por The intercostal muscles They 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 external, internal and deep layers, which are inserted into the ribs and are covered with thick connective tissue.
The function of the intercostal muscles is to assist the diaphragm in the breathing process. They are responsible for expanding the intercostal spaces to increase the individual’s thoracic capacity.
During inspiration the intercostal muscles contract causing the ribs to take a more horizontal position and allowing the entry of air. 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 injuring their vascular or neurological structures. An example of this is the placement of a chest tube, which is a direct drain into the lung.
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Origin and anatomy
The intercostal muscles form in the fetus from the fourth to the eighth week, along with the skin and the thoracic skeleton. These are three muscles that run along the costal wall and attach to the ribs.
The external intercostal muscle is the most superficial muscle sheet. Its oblique fibers run in a downward direction, from top to bottom. Anteriorly, near the sternum, 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 internal or middle intercostal muscle is located between the two anterior muscles. Its fibers have a back-to-front direction, like those of the internal intercostal muscle.
Within the chest wall muscle group are a few other muscles that support the intercostals, but they are not considered the intercostal muscles. These are the subcostalis and transversus thorax muscles.
Insertion
The outer layer of the intercostal muscles inserts on the underside of the upper rib and extends to the upper side of the lower rib.
Its fibers are oblique and run from back to front. Posteriorly they run from protruding structures of the rib called tuberosities, and terminate at the sternum, where they are continuous with a fibrous layer of connective tissue called the anterior intercostal membrane.
The inner layer of the intercostal muscles lies on the lateral aspect of the upper rib and runs a rather perpendicular course, ending on the superior aspect of the lower rib.
The fibers are located at the junction 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 inserts on the most medial aspect of the upper rib and extends to 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 pass through the intercostal space that lies 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.
The intercostal nerves are for movement and sensation. Each intercostal branch provides the sensation of that area of the superficial thorax.
Irrigation
The blood supply of the intercostal muscles is ensured 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 to form an intercostal arterial arch.
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 musculo-phrenic artery, which also supplies branches to the diaphragm and pericardium.
As for 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 aorta. 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.
functions
The main respiratory muscle is the diaphragm. It is a strong muscle that sits below the ribcage 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 vertebral 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 expand the size of the ribcage and increase the ability to take air.
In expiration, the deep muscle group of the intercostal muscles is more involved. Its function is really more visible in forced expiration, that is, when air is expelled consciously and beyond what is required for normal breathing.
When a person in a state of rest 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
DeTroyer, 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