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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 pudendal nerve is a mixed nerve of the sacral plexus, usually receiving fibers associated with S2–S4 spinal nerves. It carries sensory information from much of the perineum and motor signals to skeletal muscles involved in continence, including the external anal sphincter and external urethral sphincter. Its route includes the pudendal canal.

A description of anatomy is a pattern with variation, not a guarantee that every individual has identical branching. Dissection of 28 cadavers documented additional branches and interconnections near the external anal sphincter in some specimens.^[ref] The study also illustrates the difference between a skeletal muscle being structurally intact and its nerve supply being intact: coordinated contraction depends on both the skeletal muscle and the arriving motor signals.

The sacral plexus is a network of anterior spinal nerve rami, usually incorporating L4–S4 spinal contributions, lying against the anterior surface of the piriformis muscle. It distributes sensory and motor fibres to much of the pelvis and lower limb. Major branches include the sciatic nerve and the pudendal nerve.^[ref]

Its mixed origins explain an otherwise puzzling arrangement: the sciatic nerve carries fibres associated with several spinal nerves, and fibres from one spinal level can leave through different peripheral branches. A nerve plexus is therefore a redistribution network, rather than a new set of neurons inserted into each pathway.