the largest cutaneous branch. The saphenous nerve accompanies the femoral artery through the adductor canal, exits through the subsartorial membrane, and descends along the medial leg in company with the great saphenous vein. It provides sensation to the medial leg and medial foot as far as the medial malleolus.

Femoral nerve injury produces difficulty with hip flexion and an inability to extend the knee. Patients demonstrate sensory loss over the anterior thigh and medial leg, and the patellar reflex is absent.

<image>Panel A: Femoral nerve emerging from psoas major and passing deep to inguinal ligament alongside femoral artery (red) and vein (blue). Panel B: Immediate branching pattern in femoral triangle with motor branches to quadriceps muscles highlighted. Panel C: Saphenous nerve (yellow) descending along medial leg with great saphenous vein through adductor canal. Panel D: Dermatome overlay showing sensory territory on anterior thigh and medial leg with clinical presentation of femoral nerve injury.</image>

The Obturator Nerve

The obturator nerve arises from L2 through L4 and emerges from the medial border of the psoas major. It crosses the pelvic brim and exits the pelvis through the obturator foramen, passing through the obturator canal. Upon exiting, the nerve divides into anterior and posterior divisions.

Motor innervation reaches the adductor compartment muscles including adductor longus, adductor brevis, the adductor portion of adductor magnus, and gracilis. The posterior division also supplies obturator externus. Sensory branches provide variable cutaneous innervation to the medial thigh and articular branches to both the hip and knee joints.

Obturator nerve injury causes weakness in hip adduction, leading to a characteristic gait abnormality where the patient swings the leg outward in circumduction to avoid dragging the foot. Sensory loss occurs over the medial thigh.

<image>Panel A: Obturator nerve origin from medial psoas border (L2-L4) shown in green with course across the pelvis. Panel B: Cross-sectional inset showing passage through obturator foramen with bone rendered semi-transparent. Panel C: Anterior division supplying adductor longus, brevis, and gracilis with each muscle color-coded. Panel D: Posterior division supplying adductor magnus adductor portion and obturator externus with sensory territory on medial thigh.</image>

III. The Sacral Plexus

The sacral plexus forms from the union of the lumbosacral trunk, carrying fibers from L4 and L5, with the ventral rami of S1 through S4. The plexus lies on the anterior surface of the piriformis muscle, anterior to the sacrum.

The superior gluteal nerve emerges from L4 through S1 and innervates gluteus medius, gluteus minimus, and tensor fasciae latae. The inferior gluteal nerve, derived from L5 through S2, provides the sole motor supply to gluteus maximus. The posterior femoral cutaneous nerve arises from S1 through S3 and carries sensation from the posterior thigh.

The sciatic nerve, formed from L4 through S3, represents the largest nerve in the body and innervates the posterior thigh muscles and all structures below the knee. The pudendal nerve, derived from S2 through S4, provides both motor and sensory innervation to the perineum.

<image>Panel A: Sacral plexus formation with lumbosacral trunk (L4-L5) joining sacral roots S1-S4 on the piriformis muscle surface. Panel B: Superior gluteal nerve (blue) passing above piriformis to gluteus medius, minimus, and TFL, and inferior gluteal nerve (green) to gluteus maximus. Panel C: Sciatic nerve (large yellow trunk) exiting below piriformis with posterior femoral cutaneous nerve (orange) and pudendal nerve (red) to perineum. Panel D: Inset showing piriformis relationship from posterior view with all branches and their exit points relative to piriformis labeled.</image>

IV. The Sciatic Nerve

The sciatic nerve, derived from L4 through S3, is the largest nerve in the human body. It exits the pelvis through the greater sciatic foramen, typically passing below the piriformis muscle though anatomical variants exist where all or part of the nerve passes through or above the muscle. The nerve descends in the posterior thigh between the greater trochanter and ischial tuberosity, lying deep to gluteus maximus. It continues between adductor magnus and the long head of biceps femoris before dividing into its terminal branches.

The sciatic nerve contains two distinct divisions that may separate at various levels. The tibial division arises from the anterior sacral plexus and runs medially within the nerve trunk. The common peroneal division originates from the posterior sacral plexus and lies laterally. In some individuals, these divisions separate within the pelvis itself.

