The vertebrate body, with the exception of the head, is originally subdivided into segments or metameres. The vertebrae, ribs, andintercostal muscles can be regarded as rem-nants of such a segmentation in humans. Metamerism concerns only tissues of the mesoderm (myotomes, sclerotomes) but not derivatives of the ectoderm. Thus, there are no spinal cord segments, only the levels at which the individual spinal roots enter and emerge. However, the spinal fibers join to form the spinal nerves as they emerge through the metameric intervertebral foramina, thus creating an apparent second-ary segmentation. The sensory fibers of thespinal nerves supply stripe-shaped zones of the skin, called dermatomes in analogy to myotomes and sclerotomes. This, too, is a secondary segmentation and reflects the in-nervation of each dermatome by a single posterior root (segmental innervation).
Clinical Note: The dermatomes play an im-portant role in the diagnosis and localization ofspinal cord injuries. Loss of sensibility in certaindermatomes indicates a specific level of injury in the spinal cord. Simplified reference points are the line through the nipples, regarded as the boundary between T4 and T5, and the groin, re-garded as the boundary between L1 and L2. The first cervical spinal nerve has no sensory repre-sentation on the body surface, for the spinal gan-glion of its posterior root is absent or rudimen-tary.
There are slightly different segmental boundaries for various modalities, such as touch and pain, and for sweating and piloerection. The diagram (A) was designed according to the decrease in sensibility (hy-poesthesia) resulting from disk prolapse; itshows how the dermatomes extending around the trunk become elongated in the limbs. They may even lose their continuity with the midline (C7, L5). They become translocated to the distal limb areas during embryonic development when the limbs are budding (C).
The dermatomes overlap like roof tiles, as il-lustrated by the shift in boundaries that have been determined according to the ex-panded areas in case of posterior root pain (hypersensitivity to pain, hyperalgesia) (B). The loss of a single posterior root cannot be demonstrated for touch sensation, since the corresponding dermatome is also supplied by the neighboring posterior roots. The der-matomes for pain and temperature sensa-tion are narrower, and the loss of a posterior root can still be demonstrated when these modalities are tested.