Dermatomes and Myotomes Quiz

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Which statement best defines a dermatome?

A A layer of skin derived from mesoderm during embryology
B A region of skin supplied by a single cranial nerve
C An area of skin supplied by a single spinal nerve ✓
D A group of muscles supplied by a single spinal nerve
E An area of skin supplied by multiple adjacent spinal nerves

The answer is C: An area of skin supplied by a single spinal nerve

Explanation

Why C is correct

See Dermatomes. Option C is correct. A dermatome is an area of skin that is supplied (innervated) by a single spinal nerve, that is, one of the 31 pairs of nerves that leave the spinal cord. Sensation from that strip of skin travels back along that nerve to one level of the spinal cord. This makes dermatomes useful in clinical practice: if a patient has loss of sensation, pain or a rash in a particular band of skin, the doctor can work out which spinal nerve or cord level is involved. This is the basis of the sensory part of a neurological examination, and it helps to localise a spinal cord injury or a trapped nerve root. The dermatome maps are drawn as neat strips, but neighbouring dermatomes overlap a little, so a single damaged root often causes less loss than the map suggests. The muscle equivalent is the myotome: the group of muscles supplied by the same spinal nerve.

Dermatome: an area of skin supplied by a single spinal nerve
Pathway: sensation from that skin travels back along the nerve to one level of the spinal cord
Use: a band of lost sensation, pain or rash points to the nerve or cord level involved
Overlap: neighbouring dermatomes overlap a little

Why the other options are incorrect

A. A layer of skin derived from mesoderm during embryology: This describes an embryological origin of tissue, not a nerve supply. The dermis does come from mesoderm, but the term dermatome is about which spinal nerve supplies an area of skin. It would be the answer to a question about the origin of dermis.
B. A region of skin supplied by a single cranial nerve: Skin supplied by a cranial nerve, such as the face supplied by the trigeminal nerve, is not described as a dermatome because dermatomes belong to spinal nerves. The face and head have their own pattern. It would be the answer to a question about cranial nerve sensory territories.
D. A group of muscles supplied by a single spinal nerve: This is the definition of a myotome, the muscle equivalent of a dermatome. The key difference is that a dermatome concerns skin sensation, not muscle movement. It would be the answer to a question asking what a myotome is.
E. An area of skin supplied by multiple adjacent spinal nerves: A dermatome is defined by one spinal nerve, not by several together. Neighbouring dermatomes overlap slightly, but each is still named for a single nerve. It would be the answer only if the question asked about overlapping sensory territories.

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1
In the standard myotome screening scheme, which spinal nerve root is most closely associated with ankle dorsiflexion?
2
A 22-year-old woman develops a painful blistering rash that later crusts over. It forms a horizontal band on one side of her abdomen at the level of her umbilicus and does not cross the midline. Which spinal nerve dermatome is most likely affected?

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