Definition
A dermatome is an area of the skin that is autonomously innervated by the nerve fibres of a specific spinal cord root (spinal nerve root). The name “dermatome” comes from the Greek and is composed of the words for skin and section. The understanding of dermatomes is of great importance in medicine for various diseases.
Classification of dermatomes
The basis for the development of dermatomes is embryology. An embryo has three different cotyledons (ectoderm, mesoderm and endoderm), from which all the different tissues develop during its maturation. In the trunk area, the mesoderm first develops into the so-called primal vertebrae (somites), which are located on the side of the neural tube.
From the laterally rear part of these primeval vertebrae, subcutis and skin are finally formed. This results in a 1:1 assignment of a spinal nerve to a specific area of skin. The dermatomes are therefore also named after the nerve from which they are supplied.
In the cervical vertebrae there are 8 spinal nerves, which are designated C1 to C8, and the dermatomes are named accordingly. However, there is the only exception: A dermatome C1 does not exist because the spinal nerve fibre of the first cervical vertebra has purely motor functions and does not innervate the skin. At the trunk there are 12 spinal nerves and thus 12 dermatomes, Th1 to Th12.
The lumbar and sacral vertebrae each have 5, so we have both the spinal nerves and the dermatomes L1 to L5 and S1 to S5. This early assignment is also maintained in adults. If we imagine the human being standing bent forward, with both arms and legs pointing to the ground at an angle of about 90 degrees from the back, we can divide the body roughly into strips, resulting in the dermatomes, starting with dermatome C2 on the head and ending with dermatome S5 on the back of the buttocks.
Sensitivity
However, it is important to note that the dermatomes are not really separated by clear lines, this picture is only for better imagination. Actually, the dermatomes partially overlap. It is assumed that this overlapping is more pronounced for the sensation of touch stimuli than for the sensation of pain and temperature stimuli.
Due to this phenomenon, affected persons often do not notice if only one segment loses its function, since the innervation of the corresponding area by the adjacent spinal nerve roots is still largely guaranteed. Only when two adjacent segments fail does the impairment usually become apparent. In contrast to the dermatomes, there are also the so-called autonomous areas on the skin.
These are the supply areas of certain peripheral nerves, not the spinal nerve fibres. The reason why this is different is that the nerves that originate from the spinal cord divide and connect with nerve fibres from other nerves. This exchange of nerve fibres is also called the plexus, and it is the plexus nerves that emerge.