General characteristics and primary branches
The thoracic nerves (nervi thoracici) are represented by 12 pairs of spinal nerves designated from Th₁ to Th₁₂. One of their main features is the absence of nerve plexuses: each nerve runs in isolation. The only exception is the first thoracic nerve (Th₁), which partially participates in the formation of the brachial plexus (plexus brachialis).
By composition, each thoracic nerve is mixed. Upon leaving the vertebral canal through the intervertebral foramen, the nerve trunk immediately divides into several branches:
- Meningeal branch (r. meningeus) — returns to the vertebral canal;
- White communicating branches (rr. communicantes albi) — head towards the sympathetic trunk ganglia;
- Posterior branch (r. dorsalis);
- Anterior branch (r. anterior).
Posterior branches of the thoracic nerves
The posterior branches (rr. dorsales nn. thoracicorum) retain their mixed nature, carrying both motor and sensory nerve fibers.
Topographically, each posterior branch heads to the posterior surface of the trunk, where it invariably divides into two trunks:
- Medial branch (r. medialis);
- Lateral branch (r. lateralis).
Anterior branches: intercostal and subcostal nerves
The anterior branches of the thoracic nerves (rr. anteriores nn. thoracicorum) have significant topographical differences depending on their level of origin.
Intercostal nerves (nn. intercostales) The anterior branches from the first to the eleventh pair (Th₁–Th₁₁) are called intercostal nerves. They lie strictly in the corresponding intercostal spaces. Each such nerve runs between the internal intercostal muscle (m. intercostalis internus) and the innermost intercostal muscle (m. intercostalis intimus). The intercostal nerve is always accompanied by blood vessels of the same name, forming the neurovascular bundle of the intercostal space.
Subcostal nerve (n. subcostalis) The anterior branch of the twelfth thoracic nerve (Th₁₂) is called the subcostal nerve. Unlike the others, it is located outside the intercostal space — its trunk runs below the 12th rib. The subcostal nerve participates in the innervation of the muscles of the anterior abdominal wall, gives off cutaneous branches to the lumbar region and lower abdomen, and also takes part in the formation of the lumbar plexus.
Cutaneous branches of the anterior trunks
To provide skin sensitivity of the chest and abdomen, the intercostal nerves give off specialized cutaneous branches that are divided into two major groups.
Lateral cutaneous branches (rr. cutanei laterales) These branches pierce the muscles and enter the subcutaneous tissue along the midaxillary line. Each lateral branch divides into anterior and posterior parts. They innervate the skin of the lateral surface of the chest and abdomen. Special lateral branches:
- From Th₂ to Th₄ — may extend to the shoulder region, forming the intercostobrachial nerves (nn. intercostobrachiales);
- From Th₄ to Th₆ — reach the integument of the mammary gland, forming the lateral mammary branches (rr. mammarii laterales).
Anterior cutaneous branches (rr. cutanei anteriores) These are the terminal branches of the intercostal nerves. They emerge into the subcutaneous tissue along the parasternal line and also divide into medial and lateral branches. They provide skin sensitivity of the anterior surface of the chest and abdominal wall. Special anterior branches:
- From Th₂ to Th₄ — form the medial mammary branches (rr. mammarii mediales);
- In the abdominal region, the terminal branches form the anterior cutaneous branches of the abdomen (rr. cutanei abdominales).
Briefly: answers to topical questions
Which specific muscles do the posterior branches of the thoracic nerves innervate?
The posterior branches of the thoracic nerves innervate the deep muscles of the back, which are also called autochthonous muscles. The posterior branch of each thoracic nerve divides into a medial branch (r. medialis) and a lateral branch (r. lateralis). The specific list of innervated muscles is not provided in the given sources, but it is indicated that these branches carry both motor and sensory fibers to supply the deep back muscles and skin.
Which specific muscles of the anterior abdominal wall does the subcostal nerve innervate?
The subcostal nerve (n. subcostalis, Th₁₂) participates in the innervation of the abdominal wall muscles. It runs along the quadratus lumborum muscle and ends in the lower part of the rectus abdominis muscle and the pyramidalis muscle. The sources do not allow us to claim that the subcostal nerve alone innervates all the listed muscles of the anterior abdominal wall.
In what order (from top to bottom) are the elements of the neurovascular bundle located in the intercostal space?
In the intercostal space, the elements of the neurovascular bundle are located in the order designated by the abbreviation V–A–N (Vein, Artery, Nerve).
The bundle lies in the costal groove (sulcus costae) along the lower margin of each rib. It runs between the internal intercostal muscles (mm. intercostales interni) and the innermost intercostal muscles (mm. intercostales intimi). Anterior to the midaxillary line, the vessels and nerves are not covered by the ribs and enter the intermuscular tissue.
Which chest muscles do the muscular branches of the intercostal nerves innervate?
The muscular branches of the intercostal nerves (rr. musculares) innervate all ventral muscles of the thoracic cavity walls, as well as muscles of ventral origin on the back.
These include the following muscles:
- External intercostal muscles (mm. intercostales externi) — ventral muscles of the chest wall
- Internal intercostal muscles (mm. intercostales interni) — ventral muscles of the chest wall
- Innermost intercostal muscles (mm. intercostales intimi) — ventral muscles of the chest wall
- Subcostal muscles (mm. subcostales) — ventral muscles of the chest wall
- Transversus thoracis muscle (m. transversus thoracis) — ventral muscles of the chest wall
- Serratus posterior superior and inferior muscles (mm. serrati posteriores superior et inferior) — muscles of ventral origin on the back
- Levatores costarum muscles (mm. levatores costarum) — muscles of ventral origin on the back
To memorize the topography of the intercostal neurovascular bundle (from top to bottom), use the rule "VAN": Vein – Artery – Nerve. The nerve is always located below the vessels, which is important to remember when performing a pleural puncture.
Class and exam questions
Do thoracic nerves participate in the formation of nerve plexuses?
Generally no, they retain a strict segmental structure. The only exceptions are the first thoracic nerve (Th₁), some fibers of which are part of the brachial plexus, and the subcostal nerve (Th₁₂), which participates in the formation of the lumbar plexus.
Between which muscles does the trunk of the intercostal nerve run?
Each intercostal nerve runs between the internal intercostal muscle (m. intercostalis internus) and the innermost intercostal muscle (m. intercostalis intimus).
How does the subcostal nerve (Th₁₂) topographically differ?
Unlike the other anterior branches, it does not run in the intercostal space, but is located below the 12th rib, heading towards the muscles and skin of the abdominal wall.
What else to study on this topic
- Formation and course of the intercostobrachial nerves (nn. intercostobrachiales)
- Innervation of the mammary gland: medial and lateral branches
- Connection of the thoracic nerves with the sympathetic trunk via white communicating branches
- Topography of the subcostal nerve and its participation in the lumbar plexus
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