Sural nerve

The sural nerve is a sensory nerve located in the lower leg. It arises from branches of the tibial nerve and common peroneal nerve. The sural nerve travels down the posterior aspect of the leg, running along the calf, and passes behind the lateral malleolus (the outer ankle bone). It continues along the lateral side of the foot, providing sensory innervation to the skin of the lower leg, lateral foot, and outer toes. The sural nerve is superficial in its course, making it easily accessible for nerve biopsies or grafting procedures.

Structure and Anatomy

The sural nerve is a superficial sensory nerve located in the lower leg and foot. It provides sensory innervation to the skin of the posterolateral lower leg and lateral foot. Below is a detailed description of its anatomy, including its origin, course, and relationship with surrounding structures.

Origin

The sural nerve typically arises from the union of branches from the tibial nerve and the common peroneal nerve:

• Medial Sural Cutaneous Nerve: This branch originates from the tibial nerve in the popliteal fossa, the shallow depression located at the back of the knee.
• Lateral Sural Cutaneous Nerve (Peroneal Communicating Branch): This branch arises from the common peroneal nerve, which is a branch of the sciatic nerve, also in the popliteal fossa.

These two branches merge, often in the mid to lower part of the leg, forming the sural nerve. However, there are anatomical variations in how these branches combine.

Course and Pathway

• Proximal Course (Posterior Leg): After its formation, the sural nerve descends along the posterior aspect of the leg. It courses superficially between the gastrocnemius muscle bellies, remaining subcutaneous as it travels down the calf. This superficial location makes it easily palpable and accessible, particularly in the mid-lower leg.
• Around the Lateral Malleolus: As the nerve approaches the ankle, it runs posterior to the lateral malleolus (the bony prominence on the outer side of the ankle). This is an important anatomical landmark, as the nerve can be easily injured in this region during ankle trauma or surgery.
• Lateral Foot and Toes: After passing behind the lateral malleolus, the sural nerve continues along the lateral aspect of the foot, running just under the skin. It terminates by providing sensory innervation to the lateral border of the foot and the fifth toe.

Branches

• Medial Sural Cutaneous Nerve (from the Tibial Nerve): This branch arises from the tibial nerve and typically contributes to the formation of the sural nerve. In some cases, it continues as a separate nerve to innervate parts of the posterior leg.
• Peroneal Communicating Branch (from the Common Peroneal Nerve): This branch comes from the common peroneal nerve and joins the medial sural cutaneous nerve to form the sural nerve. It may vary in size and course.
• Lateral Dorsal Cutaneous Nerve: As the sural nerve descends along the lateral side of the foot, it becomes the lateral dorsal cutaneous nerve, which supplies the skin over the lateral side of the foot and the lateral toes.

Relationship to Neighboring Structures

• Gastrocnemius Muscle: In its proximal course, the sural nerve runs superficial to the gastrocnemius muscle, the large calf muscle. It lies between the two heads of the gastrocnemius muscle as it travels down the posterior leg.
• Small Saphenous Vein: The sural nerve often runs close to or alongside the small saphenous vein, a superficial vein in the leg that also courses posteriorly along the leg. This relationship is important during surgeries or procedures involving the posterior leg, as both structures are at risk of injury.
• Lateral Malleolus: The sural nerve passes directly posterior to the lateral malleolus, where it becomes superficial again. This makes the nerve vulnerable to trauma or compression in the ankle region.
• Peroneal Tendons: Near the ankle, the sural nerve lies close to the peroneal tendons, which wrap around the lateral malleolus. These tendons, including the peroneus longus and peroneus brevis, are responsible for eversion of the foot. Awareness of this relationship is crucial during surgical procedures around the ankle.

Variations

The course and structure of the sural nerve can vary between individuals. Some common variations include:

• Absence of the Peroneal Communicating Branch: In some individuals, the peroneal communicating branch is absent, and the sural nerve is formed entirely from the medial sural cutaneous nerve.
• Accessory Sural Nerve: In some cases, an accessory sural nerve arises from the lateral sural cutaneous nerve or other sources, providing additional innervation to the lower leg.
• Variable Merging Point: The point at which the medial sural cutaneous and peroneal communicating branches merge can vary. This may occur higher or lower in the leg, depending on the individual.

Blood Supply

The blood supply to the sural nerve comes from small branches of the posterior tibial artery and the peroneal artery. These vessels provide blood flow to the surrounding soft tissue, including the nerve itself. The small saphenous vein, which runs adjacent to the nerve, assists in venous drainage from the area.

