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The lumbosacral plexus is the connected system of lumbar plexus and sacral plexus pathways formed from anterior spinal nerve rami of lower spinal nerves. It redistributes sensory and motor axons into peripheral nerves supplying the pelvis and lower limb. Its connecting lumbosacral trunk usually carries contributions from L4 and L5 into the sacral plexus.^[ref]

For example, the femoral nerve draws mainly on upper lumbar levels, while the sciatic nerve combines lower lumbar and sacral contributions. A named peripheral nerve consequently does not correspond one-to-one with a spinal nerve. The nerve plexus reroutes existing axons; its intersections do not imply that the fibres form synapses.

The sciatic nerve is a large mixed nerve connecting the lumbosacral plexus with the lower limb. It passes through the posterior hip region into the thigh and contains the components that continue as the tibial nerve and common fibular nerve. These pathways carry motor signals to skeletal muscles and sensory information from substantial regions of the leg and foot.

Its course relative to the piriformis muscle varies among individuals. A retrospective magnetic resonance imaging study compared common and variant anatomical arrangements and did not find a statistically significant difference in the recorded diagnoses involving pain.^[ref] An unusual anatomical arrangement therefore does not by itself establish that the arrangement is causing symptoms; structure and physiological consequence require separate evidence.