The scalenus anterior muscle, along with a total of three paired scalenus muscles, is part of the deep neck musculature. It originates from cervical vertebrae 3 to 6 (C3-C6) and pulls obliquely toward the 1st rib. The scalenus anterior muscle performs three main mechanical tasks; it is involved in lateral flexion and rotation of the neck, and in its function as an auxiliary respiratory muscle, it is involved in elevation of the 1st rib.
What is scalenus anterior muscle?
The scalenus anterior muscle, like the other two scalenus muscles, is paired and belongs to the deep neck musculature. The German name “anterior rib cage muscle” indicates its function as an auxiliary respiratory muscle. The muscle originates from the C3 to C6 cervical vertebrae and pulls obliquely sideways to the 1st rib, so that when the cervical spine is contracted and fixed on both sides, traction is exerted on the first rib, causing the rib and thus the thorax to rise. This means that the process of inhalation is supported. Between the scalenus anterior muscle and the scalenus medius muscle, that is, between the anterior and middle rib cage muscles, there is a triangular gap, the scalenus gap, through which nerves and blood vessels pass. Among other vessels, the subclavian artery runs through the scalenus gap. It is an important paired artery of the body, of which the left branch arises directly from the aortic arch and the right branch branches off from the trunk (truncus brachiocephalicus). common with the right cervical artery. The two arteries supply oxygenated blood to the head, neck, shoulders, and arms. The nerves passing through the scalenus gap originate from the brachial plexus, the nerve plexus whose branches innervate the arms, shoulders, and chest.
Anatomy and structure
The scalenus anterior muscle, the anterior rib holding muscle, corresponds in anatomical structure to skeletal muscle. Its muscle tissue is composed mainly of striated red muscle, which has a high content of myoglobin because the muscle is exposed to continuous stress in many cases. Skeletal muscle is characterized by the fact that the amount of force required to contract the muscle fibers can be continuously dosed by corresponding nerve impulses. Another characteristic feature is that the tension of the skeletal muscles is largely subject to voluntary control. To fulfill its main function, the elevation of the first rib or the entire thorax, the scalenus anterior muscle requires a good counter-bearing. It therefore fans out at the upper end so that, when contracted, the unilateral tensile load is distributed among the four cervical vertebrae C3 to C6, minimizing the problem of disc herniation in the cervical region. At the lower end, the two rib retaining muscles are attached to specially designed small bumps (tubercles) on the first costal arch (tuberculum musculus scalenus anterior). Sensory and motor innervation is provided by spinal nerve branches that emerge from the spinal canal between cervical vertebrae C5 to C7.
Function and Tasks
One of the main functions of the two scaleni anteriores muscles is to actively support respiration. Simultaneous contraction of the two anterior rib holding muscles pulls the ribs upward. This expands the chest cavity so that inhalation is supported. In other functions, the two muscles significantly support lateral neck flexion and lateral rotation in the horizontal plane. Neck flexion to the right or left and head rotation to the right or left can be achieved by unilateral contraction of the right or left scalenus anterior muscle. In addition to their motor functions, the two anterior costal muscles also perform a protective function. Their oblique course from the first costal arch to the cervical vertebrae creates a triangular space between them and the scalenus medius muscle, the scalenus gap. It serves to pass and protect important blood and nerve vessels that supply the head, shoulders and arms with oxygenated blood or innervate sensory and motor functions.
Diseases
Chronic stress or other causes leading to hardening of the anterior or middle costal holding muscle can cause narrowing of the scalenus gap, putting pressure on the subclavian artery that runs in it or on the nerve fibers of the brachial plexus (nerve compression).The narrowing of the scalenus gap leads to the so-called scalenus syndrome, which can manifest itself as pain in the forearm or paresthesias in the hand. Symptoms such as tingling, numbness and cold sensations are also typical companions of scalenus syndrome. If the subclavian artery is also compressed, there is an inadequate blood supply and in many cases even a drop in blood pressure. Similar symptoms are caused by “thoracic outlet syndrome,” also known as shoulder girdle compression syndrome, in which nerve fibers, the main artery and the main vein (subclavian vein) are equally affected by compression. Triggering factors for the narrowing include enlargement (hypertrophy) of the scalenus anterior muscle. Excessive contraction of the anterior costal cervical muscle can cause a hyperadduction syndrome, which manifests itself in a strong elevation of the uppermost rib to which the muscle is attached. In the course, compression of the nerves also occurs with symptoms comparable to those described above. The symptoms usually occur at night, and women are more often affected than men. The fact that nerve compression in the neck leads to paresthesia and paralysis of the arms and shoulders is exploited by modern anesthesia for surgical procedures in the shoulder region and on the arms. In a regional anesthesia procedure, the brachial plexus is anesthetized in the scalenus gap. There is no need for general anesthesia.
