Dermatomes and Myotomes
Today we are going to be going into the dermatomes and myotomes of the human body. We will discuss what dermatomes and myotomes are, how to remember certain dermatomes and myotomes and show where they are laid out on the body!
Dermatome vs. Myotome
So first let’s talk about dermatome vs. myotome. Both myotomes and dermatomes come from somites, which are parts of the body of an embryo. These somites come in pairs which run from head to toe on the body.
A dermatome is a portion of the skin that is supplied by an individual spinal nerve. Whereas a myotome is a group of muscles that are innervated by a single spinal nerve.
To understand dermatomes, we must appreciate the body as a “map”. Each dermatome represents an area of skin that is supplied sensation by a certain nerve. We will go through each region of the body next regarding how to remember myotomes and dermatomes.
For the human head, particularly of the face, the dermatomes are innervated by the branches of the trigeminal nerve. The ophthalmic branch of the trigeminal nerve supplies the dermatome on the lateral side of the forehead. The maxillary branch supplies the cheek, and the mandibular branch supplies the lower jaw, except for the angle of the mandible.
Other areas of the head are innervated by the C2 and C3 nerve roots. The 1-2 cm area lateral to the occipital protuberance is innervated by C2. The supraclavicular fossa in the midclavicular line is innervated by the C3 nerve root.
As we progress down the spinal column, the next several levels innervate the upper extremities. These include C4, C5, C6, C7, C8, and T1.
The area overlying the acromioclavicular joint is supplied by C4. The area overlying the lateral portion of the lower edge of the deltoid is innervated by C5. This area can also be known as the area of the regimental badge.
The palmar side of the thumb is innervated by C6, the middle finger by C7, and the little finger by C8. Lastly, the medial portion of the antecubital fossa, proximal to the medial epicondyle of the humerus is innervated by T1.
Chest and Back
The next area to cover is the chest and back (torso). The dermatomes of this area are innervated by T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12.
The apex of the axilla is innervated by the T2 nerve root. The T3 nerve root will supply the area where the midclavicular line and third intercostal space intersect. The area where the midclavicular line and the fourth intercostal space at the level of the nipple is supplied by T4.
The T5 nerve root supplies the area where the midclavicular line and the fifth intercostal space intersect and spans horizontally midway between the level of the nipple and xiphoid process.
The area where the midclavicular line and the horizontal level of the xiphoid process intersects is innervated by the T6 nerve root.
The T7 nerve root supplies the area where the midclavicular line and the level at ¼ the distance between the level of the xiphoid process and the umbilicus horizontally.
The area where the midclavicular line and the level at ½ the distance between the level of the xiphoid process and the umbilicus horizontally intersection is innervated by the T8 nerve.
The T9 nerve will supply the area the midclavicular line and the level at ¾ the distance between the level of the xiphoid process and the umbilicus horizontally, and T10 innervates the area of the midclavicular line at the level of the umbilicus horizontally.
The nerve root of T11 supplies the area of the midclavicular line and the level at the mid between the umbilicus and the inguinal ligament horizontally; whereas the T12 nerve supplies the intersection of the midclavicular line and the midpoint of the inguinal ligament.
Next for the lower extremity dermatomes. These are supplied by the following nerve roots: L1, L2, L3, L4, L5, S1, S2, S3, S4, and S5.
The area over the inguinal region and the most proximal portion of the medial thigh is innervated by L1 nerve root. The middle and lateral portions of the anterior thigh are supplied by the L2 nerve.
The L3 nerve root innervates the medial epicondyle of the femur, the L4 root supplies the medial malleolus, and the L5 nerve root innervates the dorsum of the foot at the level of the third metatarsophalangeal joint.
The area of the lateral portion of the calcaneus is supplied by S1, the midpoint of the popliteal fossa by S2, the gluteal crease horizontally by S3, and the perineal area by S4 and S5 nerve roots.
And lastly here on dermatomes to summarize is all in one picture, see the below dermatome chart!
Lastly here today let’s talk about myotomes. As mentioned earlier a myotome is a group of muscles that are innervated by a single spinal nerve.
Some important myotomes and what actions that they produce to remember are the following:
- Shoulder shrugging
- Shoulder abduction, shoulder external rotation, and elbow flexion
- Wrist extension
- Elbow and finger extension and wrist flexion
- Thumb extension and finger flexion
- Finger abduction
- Hip abduction
- Hip flexion and abduction
- Knee extension
- Ankle dorsiflexion and knee extension
- Big toe extension and knee flexion
- Ankle plantarflexion, knee flexion, big toe flexion
- Bladder and rectal motor supply
Below see a nice summary chart of the high yield myotomes and what function they provide to the body!
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