The sciatic nerve is the largest and longest nerve in the human body, originating from the lumbar and sacral plexuses (L4-S3). It is a mixed nerve, carrying both sensory and motor fibers. The sciatic nerve innervates muscles and provides sensation to the lower limb.
Location
The sciatic nerve begins in the lower back at the level of the lumbar spine and sacrum. It exits the pelvis through the greater sciatic foramen, passing beneath the piriformis muscle. It then travels down the posterior thigh, running between the ischial tuberosity and greater trochanter of the femur. At the lower part of the thigh, near the popliteal fossa, it divides into its two terminal branches: the tibial nerve and the common fibular nerve. It runs deep in the posterior thigh and is protected by the gluteal muscles.
Structure and Anatomy
The sciatic nerve is a major peripheral nerve that originates in the lower spine and travels through the pelvis, down the posterior thigh, and into the lower leg. Below is a detailed description of its anatomy, including its origin, course, branches, and relationships with surrounding structures.
Origin
The sciatic nerve arises from the lumbosacral plexus, formed by the anterior rami of spinal nerves L4 to S3. It is a convergence of nerves from both the lumbar and sacral plexuses, specifically from the sacral plexus. The roots of the sciatic nerve join together in the pelvis to form a single, large nerve.
Course in the Pelvis and Gluteal Region
Exiting the Pelvis
The sciatic nerve exits the pelvis through the greater sciatic foramen, located just below the piriformis muscle. This is a crucial anatomical landmark, as the relationship between the sciatic nerve and the piriformis muscle can vary. In most individuals, the nerve passes beneath the piriformis; however, in some cases, the nerve may pass through or above the piriformis.
Gluteal Region
Once the sciatic nerve exits the pelvis, it enters the gluteal region. It runs deep beneath the gluteus maximus muscle, remaining posterior and medial in the thigh. The sciatic nerve does not give off any branches in the gluteal region, continuing its course toward the posterior thigh.
Course in the Posterior Thigh
Proximal Thigh
As the sciatic nerve descends into the posterior compartment of the thigh, it lies deep to the hamstring muscles (biceps femoris, semitendinosus, and semimembranosus). It travels along the midline of the posterior thigh, sandwiched between the hamstring muscles and the adductor magnus. The nerve remains medial to the long head of the biceps femoris and lateral to the ischial tuberosity.
Mid-Thigh
In the mid-thigh, the sciatic nerve continues its descent, remaining posterior to the femur. It provides motor innervation to the hamstring muscles, but this occurs via terminal branches from the nerve before it divides. At this point, the sciatic nerve is still a single trunk.
Distal Thigh and Popliteal Fossa
As the sciatic nerve approaches the distal part of the thigh, just before entering the popliteal fossa (the posterior aspect of the knee), it divides into two terminal branches:
- Tibial Nerve: This branch continues to descend along the posterior leg, innervating structures in the leg and foot.
- Common Fibular (Peroneal) Nerve: This branch travels laterally and descends around the neck of the fibula, innervating muscles in the lateral and anterior compartments of the leg.
Anatomical Relationships
Muscles
- Piriformis Muscle: The sciatic nerve typically exits the pelvis below the piriformis muscle, although variations exist where the nerve passes through or above the piriformis.
- Hamstrings (Biceps Femoris, Semitendinosus, and Semimembranosus): The nerve runs deep to these muscles in the posterior thigh, supplying them with motor innervation.
- Adductor Magnus: The sciatic nerve lies just lateral to this muscle as it travels through the thigh.
Bone
- Ischial Tuberosity: The nerve passes close to the ischial tuberosity as it descends into the posterior thigh. The ischial tuberosity is a key bony landmark located on the pelvis, and it is the point of origin for the hamstring muscles.
- Greater Trochanter: As it courses through the gluteal region, the sciatic nerve lies medial to the greater trochanter of the femur.
Blood Vessels
- Inferior Gluteal Artery: The sciatic nerve travels near the inferior gluteal artery and vein in the gluteal region. These vessels provide blood supply to the surrounding muscles and nerves.
- Popliteal Artery and Vein: After the sciatic nerve bifurcates, its tibial nerve branch runs in close proximity to the popliteal artery and vein in the popliteal fossa.
Variations in Anatomy
In most individuals, the sciatic nerve exits below the piriformis muscle. However, anatomical variations exist, including:
- Piriformis Syndrome: In some people, the sciatic nerve may pass through or above the piriformis muscle, leading to possible nerve compression or irritation.
- Early Bifurcation: In some cases, the sciatic nerve bifurcates early, with the tibial and common fibular nerves separating higher in the thigh or even in the gluteal region.
Branches of the Sciatic Nerve
Tibial Nerve
The tibial nerve is the larger terminal branch of the sciatic nerve. It descends down the posterior leg and enters the foot, innervating the posterior compartment muscles and plantar surface of the foot.
Common Fibular Nerve
The common fibular nerve is the smaller branch of the sciatic nerve. It wraps around the lateral side of the knee and divides into the superficial and deep fibular nerves, which innervate the lateral and anterior compartments of the leg.
