Any condition that leads to pressure on the spinal cord, or on spinal nerve roots, can give rise to symptoms in the region supplied by nerves. In such cases it is important to be able to localize the particular spinal segments, or roots, involved. For this purpose it is necessary to know which areas of skin, (and which muscles) are innervated through each segment.
Areas of skin supplied by individual spinal nerves are called dermatomes. To understand the arrangement of dermatomes it is necessary to know some facts about the development of the limbs.
The upper and lower limbs are derived from limb buds. These are, paddle-shaped, outgrowths that arise from the side-wall of the embryo. They are at first directed forward and laterally from the body of the embryo (Fig. 2.8). Each bud has a preaxial (or cranial) border and a postaxial border (Fig. 2.9). The thumb and great toe are formed on the preaxial border.
The forelimb bud is derived from the part of the body wall belonging to segments C4, C5, C6, C7, C8, T1 and T2. It is, therefore, innervated by the corresponding spinal nerves. The hindlimb bud is formed opposite the segments L2, L3, L4, L5, S1 and S2. As the limbs grow skin supplied by these nerves gets “pulled away” into the limbs. This has great effect on the arrangement of dermatomes (Figs. 2.10, 2.11). The dermatomes of the body are shown in Fig. 2.12.
Now note the following facts about dermatomes These are of clinical significance.
a. Spinal nerve C1 does not supply any area of skin.
b. The areas supplied by different spinal nerves overlap in such a way that any given area is supplied by two (or more) nerves. The overlap is less for fibres carrying sensation of touch and more for those carrrying pain and temperature.
c. Because of overlap maps of dermatomes are only approximate.
d. Because of what has been said above (about the development of the limbs) the lateral (preaxial) aspect of the upper limb is supplied (in sequence) by segments C4 to C6 and the medial aspect by segments C8 to T2. The segment C7 supplies an intermediate strip.
e. The layout becomes complex in the lower limb because a twisting of the limb during development.
f. As a rule, the arrangement of dermatomes is simple over the trunk, as successive horizontal strips of skin are supplied by each spinal nerve of the region. However, the arrangement is unusual over the pectoral region. The skin of the upper part of the pectoral region is supplied by spinal segments C3 and C4 (upto the level of the sternal angle). Below this level we find that in fact the area just below the level of the sternal angle is supplied by segment T2. This is so because nerves C5 to T1 have been pulled away into the upper limb. For the same reason there is no overlap between the areas supplied by C4 and T2.