Motor innervation in the posterior thigh supplies all the hamstring muscles. The tibial division innervates semimembranosus, semitendinosus, the long head of biceps femoris, and the hamstring portion of adductor magnus. The common peroneal division supplies the short head of biceps femoris, the only hamstring muscle innervated by this division.

<image>Panel A: Sciatic nerve exiting greater sciatic foramen below piriformis (rendered semi-transparent) coursing between greater trochanter and ischial tuberosity. Panel B: Path deep to gluteus maximus (shown in cutaway) with tibial division innervation in red to semimembranosus, semitendinosus, and biceps long head. Panel C: Common peroneal innervation (blue) to biceps short head and tibial division to adductor magnus hamstring part with typical bifurcation level in distal thigh. Panel D: Anatomical variants of sciatic nerve division showing early separation within pelvis and relationship to piriformis variations.</image>

V. The Tibial Nerve

The tibial nerve, the larger terminal branch of the sciatic nerve, continues from L4 through S3. It descends through the popliteal fossa, lying superficial to the popliteal vessels. Below the knee, it passes deep to the soleus muscle and is accompanied by the posterior tibial artery throughout its course in the leg. The nerve passes behind the medial malleolus within the tarsal tunnel and divides into the medial and lateral plantar nerves.

Motor innervation from the tibial nerve supplies all muscles of the posterior compartment of the leg and, through the plantar nerves, all intrinsic muscles of the plantar foot.

Sensory distribution includes the sural nerve, formed by the medial sural cutaneous branch of the tibial nerve combined with a contribution from the common peroneal nerve. The sural nerve provides sensation to the lateral leg and lateral foot, traveling near the small saphenous vein. The plantar nerves distribute sensory fibers in a pattern analogous to the hand, with the medial plantar nerve supplying the medial three and a half digits similar to the median nerve in the hand, and the lateral plantar nerve supplying the lateral one and a half digits similar to the ulnar nerve.

The medial plantar nerve carries motor fibers to abductor hallucis, flexor hallucis brevis, flexor digitorum brevis, and the first lumbrical. The lateral plantar nerve innervates the remaining intrinsic foot muscles.

Tibial nerve injury results in loss of plantarflexion and inability to stand on tiptoes. Toe flexion is lost, and sensory deficit occurs over the sole of the foot. The ankle reflex is absent.

<image>Panel A: Tibial nerve descending through popliteal fossa superficial to vessels shown in cross-section. Panel B: Passage deep to soleus with posterior tibial artery (red) through the posterior compartment of the leg. Panel C: Transit through tarsal tunnel behind medial malleolus (bone window view) and terminal division into medial plantar (medial 3.5 digits) and lateral plantar (lateral 1.5 digits) nerves. Panel D: Sural nerve course along lateral leg shown in separate pathway with plantar sensory map inset and clinical presentation of tibial nerve injury.</image>

VI. The Common Peroneal Nerve

The common peroneal nerve, also called the common fibular nerve, arises from L4 through S2 as the smaller terminal branch of the sciatic nerve. It courses along the tendon of biceps femoris in the lateral popliteal fossa, then wraps around the neck of the fibula in a superficial position that renders it vulnerable to injury. At this point, it divides into its deep and superficial branches.

The deep peroneal nerve enters the anterior compartment and provides motor innervation to all the dorsiflexors and toe extensors including tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius. Its sensory distribution is limited to the first web space between the great and second toes.

The superficial peroneal nerve supplies the lateral compartment, innervating fibularis longus and fibularis brevis which produce foot eversion. Sensory branches supply most of the dorsum of the foot.

Common peroneal nerve injury represents the most frequent mononeuropathy affecting the lower limb. Causes include fibular neck fractures, tight leg casts, habitual leg crossing, and prolonged squatting. The characteristic presentation includes foot drop, the inability to dorsiflex the foot, along with loss of eversion. Patients develop a steppage gait, lifting the leg high to clear the drooping foot. Sensory loss occurs over the dorsum of the foot.

