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Anatomy and Physiology of the Spine

Your spine, or spinal column, supports your body so you can stand, walk, and sit. It's also designed to move in certain ways, so you can twist and bend. It's both strong and easy to injure, especially as we age.

But the spine is more than the bones — or vertebrae — we often talk about. It's a complex structure of bones, nerves, disks, and spinal fluid that protects the delicate nerve tissue of the spinal cord.

It's helpful to understand the parts of the spine and know the terms doctors use.

What Is the Spine?

When we hear the word "spine," we usually think "backbone." Although your spine gives your back structure and flexibility, it isn't a single bone.

More than two dozen bones stacked on top of each other form the spine. The “atlas" is the bone closest to your head because it carries the weight of the skull.

Doctors group the bones of the spine according to where they're located. Top to bottom, you have 33 vertebrae:

  • 7 neck bones (C, for cervical).
  • 12 chest-level bones (T, for thoracic).
  • 5 lower back bones (L, for lumbar).
  • 5 fused bones that form the sacrum (S).
  • 4 fused bones that make up the tailbone (coccyx).

Parts of the Spine

The vertebrae

The actual name for each bony unit of the spine is vertebra. (Vertebrae is the plural form of the word.)

The body and the arch make up every vertebra. The arches align, and when stacked, form the spinal canal

There are “cushions" between each vertebra, called intervertebral disks. These disks contain cartilage, collagen fibers, and water.

Ligaments hold the vertebrae and the intervertebral disks together.

Damage to the vertebrae

Like any other bone in your body, you can fracture (or break) a vertebra. This usually happens when a force pushes on the vertebra in a fall or traumatic accident.

You can hurt the bones of your spine in other ways, too. Ligaments hold your vertebrae together. Damage to these ligaments will cause the stacked vertebrae to fall out of alignment.

Because the spine is so carefully engineered, breaks or injuries can also damage the spinal cord. Some injuries are more mild, like a bruised spinal cord. Others can be severe and life-changing, like a crushed spinal cord.

Intervertebral disks

Doctors often talk about people having a "herniated disk" or "bulging disk." Your disks are the cushions between each of your vertebrae. You have 23 intervertebral disks (IVD) throughout your spine.

Disks separate the vertebrae so they don't rub together. Disks also help to absorb shock. They are both flexible and strong, because of their gel-like center.

Like any other part of the spine, you can injure your disks. With age, disks start to lose some of their strength and flexibility. They can also slip out of place and cause problems like low back pain.

Facet joints

Just like the other bones in your body, joints connect your vertebrae. These facet joints make sure the bones of your spine work together to help you move, bend, and twist.

Like knee or shoulder joints, facet joints have cartilage. Cartilage helps prevent the bones from rubbing against each other where they meet. Facet joints also have lubricating fluid to help with this.

If someone is having issues with the joints in their back, they may get a diagnosis of facet joint syndrome. This means their joints are painful and swollen.

Spinal cord and nerves

What is the spinal cord?

Your spinal cord isn't just one nerve. Rather, it's many nerves that run from the base of the brain to the small of the back. This is a key way your brain communicates with the rest of your body.

Your spinal cord runs within the spinal canal. In this way, it's like a highway for signals to travel to and from your brain and all over your body. Though it has a big job, your spinal cord is thinner than you might think—about a half-inch thick.

What do spinal nerves do?

Spinal nerves exit from the spinal cord to send and receive signals from muscles, skin, and other organs. The nerves are able to leave the bony canal through passageways, called foramen, between the vertebrae.

Depending on which segment of the spinal cord the nerves exit from, they control different functions in your body:

  • Nerves exiting near the neck and upper back send and receive signals from the arms.
  • Nerves exiting near the chest area are in charge of internal organs.
  • Nerves exiting at the lower back control the legs and the genital area.

This is why, if someone has a spinal cord injury, it matters how high up on the spinal cord the injury happens.

Interestingly, the spinal cord and the spinal canal are about the same length before birth. But the spinal cord stops growing earlier than the spinal canal that covers it. For most people, the spinal cord stops growing around age 5.

In an adult, the spinal cord ends around the small of the back, an area known as the second lumbar vertebrae. The rest of the spinal canal only has spinal nerves, and not the spinal cord itself.

Why Choose UPMC Rehabilitation Institute for Spinal Cord Care?

As part of the largest rehabilitation network in western Pa., the UPMC Rehabilitation Institute offers specialized inpatient and transitional care for a variety of conditions, including: