 You're very likely to get questions on step one that require you to know which areas of the body are supplied by which dermatomes. As we mentioned in one of our other videos, many of the dermatomes are oddly shaped and sized so it would be quite difficult to commit all of them to memory. Rather than memorize all 30 dermatomes, it's better to learn just a handful from head to toe. If you get a question that's asking about a dermatome of an area in the body that you're not familiar with, you can infer what it might be from the handful we're about to discuss. Let's run through some of the major landmarks of the body and what dermatomes they correspond to. From head to toe, we'll start with the back of the head, which is the dermatome of spinal nerve C2. You can see that it's responsible for sensation to almost the entire back of the head. Then we move to the neck. You can think of the majority of the neck, both anterior and posterior, as being part of the C3 dermatome. Moving to the arm, we have the C6 dermatome, which is the thumb. You can remember this by remembering the mnemonic 6-shooter, because when you make a finger gun like this, your thumb points up in the air. So the thumb is the C6 dermatome. You'll notice the middle finger is C7 and the pinky is C8. And you'll notice the dermatomes work their way back up the arm, representing subsequent descending spinal nerves. And if you're familiar with the brachial plexus, you'll understand why we see that pattern. If you'd like to see us do a video on the brachial plexus, be sure to leave a comment below and let us know. You can see C7 and C8 on the diagram here, and then T1 is on the forearm. Moving on, the T4 dermatome corresponds to the nipple, as you can see here. Further down, we have T10, thoracic spinal nerve 10, whose dermatome corresponds to the belly button. You can remember this one by remembering belly but 10, where the second half of the word button is the number 10, representing T10. Moving on to the lumbar dermatomes, we have L1, which corresponds to the inguinal ligaments. If you're not familiar with the inguinal ligaments, that's the ligament that runs between the anterior superior iliac spine and the pubic tubercle. It's quite thick, and you can usually feel it on yourself or on a patient. It kind of divides the torso from the legs. And that's the ligament that runs right under the L1 dermatome. Down the little more, we have L4, which is around the area that needs the lower portion of the leg. S3 through S5 encompass the sensation to the genitals and the anus. I'd say it's less important to know exactly which areas of skin correspond to S3 versus S4 versus S5, but more important to know that this general area of the body is covered by the sacral dermatomes. So when you get a question about someone with abnormal sensation in this area, you know there's a problem with the sacral spinal nerves. For example, cauda equina syndrome, which is caused by compression of the cauda equina by a tumor, a bulging disc, a hematoma, or something similar. This can cause a symptom called saddle anesthesia, which is the inability to feel sensation in this area of the body. That's because the sacral nerve roots are some of the most affected in cauda equina syndrome. Here in the sagittal MRI, you can see an example of the cauda equina being compressed by an abscess. Moving down towards the foot, we can see that the dorsal surface of the foot is part of the L5 and S1 dermatome, while the plantar surface of the foot is part of the S1 and S2 dermatome on the lateral side and the L4 and the L5 dermatome on the medial side. And with that, we've covered all the dermatomes. But I think we're forgetting something. What about the face? The face, as you might recall, is not innervated by any spinal nerve. It's innervated by various branches of cranial nerve 5 or the trigeminal nerve. These branches are also known as V1, the ophthalmic division, V2, the maxillary division, and V3, the mandibular division. Cranial nerve 5 comes directly off the brainstem and is therefore not a spinal nerve. The V1 or ophthalmic division of cranial nerve 5 covers sensation to the forehead and the area around the eye. V2 or the maxillary division covers sensation to the middle of the face. And the mandibular division, V3, covers sensation to the bottom of the face and the jaw. So in this video, we give you a look at how to determine the sensory innervation to any area of skin in the body. Once again, it's not necessary to know all 30 dermatomes in detail. But if you're generally aware of the dermatomes we discussed in this video, you have a strong chance of getting these questions right when they come up on step one.