A column of bones called vertebrae make up the spine (spinal column). The vertebrae protect the spinal cord (a long, fragile structure contained in the spinal canal), which runs through the center of the spine. Between the vertebrae are disks composed of cartilage, which help cushion the spine and give it some flexibility.
Spinal nerves: Emerging from the spinal cord between the vertebrae are 31 pairs of spinal nerves. Each nerve emerges in two short branches (roots)—motor and sensory—which join to form a spinal nerve.
The motor roots carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles.
The sensory roots carry information to the brain from other parts of the body.
Cauda equina: The spinal cord ends about three fourths of the way down the spine, but a bundle of nerves extends beyond the cord. This bundle is called the cauda equina because it resembles a horse's tail. The cauda equina carries nerve impulses to and from the legs, lower intestine, and bladder.