The intercostal nerves are part of the somatic nervous system, and arise from the anterior rami of the thoracic spinal nerves from T1 to T11. The intercostal nerves are distributed chiefly to the thoracic

pleura The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls. The inner pleura, called the visceral pleura, covers the surface of each lung and dips b ...

and abdominal

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...

, and differ from the anterior rami of the other spinal nerves in that each pursues an independent course without plexus formation. The first two nerves supply fibers to the upper limb and thorax; the next four distribute to the walls of the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the c ...

; the lower five supply the walls of the thorax and

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

. The 7th intercostal nerve end at the xyphoid process of the sternum. The 10th intercostal nerve terminates at the

navel The navel (clinically known as the umbilicus, commonly known as the belly button or tummy button) is a protruding, flat, or hollowed area on the abdomen at the attachment site of the umbilical cord. All placental mammals have a navel, altho ...

. The 12th ( subcostal) thoracic is distributed to the walls of the abdomen and groin. Each of these fibers contains around 1300 axons. Unlike the nerves from the autonomic nervous system that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the somatic nervous system. This enables them to control the contraction of muscles, as well as provide specific sensory information regarding the skin and parietal pleura. This explains why damage to the internal wall of the thoracic cavity can be felt as a sharp pain localized in the injured region. Damage to the visceral pleura is experienced as an un-localized ache.

The 1st thoracic nerve

The anterior division of the first thoracic nerve divides into two branches. The larger branch leaves the thorax in front of the neck of the first rib, and enters the brachial plexus. The smaller branch, the first intercostal nerve, runs along the first intercostal space, and ends on the front of the chest as the first anterior cutaneous branch of the thorax. Occasionally, this anterior cutaneous branch is missing. The first intercostal nerve rarely gives off a lateral cutaneous branch; but sometimes sends a small branch to communicate with the intercostobrachial. From the second thoracic nerve it frequently receives a connecting twig, which ascends over the neck of the second rib. This nerve was first described by Kuntz in 1927. There is considerable anatomic variation, but Kuntz nerve may be present in 40-80% of the population.Marhold F, Izay B, Zacherl J, Tschabitscher M, Neumayer C. Thoracoscopic and anatomic landmarks of Kuntz's nerve: implications for sympathetic surgery. Ann Thorac Surg. 2008;86:1653Y1658.

The upper thoracic nerves: 2nd–6th

The anterior divisions of the second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the first thoracic, are confined to the walls of the thorax, and are named thoracic intercostal nerves. They pass forward in the intercostal spaces below the intercostal vessels. At the back of the chest they lie between the pleura and the posterior intercostal membranes, but soon they run between the internal intercostals and the innermost intercostals then anteriorly they lie between the pleura and the internal intercostals. Near the sternum, they cross in front of the

internal mammary artery In human anatomy, the internal thoracic artery (ITA), previously commonly known as the internal mammary artery (a name still common among surgeons), is an artery that supplies the anterior chest wall and the breasts. It is a paired artery, with on ...

and transversus thoracis muscle, pierce the intercostales interni, the anterior intercostal membranes, and

pectoralis major The pectoralis major () is a thick, fan-shaped or triangular convergent muscle, situated at the chest of the human body. It makes up the bulk of the chest muscles and lies under the breast. Beneath the pectoralis major is the pectoralis minor, ...

, and supply the integument of the front of the thorax and over the mammary gland, forming the anterior cutaneous branches of the

; the branch from the second nerve unites with the anterior supraclavicular nerves of the cervical plexus. The fourth intercostal nerve is innervated by cutaneous slowly-adapting and rapidly-adapting mechanoreceptors, especially by ones densely-packed under the areola; innervation subsequently triggers oxytocin release, which, when in the peripheral bloodstream, causes myoepithelial cell contraction and

lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...

: this is an example of a non-nerve-innervation muscular reflex.

Branches

Numerous slender muscular filaments supply the

Intercostales Intercostal muscles are many different groups of muscles that run between the ribs, and help form and move the chest wall. The intercostal muscles are mainly involved in the mechanical aspect of breathing by helping expand and shrink the size ...

, the Subcostales, the Levatores costarum, the

Serratus posterior superior The serratus posterior superior muscle is a thin, quadrilateral muscle. It is situated at the upper back part of the thorax, deep to the rhomboid muscles. Structure The serratus posterior superior muscle arises by an aponeurosis from the lo ...

, and the Transversus thoracis. At the front of the thorax some of these branches cross the costal cartilages from one intercostal space to another. * Lateral cutaneous branches (rami cutanei laterales) are derived from the intercostal nerves, about midway between the vertebræ and sternum; they pierce the

Intercostales externi The external intercostal muscles, or external intercostals (Intercostales externi) are eleven in number on both sides. Structure The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they en ...

and Serratus anterior, and divide into anterior and posterior branches. * The anterior branches run forward to the side and the forepart of the chest and skin, fourth nerve anterior branches supplying the

areola The human areola (''areola mammae'', or ) is the pigmented area on the breast around the nipple. Areola, more generally, is a small circular area on the body with a different histology from the surrounding tissue, or other small circular ...

and the mamma; those of the fifth and sixth nerves supply the upper digitations of the Obliquus externus abdominis. * The posterior branches run backward, and supply the skin over the

scapula The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eith ...

and Latissimus dorsi. The lateral cutaneous branch of the second intercostal nerve does not divide, like the others, into an anterior and a posterior branch; it is named the

intercostobrachial nerve The intercostobrachial nerves are cutaneous branches of the intercostal nerves. Second intercostal nerve The lateral cutaneous branch of the second intercostal nerve does not divide, like the others, into an anterior and a posterior branch; it i ...

The lower thoracic nerves: 7th–11th

The lower thoracic nerves: 12th

Anterior division

Lateral cutaneous branch

The lateral cutaneous branch of the last thoracic nerve is large, and undivided. It perforates the internal and the external oblique muscles, descends over the iliac crest in front of the lateral cutaneous branch of the

iliohypogastric nerve The iliohypogastric nerve is a nerve that originates from the lumbar plexus that supplies sensation to skin over the lateral gluteal and hypogastric regions and motor to the internal oblique muscles and transverse abdominal muscles. Struct ...

, and is distributed to the skin of the front part of the gluteal muscles, some of its filaments extending as low as the greater trochanter of the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

Additional images

File:Nervous system diagram-en.svg, Nervous system File:Gray530.png, Intercostal spaces, viewed from the left File:Brachial plexus.svg, Brachial plexus File:Brachial_plexus_color.svg, Brachial plexus with courses of spinal nerves shown

References

External links

* () * - "Abdominal Wall, Dissection, Lateral View" {{Authority control Spinal nerves