Final answer:
The sacral plexus innervates the posterior part of the leg and forms the sciatic nerve, which extends from the hip joint through the gluteal region into the upper posterior leg.
Step-by-step explanation:
The regions of the body innervated by the sacral plexus include the posterior part of the leg. The sacral plexus is formed by the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. One of the primary nerves from the sacral plexus is the sciatic nerve, which is composed of two nerves, the tibial and fibular nerves. This nerve extends across the hip joint, through the gluteal region, and into the upper posterior leg. Conditions such as sciatica, which can cause pain, are often associated with issues affecting the sciatic nerve.