Terminal Course and Bifurcation
The sciatic nerve terminates in the distal thigh, just above the knee joint, by bifurcating into the tibial nerve and the common fibular nerve. This bifurcation typically occurs at the level of the popliteal fossa, but in some individuals, it may occur higher in the thigh.
Function
The sciatic nerve, the largest nerve in the body, carries both motor and sensory fibers, playing a vital role in lower limb movement and sensation. The nerve’s functions are primarily executed by its two terminal branches—the tibial nerve and the common fibular nerve—but the sciatic nerve itself contributes to key functions in the posterior thigh before its division.
Motor Functions
The sciatic nerve supplies the motor innervation to the muscles in the posterior compartment of the thigh before bifurcating. After its division, the motor functions extend to the lower leg and foot through its branches.
Motor Innervation in the Posterior Thigh
The sciatic nerve provides motor innervation to the hamstring muscles in the posterior thigh. These muscles are essential for actions such as knee flexion and hip extension.
- Biceps Femoris (Long Head): The sciatic nerve supplies the long head of the biceps femoris muscle, which contributes to both knee flexion and hip extension.
- Semitendinosus: This hamstring muscle helps in flexing the knee and extending the hip, playing an important role in walking and running.
- Semimembranosus: Like the other hamstrings, this muscle assists in knee flexion and hip extension.
- Adductor Magnus (Hamstring Portion): The sciatic nerve innervates the hamstring part of the adductor magnus, which assists in thigh extension.
Motor Functions of the Tibial Nerve (Branch of the Sciatic Nerve)
After the sciatic nerve divides, the tibial nerve innervates muscles in the posterior compartment of the leg and intrinsic muscles in the foot. The tibial nerve is responsible for:
- Plantarflexion of the foot (pointing the toes downward).
- Flexion of the toes.
- Inversion of the foot.
Some specific muscles innervated by the tibial nerve include:
- Gastrocnemius and Soleus: Responsible for plantarflexion.
- Flexor Hallucis Longus and Flexor Digitorum Longus: Responsible for flexing the toes.
Motor Functions of the Common Fibular Nerve (Branch of the Sciatic Nerve)
The common fibular nerve innervates muscles in the anterior and lateral compartments of the leg, contributing to:
- Dorsiflexion of the foot (lifting the foot upward).
- Eversion of the foot (turning the sole outward).
- Extension of the toes.
Specific muscles innervated by the common fibular nerve include:
- Tibialis Anterior: Responsible for dorsiflexion.
- Fibularis Longus and Fibularis Brevis: Responsible for foot eversion.
Sensory Functions
The sensory functions of the sciatic nerve are primarily carried out by its terminal branches—the tibial nerve and common fibular nerve—which provide sensation to different regions of the leg and foot.
Sensory Functions in the Posterior Thigh
Before it bifurcates, the sciatic nerve itself does not have significant sensory branches. However, after division, its branches provide sensory innervation to large areas of the leg and foot.
Sensory Functions of the Tibial Nerve
The tibial nerve provides sensory innervation to:
- The posterior aspect of the leg.
- The plantar surface of the foot (the sole).
- The heel.
The sensory functions of the tibial nerve include detecting touch, temperature, pain, and pressure in these areas.
Sensory Functions of the Common Fibular Nerve
The common fibular nerve provides sensory innervation to:
- The lateral aspect of the leg.
- The dorsum (top) of the foot, excluding the web space between the first and second toes (which is innervated by the deep fibular nerve).
The nerve transmits sensory information such as light touch, pain, and temperature from these regions.
Reflex Functions
The sciatic nerve contributes to important reflexes that help maintain balance and coordination of movement. These include:
- Knee Flexor Reflex: The sciatic nerve innervates the hamstring muscles responsible for knee flexion, which is part of the withdrawal reflex that helps protect the body from harmful stimuli.
- Ankle Reflex: The tibial nerve (branch of the sciatic nerve) controls the Achilles tendon reflex, a key indicator of the integrity of the sciatic nerve.
Clinical Significance
The sciatic nerve is clinically significant due to its large size and critical role in motor and sensory function in the lower limb. Injury to this nerve can lead to significant impairments.
Sciatica
Sciatica refers to pain along the sciatic nerve pathway, often caused by compression of the nerve roots in the lumbar spine, such as from a herniated disc, spinal stenosis, or degenerative disc disease. Symptoms include sharp pain, numbness, tingling, or weakness radiating from the lower back to the posterior leg.
Nerve Injury
Injury to the sciatic nerve, often from trauma, surgery (e.g., hip replacements), or injection mishaps, can lead to motor and sensory deficits. Patients may experience weakness in leg movements, especially knee flexion and foot dorsiflexion, as well as sensory loss in the posterior thigh, leg, and foot.
Piriformis Syndrome
In some individuals, the sciatic nerve passes through or above the piriformis muscle, causing irritation or compression, leading to piriformis syndrome. Symptoms mimic sciatica and include pain in the buttocks and radiating pain down the leg.
Foot Drop
Injury to the common fibular nerve branch of the sciatic nerve can lead to foot drop, a condition where the individual cannot dorsiflex the foot, leading to difficulty walking and an abnormal gait.
