The spinal cord is a tube-like structure filled with a bundle of nerves and cerebrospinal fluid, which protects and nourishes the cord. Other protectors of the spinal cord include linings called meninges and vertebral bones. The spinal cord is about an inch across at its widest point and about 18 inches long.
Illustration of spinal cord and nerves
The three types of membranes that surround the spinal cord are referred to as meninges. From the outer layer to the innermost layer, they are dura mater, arachnoid mater, and pia mater. These membranes can sometimes be damaged by disease or trauma. Arachnoiditis is a caused by an inflammation of the arachnoid lining resulting in severe stinging and burning pain. It can occur after surgery and can cause scarring of nerves.
Nerves exit the spinal column in pairs and branch out like a delicate web throughout the rest of the body. Each area of the body is controlled by specific spinal nerves. The placement is fairly logical. For example, nerves in the cervical spine (neck area) branch out into your arms, which is why sometimes a neck issue can lead to pain radiating down your arms. Nerves in the thoracic govern the middle of the body, those in lumbar spine extend into the outer legs, and the sacral nerves control the middle of legs and organ functions of the pelvis.
All nerves ultimately connect to the brain
There are basically two major types of nerves: sensory and motor. Sensory nerves send information such as touch, temperature, and pain to the brain and spinal cord. Motor nerves send signals from the brain back into the muscles, causing them to contract either voluntarily or reflexively.
The nerves of the peripheral nervous system (PNS) extend down the spinal canal and branch out in 31 pairs at openings in the vertebrae called foraminae. They are messengers to and from your brain(or central nervous system), sending pain signals and initiating movement—like, ‘Hey, take your hand off the stove, it’s hot!’ These nerves reflexively cause your spine to twist and turn when you walk to keep you in balance. And they keep you glued to your car seat as you turn a corner at high speeds!
The peripheral nervous system (PNS) is the collective of the millions of nerves throughout your torso and limbs. The PNS nerves convey messages to your central nervous system (CNS), which is the brain and spinal cord.
In case you’re wondering, cranial nerves (the ones in your head) supply the sense organs and muscles in your head.
The Back and Beyond
When the problem is in one part of your body yet pain is felt elsewhere, health professionals call it referred pain.
As we mentioned, the nerves that exit the spinal cord do so in pairs: one is a sensory nerve; the other is a motor nerve. It’s probably no surprise to learn that motor nerves drive movement and bodily function. If you damage a motor nerve, you might have a weakness in a muscle or loss of function—for example, loss of bladder control. If, however, you can’t feel the prick of a pin in your foot, you’ve lost some sensation, which indicates a problem with your sensory nerves, which govern pressure, pain, temperature, and other such sensations. This is why a doctor might gently poke you with a pin and ask about your bowel movements. If you can’t feel the pin or have had a problem with bowel movements, it’s a symptom of nerve damage.
A problem with a sensory nerve can sometimes feel like a sharp, electrical pain, which is why good athletic instructors will tell their students to stop if they ever feel this kind of pain. It’s not a good idea to persist in any activities that result in this sensation because it could cause further nerve damage.
The spinal cord ends in the lumbar spine, where the nerves extend in a bundle of strands called cauda equina, so called because the mass looks like a horse tail. The nerves here provide motor and sensory function to the legs, intestines, genitals, and bladder. Suspected compression of these nerves is considered an emergency situation and requires immediate attention.
Jason Highsmith, MD is a practicing neurosurgeon in Charleston, NC and the author of The Complete Idiot’s Guide to Back Pain.