Pelvic exit and thigh topography
Originating from the roots of the sacral plexus, the sciatic nerve (n. ischiadicus) leaves the pelvic cavity through the infrapiriform foramen (foramen infrapiriforme). Immediately upon exit, it lies deep to the gluteus maximus muscle.
Descending along the posterior aspect of the thigh, the nerve trunk gives off a number of motor branches. These pass to the posterior muscle group, which includes the biceps femoris (m. biceps femoris), semitendinosus (m. semitendinosus), and semimembranosus (m. semimembranosus) muscles. In addition, the adductor magnus (m. adductor magnus) receives innervation from the sciatic nerve. Along its course in the thigh, the nerve also gives off sensory articular branches that supply the hip and knee joints.
Bifurcation of the sciatic nerve
Upon reaching the popliteal fossa (and in some anatomical variants, significantly higher up), the main trunk of the sciatic nerve divides into two major terminal branches:
- Tibial nerve (n. tibialis).
- Common fibular nerve (n. fibularis communis / peroneus).
Common fibular nerve and its branches
Common fibular nerve (segments L₄–S₂) courses laterally and winds around the neck of the fibula. Even before its main division, it gives off the lateral sural cutaneous nerve (n. cutaneus surae lateralis), which supplies the skin of the anterolateral aspect of the leg. Subsequently, the common trunk divides into two branches:
- Superficial fibular nerve (n. fibularis superficialis). It descends within the lateral compartment muscles of the leg, after which it pierces the fascia and enters the subcutaneous tissue. It is responsible for the cutaneous sensibility of the anterolateral leg, as well as most of the dorsum of the foot and the dorsal aspect of the toes (except for the first web space). Its terminal branches are the dorsal digital nerves of the foot (nn. digitales dorsales pedis).
- Deep fibular nerve (n. fibularis profundus). It descends along the anterior surface of the interosseous membrane. Its area of cutaneous innervation is extremely limited: a narrow skin strip strictly between the 1st and 2nd toes (the first web space).
Tibial nerve and innervation of the sole
Tibial nerve (n. tibialis, segments L₄–S₃) continues its course through the popliteal fossa and the posterior leg. It gives off the medial sural cutaneous nerve (n. cutaneus surae medialis), which then joins the lateral sural cutaneous nerve to form the single sural nerve (n. suralis). This nerve supplies the skin of the posterolateral leg, the lateral margin of the foot, and the 5th toe.
Passing posterior to the medial malleolus, the tibial nerve trunk enters the foot and divides into two branches:
- Medial plantar nerve (n. plantaris medialis). It supplies the skin of the medial sole, digits I to III, and the medial half of digit IV. During its branching, it gives off common (nn. digitales plantares communes) and proper (nn. digitales plantares proprii) plantar digital nerves.
- Lateral plantar nerve (n. plantaris lateralis). It provides sensation to the skin of the lateral sole, digit V, and the lateral half of digit IV, giving off proper plantar digital nerves.
Summary: cutaneous innervation of the dorsum of the foot
Cutaneous innervation of the dorsal surface of the foot is supplied by several nerves simultaneously, with clearly demarcated zones:
- Main area of the dorsal foot: innervated by the superficial fibular nerve (n. fibularis superficialis).
- First web space: narrow zone of responsibility of the deep fibular nerve (n. fibularis profundus).
- Lateral margin of the foot: innervation zone of the sural nerve (n. suralis).
- Medial margin of the foot: supplied by the saphenous nerve (n. saphenus).
Briefly: answers to topical questions
Which leg muscles are innervated by the tibial nerve?
The tibial nerve innervates the posterior compartment muscles of the leg, which act as plantar flexors of the foot and flexors of the toes. These include:
- Popliteus muscle (m. popliteus)
- Plantaris muscle
- Triceps surae muscle (includes the gastrocnemius and soleus muscles)
- Tibialis posterior muscle
- Flexor digitorum longus
- Flexor hallucis longus
Which muscles are innervated by the deep fibular nerve?
The deep fibular nerve innervates the anterior extensor muscles of the foot and toes. These include:
- Tibialis anterior muscle
- Extensor digitorum longus
- Extensor digitorum brevis
- Extensor hallucis longus (long extensor of the great toe)
- Extensor hallucis brevis (short extensor of the great toe)
Which muscles are innervated by the superficial fibular nerve?
The superficial fibular nerve innervates the fibular (peroneal) muscles of the leg, specifically:
- Fibularis longus muscle (m. peroneus longus or m. fibularis longus) — it performs plantarflexion and eversion of the foot, lowering its medial margin.
Which intrinsic foot muscles are innervated by the plantar nerves?
The plantar nerves innervate the intrinsic muscles of the foot, dividing into medial and lateral branches.
| Nerve | Innervated muscles |
|---|---|
| Medial plantar nerve (n. plantaris medialis) | Muscles of the great toe, flexor digitorum brevis, two medial lumbricals |
| Lateral plantar nerve (n. plantaris lateralis) | Muscles of the 5th toe, adductor hallucis muscle (m. adductor hallucis), quadratus plantae (m. quadratus plantae), two lateral lumbrical muscles (mm. lumbricales), all interosseous muscles (mm. interossei) |
For quick memorization of the dorsal foot innervation: "Deep hides in the first cleft" (the deep fibular nerve innervates only the 1st web space), and "Superficial covers everything else" (the main part of the dorsum of the foot).
Class and exam questions
At what level does the sciatic nerve divide into its branches?
Classically, the division into the tibial and common fibular nerves occurs in the popliteal fossa. However, bifurcation can also happen higher up along the course of the nerve on the posterior thigh.
Which nerve innervates the first interdigital cleft on the foot?
The skin in the area of the first web space (between the 1st and 2nd toes) is innervated by the terminal branches of the deep fibular nerve (n. fibularis profundus).
Which nerves form the sural nerve?
The sural nerve (n. suralis) is formed in the leg by the union of the medial sural cutaneous nerve (from the tibial nerve) and the lateral sural cutaneous nerve (from the common fibular nerve).
What else to study on this topic
- Anatomy of the infrapiriform foramen and adjacent fasciae
- Topography of the popliteal fossa
- Structure of the interosseous membrane of the leg
- Branching pattern of the common and proper plantar nerves
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