Segmental Innervation of Skin

The area of skin supplied by a single spinal nerve, and therefore, a single spinal segment, is called dermatome. With the exception of the first cervical nerve (C1), all of the spinal nerves are associated with specific dermatomes.

In the trunk the arrangement of dermatomes is simple because the thoracic and upper lumbar spinal nerve supplying it are arranged segmentally. On the trunk the dermatome extends around the body from the posterior median line to the anterior median line. Figures 5.1 and 5.2 depict the dermatomal maps for the anterior and posterior surfaces of the body. Adjacent dermatomes overlap considerably (Fig. 5.3); therefore to produce a region of complete anaesthesia at least three contiguous nerves have to be severed. Further, it is important to note that the area of tactile loss is always larger than the area of loss of thermal and painful sensations. This is due to the fact that the degree of overlap of fibres carrying pain and temperature sensations is much more extensive than the overlap of fibres carrying tactile sensations.

In the limbs the arrangement of dermatomes is complicated and this is because of two reasons: (a) the embryo-logical rotation of the limbs as they grow out from the trunk, and (b) the spinal nerves that supply them form plexuses.

In early embryonic development, the upper and lower limbs appear as paired paddle-shaped limb buds. Each bud has preaxial and postaxial borders, with former being towards the head. Later in the development the ends become expanded and flattened to form hand and foot plates, in which the digits develop. The digits along the preaxial border are the thumb and big toe. The limbs then rotate. The upper limb rotates laterally so that its preaxial border and thumb come to lie on the lateral side. On the other hand, the lower limb rotates medially and its preaxial border and big toe come to lie on the medial side. It is now easier to understand why the dermatomes innervated by spinal nerves of brachial plexus are arranged consecutively down on the lateral side and up on the medial side of the upper limb, and dermatomes innervated by spinal nerves of lumbosacral plexus are arranged down the medial side and up the lateral side and back of the lower limb (Fig. 5.3 A, B).

The spinal nerves that supply the upper limb form plexuses from which individual named nerves arise. Each of these named nerves may contain fibres derived from a number of spinal nerves so that the area of skin supplied by one of the named nerves will be quite different from the area supplied by the individual spinal nerves.

In the head and face region, each of the three divisions of the trigeminal nerve supply a precise area of skin and there is little or no overlap to the cutaneous area of the another division (Fig. 9.8).

Points to Note

Segmental Innervation of Muscles

The skeletal muscles are also segmentally innervated. Most of these muscles are innervated by more than one spinal nerve and therefore by the same number of spinal segments. Therefore, to paralyze a muscle completely it would be necessary to section several spinal nerves or damage several segments of the spinal cord.

It is important to note the following facts carefully:

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Development of the nervous system Peripheral nerves and ganglia Autonomic nervous system Reticular formation and limbic system Special senses and their neural pathways Diencephalon and third ventricle

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