Spinal Nerve Roots and Dermatomes

Part 1- Nerve Pain

Nerve cells.
Nerve cells. PASIEKA/Science Photo Library/Getty Images

Common spinal conditions such as herniated disc, stenosis, as well as piriformis syndrome (which is not a back condition per se, but often yields symptoms that mimic them) can lead to pain that goes down one leg (sciatica) or one arm.

One key underlying feature of most of these conditions is an irritated or compressed spinal nerve root.

Related: Spinal Nerve Root Definition

Learning to distinguish between nerve and other types of pain (such as the kind related to inflammation) can help you recognize serious problems that require medical attention.

Generally, when you have nerve type symptoms down one arm or leg, it's time to see the doctor about your neck or back.

Such symptoms include (as I mentioned above) pain that goes down one leg or arm.  They can also include electrical sensations like pins and needles, shock and/or burning, as well as motor symptoms such as weakness and/or numbness.

And, by the way, if you get "saddle amnesia" or you have problems with your bladder or bowels, this may indicate cauda equina syndrome which may constitute a medical emergency.  


Dermatome map
Dermatome map. BSIP/UIGM/Universal Images Group/Getty Images

So how does a problem located in your spine (i.e., your spinal nerve root) make for pain, weakness, numbness and/or electrical sensations down an arm or leg?

The answer is dermatomes or zones of skin whose sensory input are "assigned," so to speak to individual spinal nerves.  Often called nerve roots, dermatomes are a bit like highways through which information about felt sensations in your skin related to your central nervous system.

Within a dermatome, things you physically feel such as touch, pressure, heat, and cold are transmitted through dermatomes to the central nervous system via a single nerve root.  When a spinal nerve root becomes compressed or irritated - usually because it comes into contact with another spinal structure - symptoms of radiculopathy often result.

Vertebrae, Spinal Cord Levels and Dermatomes

Spinal column, spinal canal and spinal nerve roots.
Spinal column, spinal canal and spinal nerve roots. MedicalRF.com/Getty Images

Each vertebra represents a level of the spine, and therefore of the spinal cord. At each level, some of the spinal cord branches away from the center to become individual nerves. The individual nerves branch out further and further. The nerves collectively form a network all over the human body.

The nerve root is the place where the spinal cord first branches out. There is nerve root to either side of the spinal cord at each vertebral level.

As we discussed, the area of skin that each nerve serves (or supplies) is called a dermatome. Each nerve root has an associated dermatome, and each dermatome is supplied by many branches of that nerve root.

When doctors look for radiculopathy in their examination, they test (among other things) the dermatomes for sensation and the myotomes for any movement impairment. Using specific manual tests, she determines the spinal level from which the symptoms arise. Manual exams are quite often accompanied by diagnostic imaging tests such as MRI.

Note: While imaging such as MRI can show abnormalities of the spinal nerve root, a complete physical examination by a doctor is often needed to determine if that spinal nerve root is the source of your symptoms.


Spinal reflex arc shows paths of sensory and motor nerves.
Motor nerves come from the spinal cord and go to a muscle to produce movement. roxanabalint

While we're on the subject, let's discuss myotomes. Myotomes are very similar to dermatomes except that instead of describing zones of skin sensation, they describe groups of muscles that receive motor impulses delivered through a single (or shared) nerve root.

FYI, Motor impulses have to do with your muscles; a motor impulse is the relay signal that causes movement (often a reaction to the sensations you feel.)

Related: What is a Myotome? 

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