The intercostal nerves are part of the

somatic nervous system The somatic nervous system (SNS), also known as voluntary nervous system, is a part of the peripheral nervous system (PNS) that links brain and spinal cord to skeletal muscles under conscious control, as well as to sensory receptors in the skin ...

, 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 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 mesotheli ...

, 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 The upper Limb (anatomy), limbs or upper extremities are the forelimbs of an upright posture, upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digit (anatomy), digits, including all the musc ...

and thorax; the next four distribute to the walls of the

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of 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 di ...

; the lower five supply the walls of the thorax and

abdomen The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...

. The 7th intercostal nerve ends at the xyphoid process of the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

. The 10th intercostal nerve terminates at the

navel The navel (clinically known as the umbilicus; : umbilici or umbilicuses; also 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. Structure The u ...

. The 12th ( subcostal) thoracic is distributed to the walls of the abdomen and

groin In human anatomy, the groin, also known as the inguinal region or iliac region, is the junctional area between the torso and the thigh. The groin is at the front of the body on either side of the pubic tubercle, where the lower part of the abdom ...

. Each of these fibers contains around 1300 axons. Unlike the nerves from the

autonomic nervous system The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...

that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the

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

mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into action potential, electrical signals tha ...

s, especially by ones densely-packed under the areola; innervation subsequently triggers

oxytocin Oxytocin is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. Present in animals since early stages of evolution, in humans it plays roles in behavior that include Human bonding, ...

release, which, when in the peripheral bloodstream, causes

myoepithelial cell Myoepithelial cells (sometimes referred to as myoepithelium) are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the lumen (anatomy), luminal cells. These may be positive for ACTA2, alph ...

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 process ...

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

Branches

Numerous slender muscular filaments supply the Intercostales, the Subcostales, the Levatores costarum, the Serratus posterior superior, 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 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. More generally, an areola is a small circular area on the Human body, body with a different histology from the surrounding Tissue (biology), tissue ...

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 and

Latissimus dorsi The latissimus dorsi () is a large, flat muscle on the back that stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The word latissimus dorsi (plural: ''latissimi dorsi'') comes from L ...

. 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 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 The crest of the ilium (or iliac crest) is the superior border of the wing of ilium and the superolateral margin of the greater pelvis. Structure The iliac crest stretches posteriorly from the anterior superior iliac spine (ASIS) to the posterio ...

in front of the lateral cutaneous branch of the iliohypogastric nerve, and is distributed to the skin of the front part of the

gluteal muscles The gluteal muscles, often called glutes, are a group of three muscles which make up the gluteal region commonly known as the buttocks: the gluteus maximus muscle, gluteus maximus, gluteus medius muscle, gluteus medius and gluteus minimus muscl ...

, some of its filaments extending as low as the greater trochanter of the

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

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 Thoracic nerves