<image>Panel A: Common peroneal nerve course in lateral popliteal fossa along biceps femoris tendon wrapping around fibular neck with vulnerability zone highlighted. Panel B: Division into deep peroneal branch entering anterior compartment innervating dorsiflexors (tibialis anterior, extensor hallucis longus, extensor digitorum longus). Panel C: Superficial peroneal branch in lateral compartment innervating fibularis longus and brevis for foot eversion. Panel D: Sensory territory mapping on dorsal foot surface showing deep peroneal first web space (small area) versus extensive superficial peroneal distribution.</image>

VII. Arterial Supply of the Lower Limb

The arterial supply to the lower limb originates from the abdominal aorta, which bifurcates into the common iliac arteries at the level of L4. Each common iliac artery divides at the sacroiliac joint into the internal iliac artery, supplying pelvic structures, and the external iliac artery, which continues to the lower limb.

The external iliac artery becomes the femoral artery as it passes deep to the inguinal ligament. The femoral artery courses through the femoral triangle, positioned lateral to the femoral vein. It then traverses the adductor canal and exits through the adductor hiatus to become the popliteal artery.

The femoral artery gives off several branches including the superficial epigastric artery to the anterior abdominal wall, the superficial circumflex iliac artery to the lateral hip, and the external pudendal arteries to the external genitalia. The profunda femoris artery, the largest branch arising approximately four centimeters below the inguinal ligament, represents the main blood supply to the thigh muscles. It gives rise to the medial and lateral circumflex femoral arteries and the perforating branches that supply the posterior thigh. The descending genicular artery participates in the anastomosis around the knee.

The medial circumflex femoral artery provides the crucial blood supply to the femoral head, making it clinically significant in femoral neck fractures. The lateral circumflex femoral artery contributes to the anastomotic network around the hip.

<image>Panel A: Femoral artery course from inguinal ligament through femoral triangle with cross-section showing relationship to vein and nerve. Panel B: Passage through adductor canal and origin of profunda femoris with medial and lateral circumflex femoral branches forming hip anastomosis. Panel C: Perforating branches to posterior compartment and descending genicular artery contributing to knee anastomosis. Panel D: Inset showing femoral head blood supply emphasizing medial circumflex importance and clinical significance in femoral neck fractures.</image>

The Popliteal and Leg Arteries

The popliteal artery represents the continuation of the femoral artery at the adductor hiatus. It lies as the deepest structure in the popliteal fossa, positioned directly against the posterior surface of the femur and knee joint capsule. The artery terminates at the lower border of the popliteus muscle.

Branches of the popliteal artery include the superior genicular arteries, both medial and lateral, the middle genicular artery which supplies the joint capsule and cruciate ligaments, the inferior genicular arteries, and the sural arteries to the calf muscles. These genicular branches form an extensive anastomotic network around the knee that maintains circulation when the knee is flexed.

The popliteal artery terminates by dividing into the anterior tibial artery and the tibioperoneal trunk, which quickly divides into the posterior tibial and peroneal arteries.

The anterior tibial artery passes through the superior opening in the interosseous membrane to enter the anterior compartment. It descends on the anterior surface of the membrane, accompanied by the deep peroneal nerve. At the ankle, it becomes the dorsalis pedis artery, passing anterior to the extensor retinaculum. The dorsalis pedis pulse is palpated lateral to the tendon of extensor hallucis longus. This artery gives off the arcuate artery and the deep plantar artery, which joins with the lateral plantar artery to form the plantar arch.

The posterior tibial artery, the larger terminal branch, descends in the posterior compartment accompanied by the tibial nerve. It passes behind the medial malleolus within the tarsal tunnel, where its pulse can be palpated. The artery divides into the medial and lateral plantar arteries that supply the sole of the foot.

The peroneal artery, a branch of either the posterior tibial artery or the tibioperoneal trunk, supplies the lateral compartment and deep posterior group. It serves as an important collateral pathway when other leg vessels are diseased.

