Final answer:
The spinal nerves between L4 and S4 form the sacral plexus, which gives rise to the sciatic nerve that innervates the pelvic area and posterior lower limbs.
Step-by-step explanation:
The spinal nerves exiting the cord from the levels of L4 to S4 form the sacral plexus. This nerve plexus is responsible for innervating the pelvic region and the posterior portion of the lower limbs. One of the major systemic nerves arising from this plexus is the sciatic nerve, which combines the tibial nerve and the fibular nerve, and is known for its association with the condition called sciatica.
The sacral plexus arises from the lumbosacral enlargement of the spinal cord, and the nerves that contribute to this plexus include lower lumbar and sacral spinal nerves. The exit points for these nerves are through the intervertebral foramina, and these nerves then reorganize into plexuses. The sciatic nerve, a significant branch of the sacral plexus, extends across the hip joint and down towards the lower leg, becoming the tibial and fibular nerves which respectively supply the posterior and anterior parts of the lower leg.