NM Dermatomes & Myotomes
The Motor and Sensory of Nerve Roots
The first thing to get down is defining a NERVE ROOT. We have all heard of nerve roots and we toss the term around easily, but can you actually define it? Where does it come from? What does it do? What is the anatomy?
You should know all of the answers to those questions.
A nerve root is the first lower motor neuron extension from the spinal cord. We will extensively use knowledge of upper motor neurons (UMNs) and lower motor neurons (LMNs) on this test. An UMN is a nerve cell located within the brain and/or the spinal cord. A LMN is a nerve cell that is anywhere else in the body. Examples of LMNs include peripheral nerves (like the sciatic nerve or the femoral nerve), cranial nerves, and nerve roots.
Each nerve root exits through the intervertebral foramen (between two vertebrae) in the spine. A zoomed-in version of the lumbar spine with it's nerve roots is below:
Another major concept we will use in the Neuromuscular category for the test is motor and sensory. Each nerve root coming from the spine carries a motor component (known as a myotome) and a sensory component (known as a dermatome). The myotome of each nerve root innervates the motor action for a certain group of muscles. The dermatome of each nerve root innervates the sensation for a certain area of skin.
If you know your myotomes and dermatomes on the test, suddenly a myriad of questions become available to answer, because you can use this information within the context of several questions (including Musculoskeletal questions!).
We will review this information in the program. Examples of using your dermatome & myotome knowledge include spinal cord injury questions, brachial plexus questions, pediatric condition questions, and neuroanatomy questions.
The difficulty is in knowing your myotomes and dermatomes well, especially as there are multiple sources with potentially conflicting information. Dermatomes are not "perfectly" sectioned and can overlap each other (which is why we find multiple sources with multiple answers), however, we can work around this ambiguity for the test.
First, you want to try to use myotomes whenever you can. These are more "concrete" and "black and white" because a motion is a motion. If you do have to rely on knowledge of dermatomes, stick with the versions in this program first. If that doesn't work, then you can explore other options and sources. However, you'll find that usually the STRATEGY is your answer when there is conflicting information. Pay attention to the question and answers and be able to say what is wrong with the others and suddenly these slight variations are not as big of an issue.
To best learn your dermatomes and myotomes, you'll want to break them up into upper extremity and lower extremity.
The Upper Extremity
Remember we said nerve roots come from the spinal cord. The upper extremity is supplied mainly by the nerve roots from the cervical spinal cord, therefore we are dealing with our "Cs" for the upper extremity.
*Remember that dermatomes and myotomes are not set in stone. There are multiple sources and multiple "answers" for these, however we'll present a version that we believe is best to use on the test. Try to use these versions first when you're answering questions.
Let's start with the myotomes because they are more concrete. When we talk about myotomes, we're always talking about a muscle action.
Upper Extremity Myotomes
If you sit and study these myotomes for a moment, you might notice that they tend to go "down the arm". We start at the head and neck, then we move down to the shoulder, then to the elbow, and then to the wrist and hand. That can help you recall your myotomes on the test because they go in order. Definitely take into account C6 and C7 because they have two muscle actions (which is a bit abnormal).
Next, let's move onto the dermatomes. These tend to be a little more difficult because dermatomes cover an extensive area of skin that doesn't always appear logical on first look.
We listed the dermatomes in a "smaller" area so that it is easier to remember. However, you need to be pulling out pictures of your dermatomes as you're studying to enhance the meaning of the table below.
Upper Extremity Dermatomes
Again, study these dermatomes (with a picture!). Compare them to your myotomes. You should find that some of them "correlate". For instance, our C4 myotomes is "shoulder elevation". Our C4 dermatome is "superior shoulder". The upper trapezius muscles is the main shoulder elevation muscle, which happens to run across the superior shoulder. It makes sense. It fits that the C4 myotome and dermatome are relatively in the same area.
You can use that information on the test.
Even if you forget some of your myotomes and dermatomes (which you probably won't, because we'll DRILL them into your head), you can rely on your logic. If the question talks about numbness and tingling in the posterior hand, then logically the answer is probably NOT C4 dermatome. C4 is located near the shoulder. Even if you couldn't remember your C4 dermatome, you might remember your C4 myotome (shoulder shrug), which is nowhere near the posterior hand. It wouldn't make sense. So cross off that answer.
When you pull out a picture of your dermatomes, pay attention to anterior versus posterior. You'll notice that there is a single prominent dermatome on the posterior side of the upper extremity. What is it?
Takeaway Message:
You can help yourself remember the dermatome by thinking through the myotome first.
The Lower Extremity
Nerve roots supplying the lower extremity come from the lumbar and sacral segments of the spinal cord (the "Ls" and the "Ss").
*Remember that dermatomes and myotomes are not set in stone. There are multiple sources and multiple "answers" for these, however we'll present a version that we believe is best to use on the test. Try to use these versions first when you're answering questions.
As you study the myotomes and the dermatomes for the lower extremity, notice that we have a pattern, again. The myotomes start at the hip, then go to the knee, then the ankle, but then they "flip" over the bottom of the foot and go back up to the ankle, then the knee. You can sort of trace a "line" by going joint to joint, starting anteriorly and working posteriorly. (There is a video at the end of the lesson blog to help you visualize this.)
Also pay attention again to the posterior aspect. What myotome(s) is/are posterior? What dermatome(s) is/are posterior? Remember this! It can help you out!