In human anatomy, the sciatic nerve is formed by the union of which two major nerves in the lower limb?

Introduction / Context:
The sciatic nerve is the largest nerve in the human body and plays a crucial role in motor and sensory innervation of the lower limb. It is commonly mentioned in clinical conditions such as sciatica, where pain radiates along its course. Understanding which nerves contribute to the sciatic nerve helps in learning both anatomy and the distribution of symptoms in nerve injuries.

Given Data / Assumptions:

• The question asks which two nerves combine to form the sciatic nerve.
• Options include combinations of common fibular, tibial, pudendal, and posterior femoral cutaneous nerves.
• We assume knowledge of the lumbosacral plexus and major branches to the lower limb.

Concept / Approach:
The sciatic nerve originates from the lumbosacral plexus, primarily from spinal segments L4 to S3. Within the sciatic nerve trunk, two major components run together: the tibial part and the common fibular (also called common peroneal) part. Although they travel together in a common sheath as the sciatic nerve through the posterior thigh, they usually split near the popliteal region into the separate tibial and common fibular nerves. The pudendal and posterior femoral cutaneous nerves are separate branches that serve the perineum and posterior thigh skin, respectively, and are not the two components forming the sciatic nerve trunk.

Step-by-Step Solution:
Step 1: Recall that the sciatic nerve is made up of two main components travelling together: a tibial component and a common fibular component. Step 2: Recognise that the tibial nerve is responsible for most of the posterior compartment of the leg and intrinsic foot muscles. Step 3: Recognise that the common fibular nerve innervates the lateral and anterior compartments of the leg and contributes to foot dorsiflexion and eversion. Step 4: Match this knowledge with the option that lists the common fibular and tibial nerves as the two forming the sciatic nerve.

Verification / Alternative check:
Anatomy atlases and nerve diagrams clearly label the sciatic nerve as containing two distinct parts: a medial tibial part and a lateral common fibular part. Surgical and clinical descriptions of sciatic nerve injury also mention that damage can affect both of these components. In contrast, the pudendal nerve is charted as a separate branch from the sacral plexus that loops around the ischial spine to innervate perineal structures, not forming part of the sciatic trunk.

Why Other Options Are Wrong:

• Pudendal and posterior femoral cutaneous: Both are separate nerves from the sacral plexus and do not unite to form the sciatic nerve.
• Posterior femoral cutaneous and tibial: The posterior femoral cutaneous nerve supplies skin on the back of the thigh and is not fused with the tibial nerve as part of the sciatic trunk.
• Pudendal and common fibular: The pudendal nerve does not fuse with the common fibular nerve; instead, it supplies the perineum and external genitalia.

Common Pitfalls:
Learners may confuse different sacral plexus branches because they arise from similar spinal segments. A useful strategy is to associate the sciatic nerve specifically with the two large mixed nerves that eventually innervate most of the leg and foot muscles: the tibial and common fibular nerves. Remembering that pudendal deals with pelvic and perineal structures and that posterior femoral cutaneous is purely sensory to thigh skin helps avoid mixing them up with the sciatic nerve components.

Final Answer:
The sciatic nerve is formed by the union of the common fibular (peroneal) nerve and the tibial nerve within a single nerve trunk.