The intercostal muscles are a complex muscular system of the chest wall that occupies the spaces between two ribs, or intercostal spaces. There are eleven pairs of muscles, one for each intercostal space, which are located from the superficial to the deep.
Its structure is made up of three layers of muscular sheets that are the outer, inner and deep layers, which are inserted into the ribs and are covered with a thick connective tissue.
By OpenStax - https://cnx.org/contents/:/Preface, CC BY 4.0, The function of the intercostal muscles is to help the diaphragm in the process of breathing. They are responsible for expanding the intercostal spaces to increase the thoracic capacity of the individual.
During inspiration the intercostal muscles contract causing the ribs to take a more horizontal position and allowing air to enter. They are considered accessory respiratory muscles.
It is important in medical practice to know their irrigation and innervation, since there are some invasive procedures that require these muscles to be penetrated without damaging their vascular or neurological structures. An example of this is the placement of a chest tube, which is a direct drain into the lung.
Origin and anatomy
The intercostal muscles are formed in the fetus from the fourth to the eighth week, along with the skin and the thoracic skeleton. These are three muscles that run down the rib wall and attach to the ribs.
By CFCF - Own work, CC BY-SA 4.0, The external intercostal muscle is the most superficial muscle sheet. Its oblique fibers run in a downward direction, from top to bottom. In the anterior part, near the breastbone, the muscle is attached to a thick layer of fibrous tissue.
The deep intercostal muscle is the deepest layer of the three. The path of its fibers is from back to front and from bottom to top.
The medial or medial intercostal muscle is located between the two anterior muscles. Its fibers are directed from back to front, like those of the internal intercostal muscle.
Within the chest wall muscle group are some other muscles that support the intercostals, but they are not considered intercostal muscles. These are the subcostal and transverse muscles of the thorax.
Insertion
The outer layer of the intercostal muscles inserts on the underside of the upper rib and reaches the upper side of the lower rib.
Its fibers are oblique and run from back to front. Later they go from projecting structures of the rib called tuberosities, and end in the sternum, where they continue with a fibrous layer of connective tissue called the anterior intercostal membrane.
By Braus, Hermann - Anatomie des Menschen: ein Lehrbuch für Studierende und Ärzte, Public Domain, The inner layer of the intercostal muscles is located on the lateral aspect of the upper rib and runs rather perpendicularly, ending on the upper aspect of the lower rib.
The fibers are located at the junctions of the ribs with the sternum, in front, and posteriorly in the posterior connective tissue that joins the rib cage, called the posterior intercostal membrane.
The deepest layer of intercostal muscles attaches to the most medial aspect of the upper rib and reaches the innermost aspect of the lower rib. It is covered by a connective tissue called endothoracic fascia.
Innervation
The intercostal muscles are innervated by the intercostal nerves, which run through the intercostal space between the internal intercostal and deep intercostal muscles.
These nerves originate directly from branches that come from the spinal cord. They are nerves of the dorsal or thoracic division and go from T1 to T11 leaving nerve branches for each intercostal space.
By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See «Book» section below) Bartleby.com: Gray's Anatomy, Plate 530, Public Domain, https://commons.wikimedia.org/w/index. php? curid = 541391
The intercostal nerves are for movement and sensation. Each intercostal branch provides the sensitivity of that area of the superficial thorax.
Irrigation
The blood supply of the intercostal muscles is guaranteed by a complex and powerful blood system that provides arterial branches for each intercostal space called intercostal arteries.
The intercostal arteries are located to give two anterior and one posterior branches which join laterally forming an intercostal arterial arch.
By Internet Archive Book Images - https://www.flickr.com/photos/internetarchivebookimages/14598327757/Source book page: https://archive.org/stream/platesofarteries00tied/platesofarteries00tied#page/n103/mode/1up, No restrictions, The anterior intercostal arteries in the first two intercostal spaces are a branch of the superior intercostal artery, which in turn is a branch of the powerful costocervical trunk. The anterior thoracic artery provides the blood supply for the next six intercostal spaces.
The last intercostal spaces are supplied by the musculophrenic artery, which also provides branches for the diaphragm and pericardium.
Regarding the posterior intercostal arteries, the first two intercostal spaces are also direct branches of the superior intercostal artery.
The remaining nine spaces receive intercostal branches directly from the thoracic aortic artery. These branches also help in the irrigation of the pleura and even the lung.
Each intercostal artery is accompanied by its respective vein and nerve and are located on the upper face of the lower rib.
Features
The main respiratory muscle is the diaphragm. It is a strong muscle that sits below the rib cage and divides the thorax from the abdomen. However, most of the chest wall muscles, both anterior and posterior, are involved in the breathing process.
The intercostal muscles have accessory functions in the mechanism of respiration. The three muscle groups that make them up act synergistically to maintain their function in both inspiratory and expiratory movements.
When contracting during inspiration, the intercostal muscles move the ribs, which are articulated bones with the sternum in front and the spinal column behind. That is, the intercostal muscles contract when the individual takes a breath.
The movement they achieve is to change the costal position. In this way, the ribs go from an oblique position to a more horizontal one. This movement manages to enlarge the size of the rib cage and increase the ability to take in air.
On expiration, the deep muscle group of the intercostal muscles is more involved. Actually its function is more visible in forced expiration, that is, when air is consciously expelled and beyond what is required for normal breathing.
By Bobjgalindo - Own work, CC BY-SA 4.0, When a person in a resting state is forced to use these muscles in order to breathe normally, they should be studied for a respiratory disease such as asthma.
References
- Tang A, Bordoni B. (2019). Anatomy, Thorax, Muscles. StatPearls. Treasure Island (FL) Taken from: ncbi.nlm.nih.gov
- De Troyer, A; Kirkwood, P; Wilson (2005) Respiratory Action of the Intercostal Muscles. Physiological reviews. Vol. 85. No. 2
- De Troyer, A., Kelly, S., Macklem, PT, & Zin, WA (1985). Mechanics of intercostal space and actions of external and internal intercostal muscles. The Journal of clinical investigation. Taken from: ncbi.nlm.nih.gov
- Wilson, TA, Legrand, A., Gevenois, PA, & De Troyer, A. (2001). Respiratory effects of the external and internal intercostal muscles in humans. The Journal of physiology. Taken from: ncbi.nlm.nih.gov
- Rendina, EA; Ciccone, AM. (2007) The intercostal space. Thoracic surgery clinics. Taken from: nlm.nih.gov