A is a region of skin that sends sensory information through a single pair of spinal nerves

The spinal cord and its associated spinal nerves are supplied by a single anterior spinal artery and 2 posterior spinal arteries. The anterior spinal artery supplies the anterior two thirds of the cord. The posterior spinal arteries supply the dorsal columns. All three spinal arteries arise from the vertebral arteries in the skull and descend through the base of the skull. Segmental branches of the thoracic and abdominal aorta have radicular branches that anastomose with the spinal arteries to provide additional blood supply to the spinal arteries. One of the largest radicular branches, the great radicular artery or artery of Adamkiewicz, supplies the anterior spinal artery, which enters the spinal cord between T5 and L1, with the most common entry point between T9 and T12.

Dermatomes are derived from the outer portion of an embryo from which the skin and subcutaneous tissues are developed and become the areas of skin supplied by the branches of a single dorsal root ganglion. In the developing embryo, dermatomes arise from somitic mesoderm, which develops from the middle layer of embryonic tissue lateral to the developing neural tube. Dermatomes are arranged with basic segmental pattern in the vertebrate trunk, although some overlap exists with similar areas above and below.

Dermatomes of the head, face, and neck

Below, Image 1 depicts and Table 1 describes the head, face, and neck dermatomes.

Table 1. Dermatomes of the Head and Neck (Open Table in a new window)

Dermatomes of the trunk

The dermatomes of the trunk are relatively evenly spaced out; however, considerable overlap of innervations between adjacent dermatomes often occurs. Thus, a loss of afferent nerve function by one spinal nerve would not generally cause complete loss of sensation, but a decrease in sensation may be experienced.

Below, Image 2 depicts and Table 2 describes the trunk dermatomes.

Table 2. Dermatomes of the Trunk (Open Table in a new window)

Dermatomes of the extremities

The organization of dermatomes in the limbs is more complex than that of the dermatomal distribution in the trunk as a result of the limb buds and corresponding dermatomes being "pulled out" during early embryologic development.

The medial, intermediate, and lateral supraclavicular nerves from the cervical plexus supply the dermatomal distribution to the root of the neck, upper pectoral, deltoid, and the outer trapezius areas. The posterior divisions of the upper 3 thoracic nerves supply the region over the trapezius area to the spine of the scapula. The brachial plexus gives rise to most of the rest of the cutaneous innervation of the upper extremity.

Contrary to the considerable overlap of the dermatomes of the trunk, the overlap between the peripheral nerves of the limbs (upper and lower extremities) is far less extensive (see the following image). Thus, in the limbs, complete interruption of a single peripheral nerve typically produces changes in sensation that are, indeed, appreciated by a patient.

Table 3 describes the upper extremity dermatomes.

Table 3. Dermatomes of the Upper Extremity (Open Table in a new window)

Dermatomal distribution in the lower extremity has a spiral arrangement stemming from the rotation of the limb as an adaptation to the erect position during development (see the following image).

NOTE: Pain due to pleurisy, peritonitis, or gallbladder disease can often be referred to the skin over the point of the shoulder, halfway down the lateral side of the deltoid muscle. This is because this area of skin is supplied by the supraclavicular nerves (C3 and C4), and the pain generated from pleurisy, peritonitis, and/or gallbladder disease is carried from the diaphragmatic pleura and peritoneum via afferent fibers of the phrenic nerve (C3-C5). [10]

Below, Table 4 describes the lower extremity dermatomes.

Table 4. Dermatomes of the Lower Extremity and Genitalia (Open Table in a new window)