Merck Manual

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How the Spine Is Organized

How the Spine Is Organized

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.

Like the brain, the spinal cord is covered by three layers of tissue (meninges).

How the Spine Is Organized
How the Spine Is Organized

Spinal nerves: Emerging from the spinal cord between the vertebrae are 31 pairs of spinal nerves. Each nerve emerges in two short branches (roots):

  • One at the front (motor or anterior root) of the spinal cord

  • One at the back (sensory or posterior root) of the spinal cord

The motor roots carry commands from the brain and spinal cord primarily to skeletal muscles to control movement.

The sensory roots carry sensory information (pain, temperature, vibration, limb position) 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, both motor and sensory, to and from the legs.