The Spine’s Self-Defense System

The spinal cord’s critical role means it needs to be defended and protected

Peer Reviewed

If you had a remote control that managed everything about your house, your car, your life, you wouldn’t just leave it out in the open to be damaged by the elements, accidents or anything else. It’s the same with your central nervous system. The most prominent protection for the brain is the skull, but for the spinal cord…well, it’s a little more complicated than that.

Spine defenseThe spinal cord is too important to be left undefended.

As the body’s nerve center, the spinal cord controls nearly all voluntary and involuntary movements throughout the torso, arms, and legs. It also receives sensory input from throughout the torso and limbs. With so much at stake, the body has developed a complex system of defenses to protect the spinal cord and its vital two-way flow of sensory information.

The spinal cord is a bundle of nerves about an inch and a quarter wide at its broadest point. Its array of defenses starts with the cerebrospinal fluid (CSF) that cushions and nourishes it.

Moving outward, the spinal cord is further protected by a tube-like structure known as the meninges. Three layered membranes make up the meninges, each with a different protective role:

  • The dura mater
  • The arachnoid mater
  • The pia mater

But the spine’s defenses don’t stop there. Other spinal defenses include the vertebrae (backbones) themselves, intervertebral discs, and various muscles and ligaments.

Meninges

The meninges could be considered the spinal cord’s first line of defense. They are the three layers of membranes that surround the spinal cord all the way from the brainstem at the top to the coccyx at the very bottom of the spine. The innermost layer is the pia mater, which attaches directly to the spinal cord. Composed mainly of collagen, the pia mater’s elasticity helps the spinal cord maintain its shape.

The arachnoid mater is the middle layer of the three meninges. Its primary role is to maintain the nearby cerebrospinal fluid (CSF) that fills the subarachnoid space between the pia mater and the arachnoid mater. The most common condition involving the arachnoid mater is arachnoiditis, a painful inflammation of the membrane that requires medical treatment. Arachnoiditis can lead to disability if it is progressive. There are few effective treatments for arachnoiditis.

Dura Mater

The outer meningeal layer, the dura mater, is the toughest. The epidural space separates this fibrous membrane from the wall of the vertebral canal. The dura mater rests on the arachnoid mater, with a small amount of lubricating fluid between them.

Although the dura mater is generally tough, tears can occur. Dura tears can result from injury, epidural injections, lumbar punctures, or complications of spinal surgery. Some people can even develop these tears spontaneously, perhaps as a result of a connective tissue disorder.

When the dura tears, CSF leaks out of the subarachnoid space, leaving the brain and spine with less cushioning and support. Postural headaches--which are worse standing than sitting or laying down and can often be severe—are the most common symptom of CSF leaks. Fortunately, these tears usually heal quickly with bedrest.

Cerebrospinal Fluid (CSF)

CSF cerebrospinal fluidCerebrospinal fluid cushions and nourishes the spinal cord.

Cerebrospinal fluid (CSF) is a clear liquid that fills the space between the pia mater and the arachnoid mater. Produced in the brain’s ventricles, CSF’s main function is to protect and nourish the spinal cord and brain. CSF also removes waste products from the brain.

Unlike the spine’s other defenses, CSF also plays a role in helping physicians diagnose disease. If your doctor suspects you have a serious infection or disorder of the central nervous system, he or she will perform a lumbar puncture, or spinal tap. The CSF can contain evidence of inflammation or infection due to waste products that the central nervous system sheds into the CSF.

In this procedure, your doctor will insert a needle between two vertebrae in your lower back into the subarachnoid space to remove a small amount of CSF. That fluid will then be sent to the lab and analyzed for disease. Lumbar puncture can be used to diagnose a variety of conditions. These include:

  • Infectious diseases of the brain and spinal cord, including meningitis and encephalitis
  • Widespread nervous system disorders, such as multiple sclerosis (MS) and Guillain-Barré syndrome
  • Bleeding in or around the brain
  • Brain tumors

If your doctor needs to make diagnostic images of how well CSF is moving through the subarachnoid space, he or she will inject dye (myelography) or radioactive substances (cisternography) into your subarachnoid space before imaging.

Lumbar puncture can also be used to measure the pressure of your cerebrospinal fluid. Your doctor can also inject spinal anesthetics or chemotherapy drugs using the lumbar puncture technique.

Vertebrae

The 33 bones that make up the spine form a rigid framework that shapes the body and protects the spinal cord. The rest of the body organizes itself around the spine and spinal cord.

In the profile view of the normal spine, the cervical vertebrae curve slightly inward (this curve is called lordosis), while the mid-range thoracic spine curves outward (kyphosis) before meeting the inwardly curved (lordosis again) lumbar spine. This elegant structure means the spinal cord is both flexible and balanced.

Each vertebra has a complex shape whose precise configuration is determined by its location in the spine. The vertebrae’s main parts are the anterior vertebral body, the posterior vertebral arch, the intervertebral foramina where the spinal nerves enter and exit, and the vertebral foramen that accommodates the spinal cord.

Due to their complexity, many common conditions of the vertebrae involve nerves that get pinched or compressed as they leave the spine. Pinched nerves often result from bone spurs on the vertebrae that crowd the spinal canal or from herniated discs.

Intervertebral Discs

Herniated disc illustrationHerniations are the most common problem affecting intervertebral discs.

The intervertebral discs are best known as the spine’s shock absorbers. These discs fill the space between two vertebrae so they don’t make direct contact with each other. The end plate of each vertebrae is coated with protective cartilage that anchors the discs in place.

The cushiony, gel-like part of each disc is called the nucleus pulposus. The tough covering, the annulus fibrosus, wraps around each disc to protect and shape it. Discs do not contain blood vessels. Instead, they are nourished by the end plates of nearby vertebrae.

Herniated discs are probably the most common disc-related ailment. When a disc herniates, the annulus fibrosus tears. That tear allows the protective gel of the nucleus pulposus to bulge into the spinal canal.

Since there’s no room in the spinal canal for anything except the spinal cord and CSF, the gel creates pressure on the surrounding nerve or even the spinal cord itself (it’s called myelopathy when the spinal cord is compressed). Fortunately, nonoperative treatments are often quite successful in healing herniated discs, no surgery required.

From the CSF to the dura mater to the spinal bones and discs, the spine’s defenses are elaborate, effective and complicated. That’s why, if something goes wrong with them, consulting an experienced spine specialist promptly can yield the best results. Find a spine specialist near you who can help.

Updated on: 04/20/20
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Spinal Anatomy Center
Vincent C. Traynelis, MD
Lali Sekhon, MD, PhD, FAANS, FRACS, FACS
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