<image>Panel A: Popliteal artery in popliteal fossa as deepest structure shown in cross-section against femur with genicular branches forming knee anastomosis network. Panel B: Division into anterior tibial artery passing through interosseous membrane and descending anteriorly to become dorsalis pedis at the ankle. Panel C: Posterior tibial artery descending posteriorly with tibial nerve through tarsal tunnel to medial and lateral plantar arteries, and peroneal artery to lateral structures. Panel D: Pulse point locations marked for popliteal, dorsalis pedis, and posterior tibial arteries with palpation technique indicated.</image>

VIII. Venous Drainage

The deep veins of the lower limb follow the corresponding arteries as venae comitantes, paired veins accompanying the arterial trunks. They bear the same names as their associated arteries and ultimately drain into the femoral vein, then the external iliac vein, the common iliac vein, and finally the inferior vena cava.

The popliteal vein forms from the confluence of the anterior and posterior tibial veins and receives the small saphenous vein. The femoral vein continues from the popliteal vein at the adductor hiatus and receives the profunda femoris vein and the great saphenous vein.

The superficial venous system consists of two major vessels. The great saphenous vein, the longest vein in the body, begins at the medial dorsal venous arch of the foot. It ascends anterior to the medial malleolus, then along the medial aspect of the leg and thigh. It passes through an opening in the fascia lata called the saphenous opening, located approximately four centimeters inferolateral to the pubic tubercle, to drain into the femoral vein.

The small saphenous vein begins at the lateral dorsal venous arch, passes behind the lateral malleolus, and ascends along the posterior calf. It drains into the popliteal vein at a variable level.

The superficial veins contain valves that prevent retrograde flow. Incompetent valves lead to varicose veins. The great saphenous vein serves as an important conduit for coronary artery bypass grafting. Deep vein thrombosis occurs most commonly in the deep venous system.

<image>Panel A: Deep venous system following arteries as venae comitantes draining to femoral vein, external iliac, and ultimately IVC. Panel B: Great saphenous vein (blue) with medial course from foot, anterior to medial malleolus, up medial leg and thigh, through saphenous opening to femoral vein. Panel C: Small saphenous vein (lighter blue) along posterior calf, behind lateral malleolus, draining to popliteal vein at variable level. Panel D: Valve mechanism inset showing competent versus incompetent valves, and common DVT locations marked with warning indicators.</image>

IX. Lymphatic Drainage

Lymphatic vessels in the lower limb follow two parallel pathways. Superficial lymphatics accompany the superficial veins and drain to the superficial inguinal lymph nodes. Deep lymphatics follow the deep vessels and drain first to popliteal nodes before continuing to deep inguinal nodes.

The popliteal nodes lie within the popliteal fossa and receive drainage from deep leg structures and the lateral aspect of the foot.

The inguinal lymph nodes divide into superficial and deep groups. The superficial inguinal nodes include a horizontal group arranged below the inguinal ligament that receives lymph from the lower abdominal wall, perineum, and external genitalia. A vertical group aligned along the terminal great saphenous vein drains the superficial structures of the entire lower limb. The deep inguinal nodes lie medial to the femoral vein and receive lymph from deep lower limb structures as well as efferents from the superficial nodes.

Efferent vessels from the inguinal nodes drain to the external iliac nodes, then to the common iliac nodes, and finally to the lumbar nodes that form the lumbar lymph trunk draining into the cisterna chyli.

<image>Panel A: Superficial lymphatics (green channels) following great saphenous vein to superficial inguinal nodes with horizontal and vertical groups labeled. Panel B: Deep lymphatics following deep vessels through popliteal nodes in popliteal fossa to deep inguinal nodes medial to femoral vein. Panel C: Efferent pathway through external iliac, common iliac, and lumbar nodes to cisterna chyli. Panel D: Drainage territory mapping on limb surface showing superficial versus deep lymphatic distribution patterns.</image>

X. Dermatomes of the Lower Limb

Understanding the dermatomal distribution enables localization of spinal nerve root pathology. The L1 dermatome covers the inguinal region. L2 supplies the anterior upper thigh, and L3 extends from the anterior thigh to the knee. L4 covers the medial leg including the great toe. L5 supplies the lateral leg, dorsum of the foot, and middle toes. S1 covers the lateral foot, little toe, and posterior calf. S2 supplies the posterior thigh, while S3 through S5 innervate the perianal region.

These dermatomal patterns help clinicians identify the level of nerve root compression in conditions such as lumbar disc herniation.

<image>Panel A: Anterior view of lower limb dermatomes with L1 (inguinal, light purple), L2 (anterior upper thigh, blue), L3 (anterior thigh to knee, green), and L4 (medial leg including great toe, yellow). Panel B: Lateral view showing L5 (lateral leg, dorsum of foot, orange) and S1 (lateral foot, little toe, posterior calf, red) distributions. Panel C: Posterior view with S1 (posterior calf), S2 (posterior thigh, pink), and vertebral level correlation diagram alongside. Panel D: Summary dermatome chart with clinical landmarks for disc herniation localization (L4-L5 and L5-S1 levels emphasized).</image>

XI. Clinical Correlations

Femoral nerve injury occurs from pelvic fractures, hip surgery, or compression from hematoma in the iliacus muscle. Patients present with weakness of hip flexion and complete absence of knee extension. The patellar reflex is absent, and sensory loss affects the anterior thigh and medial leg along the saphenous nerve distribution.

Sciatic nerve injury results from posterior hip dislocation or improperly placed intramuscular injections in the gluteal region. Patients demonstrate weakness in all muscles below the knee plus the hamstrings. Foot drop accompanies sensory loss throughout the leg and foot below the knee.

Common peroneal nerve palsy represents the most frequently occurring mononeuropathy in the lower limb. Causes include fibular neck fractures, excessively tight casts, habitual leg crossing compressing the nerve against the fibular neck, and prolonged squatting. The characteristic presentation includes foot drop with inability to dorsiflex or evert the foot. Patients compensate with a high-stepping gait.

Meralgia paresthetica results from entrapment of the lateral femoral cutaneous nerve where it passes beneath the inguinal ligament near the anterior superior iliac spine. Patients experience numbness and burning dysesthesia over the lateral thigh. Common causes include obesity, tight belts or clothing, and pregnancy.

Tarsal tunnel syndrome involves compression of the tibial nerve behind the medial malleolus within the tarsal tunnel. Patients report plantar foot pain and paresthesias. Causes include fractures, ganglion cysts, and tenosynovitis of adjacent tendons.

Peripheral vascular disease results from atherosclerosis affecting the lower limb arteries. Claudication describes the characteristic pain that develops with walking and resolves with rest, resulting from inadequate blood flow to exercising muscles. Critical limb ischemia represents severe disease manifesting as rest pain, non-healing ulcers, and gangrene. Clinical examination reveals absent pulses at the femoral, popliteal, dorsalis pedis, and posterior tibial sites.

<image>Panel A: Foot drop posture from peroneal palsy with steppage gait illustration and fibular neck vulnerability site highlighted. Panel B: Meralgia paresthetica sensory loss area on lateral thigh with inguinal ligament entrapment site near ASIS. Panel C: Tarsal tunnel syndrome location behind medial malleolus with compressed nerve illustration and plantar symptoms. Panel D: Peripheral vascular disease diagram showing atherosclerotic narrowing with pulse examination points marked on limb outline and claudication zones indicated.</image>

Summary

• The lumbar plexus forms from L1 through L4 and gives rise to the femoral nerve supplying the anterior thigh and the obturator nerve supplying the medial thigh adductors
• The sacral plexus forms from L4 through S3 and produces the sciatic nerve, the largest nerve in the body, which divides into the tibial and common peroneal nerves
• The common peroneal nerve wraps around the fibular neck where it is vulnerable to injury; damage causes foot drop
• Arterial supply flows from the external iliac to the femoral artery, then to the popliteal artery which divides into the anterior and posterior tibial arteries
• The great saphenous vein courses along the medial limb and drains to the femoral vein; the small saphenous vein runs posterior to drain to the popliteal vein
• Superficial inguinal lymph nodes receive superficial drainage while deep inguinal nodes receive drainage from deep structures and popliteal nodes

Key Terms

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