Function

The sural nerve is primarily a sensory nerve that provides sensation to the posterolateral leg, lateral foot, and lateral toes. Its role is to transmit sensory information such as touch, pain, temperature, and pressure from these areas to the central nervous system. Although it does not have motor functions, its sensory role is crucial for balance, protective reflexes, and postural adjustments. Below is a detailed description of the sural nerve’s function.

Sensory Function

The sural nerve is exclusively sensory, meaning it carries information from the skin and tissues back to the spinal cord and brain, helping the body perceive and respond to external stimuli. This sensory feedback is essential for activities such as walking, running, and maintaining balance.

• Posterolateral Leg Sensation: The sural nerve supplies sensory innervation to the posterolateral aspect of the lower leg. This includes the skin over the calf and down to the ankle. The sural nerve transmits sensory signals related to touch, pressure, temperature, and pain from this region, enabling the body to respond to stimuli that could affect gait or posture.
• Lateral Foot Sensation: One of the primary roles of the sural nerve is to provide sensation to the lateral aspect of the foot, including the outer edge and side of the sole. This area is important for balance and weight distribution, particularly during activities like walking or running, as the nerve helps detect changes in pressure and terrain.
• Lateral Toes Sensation: The sural nerve continues into the lateral toes, supplying sensory fibers to the fifth toe (little toe) and sometimes part of the fourth toe. The nerve allows for the perception of pressure, temperature, and touch in this region, which is essential for fine control of foot positioning and balance.

Role in Gait and Balance

Though the sural nerve is purely sensory, it plays a critical role in maintaining balance and stability during walking and standing. The sensory feedback provided by the sural nerve allows the body to adjust to changes in terrain, foot pressure, and body positioning.

• Pressure and Weight Distribution: The sural nerve helps detect changes in pressure along the lateral side of the foot and leg, which is particularly important when walking or running on uneven surfaces. The information transmitted by the nerve allows the body to make postural adjustments to maintain balance.
• Postural Adjustments: Sensory input from the sural nerve is critical for making quick adjustments in response to external stimuli, such as changes in ground texture or slope. The nerve detects alterations in pressure and tactile sensation, allowing the nervous system to rapidly coordinate muscle responses that maintain proper foot positioning.
• Protective Reflexes: Sensory feedback from the sural nerve helps the body detect potentially harmful stimuli, such as sharp objects or extreme temperatures, allowing for protective reflexes like withdrawing the foot or shifting weight. These reflexes help prevent injuries to the foot and leg.

Sensory Feedback for Terrain Adaptation

The sural nerve plays an essential role in helping the body adapt to various terrains during locomotion. For example, when walking on uneven or sloped surfaces, the sural nerve provides sensory feedback that allows the brain to process changes in pressure along the lateral side of the foot and adjust muscle tension to maintain stability.

Proprioceptive Feedback: The sural nerve provides proprioceptive information (sense of position) from the lateral part of the leg and foot. This allows the central nervous system to keep track of foot placement and body alignment, ensuring smooth transitions between steps and proper coordination during complex movements like jumping, climbing, or rapid directional changes.

Interaction with Other Nerves

The sural nerve works in conjunction with other sensory nerves, such as the medial plantar nerve and lateral plantar nerve, which cover different areas of the foot. Together, these nerves ensure complete sensory coverage of the foot, allowing for more accurate feedback about foot pressure, touch, and temperature across the entire plantar surface.

• Complementary Function with the Medial Plantar Nerve: The medial plantar nerve provides sensation to the medial portion of the foot, while the sural nerve covers the lateral side. These two nerves collaborate to ensure that the entire sole of the foot is sensitive to external stimuli, enabling the body to make coordinated movements and adjustments.
• Collaboration with the Lateral Plantar Nerve: The sural nerve complements the lateral plantar nerve, which innervates deeper parts of the foot. Together, they help maintain sensitivity and balance during complex activities that involve weight shifting or lateral movements of the foot.

Clinical Significance

The sural nerve is clinically significant due to its superficial location and role in providing sensory innervation to the posterolateral leg and lateral foot. It is commonly involved in nerve injuries, particularly during ankle sprains, fractures, or surgeries, as it runs close to the surface near the lateral malleolus. Damage to the sural nerve can lead to numbness, tingling, burning sensations, or loss of sensation in the areas it supplies, affecting balance and gait.

The sural nerve is often used in nerve grafting procedures because of its accessible location and sensory-only function, making it an ideal donor nerve. Additionally, it can be involved in sural nerve entrapment, which may cause persistent pain or discomfort along its distribution, especially in athletes. Testing the sural nerve is also useful in diagnosing neuropathies, including peripheral neuropathy in diabetes.