A nerve plexus is a complicated intertwining of many nerve fibers. Two nervous projections from the spinal cord, called the dorsal root (dorsal roughly means back) and the ventral root (ventral roughly means front) combine to form a single spinal nerve. This spinal nerve then divides into two separate branches, the dorsal branch (also called the dorsal ramus) and the ventral branch (also called the ventral ramus). A number of the ventral rami (rami is plural of ramus) combine together, exchanging nerve fibers, and creating a complex intertangled web of nerve fibers called a nerve plexus. The major nerve plexuses are the cervical plexus, the brachial plexus, and the lumbosacral plexus.

The cervical plexus provides nerves which serve the muscles and skin of the neck, upper shoulders, chest, and part of the head. The nerve which operates the diaphragm also emerges from the cervical plexus. The brachial plexus allows the lower shoulder and the entire arm to function. The lumbosacral plexus sends nerves to the skin and the muscles of the thigh and leg, including the body's largest nerve, the sciatic nerve.