Nerve · Pelvis

Lumbosacral trunk

Truncus lumbosacralis

Not a nerve to anything, but the bridge between two plexuses. The lower part of the fourth lumbar nerve joins the whole of the fifth to form a thick cord that descends over the wing of the sacrum, crosses the pelvic brim and sinks into the pelvis to join the first three sacral nerves in the sacral plexus. Without it the lumbar and sacral plexuses would be separate networks; with it, the nerve supply of the whole lower limb is one continuous sheet, which is why fibres from a lumbar segment can end up in the sciatic nerve. It lies directly on bone at the pelvic brim, with the obturator nerve crossing in front of it.