Thoracic Nerves

The dorsal rami of the thoracic nerves run backward near the zygapophyseal joints, which they supply, and divide into medial and lateral branches. Both sets of branches pass through the groups of muscles constituting the erector spinae and give off branches to them. The terminations of the upper six or seven medial branches innervate the skin adjacent to the corresponding spinous processes, but the lower five or six often fail to reach the skin. The terminations of all the lateral branches usually pierce the thoracolumbar fascia over the erector spinae muscles and divide into medial and lateral cutaneous branches, which innervate much of the skin of the posterior thoracic wall and upper lumbar regions.

The ventral rami of most of the thoracic nerves, unlike those in other regions, do not form plexuses. They retain their segmental character, and each pair runs separately in the corresponding intercostal spaces as the intercostal nerves. The first pair, however, divides into larger and smaller branches; the former, usually joined by twigs from the second pair, participate in the formation of the brachial plexuses (Plate 4-13), whereas the smaller branches are the first pair of intercostal nerves. The last (twelfth) pair of ventral rami course below the lowest ribs and are therefore termed the subcostal nerves.

The intercostal nerves are distributed mainly to structures in the thoracic and abdominal walls. The upper six pairs are limited to the thoracic parietes, whereas the lower five pairs extend from the thoracic into the abdominal walls and also contribute fibers to the diaphragm. The intercostal nerves give off muscular, anterior and lateral cutaneous, mammary, and collateral branches, and supply filaments to adjacent vessels, periosteum, parietal pleura, and peritoneum.

The upper six pairs supply muscular branches to the corresponding intercostal muscles and also to the subcostal, serratus posterior superior, and transverse thoracic muscles. The lower five pairs supply the lower intercostal muscles and the subcostal, serratus posterior inferior, transverse, oblique, and rectus abdominal muscles. Fascicles from the lower intercostal nerves also enter the margins of the diaphragm, but they are sensory. The subcostal nerves supply the pyramidalis muscles.

The anterior cutaneous branches supply the front of the thorax. The lateral cutaneous branches pierce the internal and external intercostal muscles and end by dividing into branches that extend forward and backward to innervate the skin covering the lateral sides of the thorax and abdomen. The small lateral branch of the first intercostal nerve supplies the skin of the axilla, and the lateral branch of the second is the intercostobrachial nerve, which is distributed to the skin on the medial side of the arm. The lateral cutaneous branch of the subcostal nerve pierces the internal and external oblique abdominal muscles and descends over the iliac crest to supply the skin of the anterior part of the gluteal region.

The mammary glands receive filaments from the lateral and anterior cutaneous branches of the fourth, fifth, and sixth intercostal nerves, which convey autonomic and sensory fibers to and from the glands.

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