Scalene muscles - via Anatomy in Motion (THANK YOU!)
Scalene muscles are three paired muscles of the neck, located in the front on either side of the throat, just lateral to the sternocleidomastoid. There is an anterior scalene (scalenus anterior), a medial scalene (scalenus medius), and a posterior scalene (scalenus posterior). They derive their name from the Greek word skalenos and the later Latin scalenus meaning "uneven", similar to the scalene triangle in mathematics, which has all sides of unequal length. These muscles not only have different lengths but also considerable variety in their attachments and fiber arrangements. As you will see from the descriptions below, these muscles are in a very crowded place and are related to many important structures, namely nerves and arteries, that run through the neck.
The scalenes run deep to the sternocleidomastoid. They all start at the cervical vertebra and run to the first to second ribs. The anterior scalene runs almost vertically and its upper part is concealed by the SCM and the lower part is concealed by the clavicle. Along its medial border runs the carotid artery. The internal jugular vein, the intermediate tendon of the omohyoid, the phrenic nerve; and the transverse cervical and scapular arteries all lie between the anterior scalene and the sternocleidomastoid (in front of scalene behind the SCM) Between the muscle and the clavicle runs the subclavian vein. The rear of the muscle, its posterior border, makes contact with the brachial plexus nerve roots, which run between it and the medial scalene.
Together with the first rib these muscles form a triangle known as the scalene triangle or interscalene triangle1 through which the brachial plexus nerves and the subclavian artery pass.
Also behind the anterior scalene are the pleura of the lungs and the superior intercostal artery.
Just behind the anterior scalene is the scalenus medius, referring to the "middle" muscle. This muscle forms part of the floor of the posterior triangle of the neck2. The front of the muscle runs close the the brachial plexus and the upper two thoracic nerve roots run through it. It makes contact with the levator scapulae in the rear, and the dorsal scapular nerve and transverse cervical artery pass between the two. The upper two roots of the long thoracic nerve go through the muscle. Only the anterior and medial scalene can be palpated. The posterior scalene is much shorter than the other two, and only starts at the lower cervical vertebra, where it attaches via two three tendinous slips. Whereas the first two attach to the first rib, the medius attaches to the second rib. 1,2,3,4,5,6.7
Some texts refer to a fourth scalene muscle, the scalenus minor. This variant does not always occur on both sides of the neck, but may be present in up one-third of people. This normal variation may have implications in thoracic outlet syndromes as does the scalenus anterior, resulting in a syndrome known as Scalenus Anterior sydrome or Scalenus Anticus syndrome (another name for the anterior muscle). The brachial plexus and the subclavian artery, as mentioned above, pass between the anterior scalene and the middle scalene. When present, the minimus inserts between the scalenus anterior and medius, passing behind the subclavian artery while the scalene anterior passes over and in front of it.7,8
At the top of the lungs is a the suprapleural membrane, which is a dense fascial layer also called Sibson's fascia. This fascia is attached to the inner border of the first rib and the costal cartilage. The pleura of the lungs attach to this fascia underneath. The fascia attaches to the transverse process of the C7 vertebra and when muscle fibers are found in it, it is called the pleuralis muscle, which is another name for the scalenus minimus. So this suprapleural membrane could be regarded as a flattened out tendon of the scalenus minimus, meaning that the scalenus minimus is attached to the pleura of the lungs, or the pleural dome and then beyond to the first rib, lying behind the anterior scalene and the groove of the subclavian artery. The scalenus muscle is a reinforcement of Sibson's fascia, which serves to stiffen the thoracic inlet and the neck structures above it so that they are not "puffed" up and down during forced respiration.8
The scalenes are clearly individual muscles but the all work together as a functional unit. They are usually considered accessory muscles of inspirations, as they work to elevate and fix the first and second ribs, while serving to fix them during quiet breathing, becoming guy-wires from the neck. It was thought that they were only active during labored or forceful breathing. However, measurement of their activity with concentric needles electrodes have demonstrated their activity even during quiet, normal breathing, even when the intake of breath is quite small. This has caused some researchers to drop the "accessory" label and consider them primary muscles of inspiration.
During normal diaphragmatic breathing, the ribs are elevated by the intercostal muscles and the scalenes. The orientation of the ribs causes them, when elevated, to expand the chest to the sides and front which increases the thoracic volume available for the lungs to expand into, although a most of this expansion is into the abdominal space which is made available by the contraction of the diaphragm downward. Their exact role in breathing is difficult to resolve.
The actions of the scalene muscles as movers of the neck and head are variously reported. They stabilize the cervical spine against lateral movement. The most common moving action attributed to them unilaterally is contralateral rotation of the cervical spine (rotation of head to the opposite side of working muscle). They have also been reported to be ipsilateral rotators (rotation to same side as working muscle). Bilaterally they are reported to be flexors of the neck. Their action in this regard depends on whether the thorax is fixed or the neck is fixed.1,2,3,4,5,6.7
Whether they are always active during breathing or not, the scalenes may become overactive in quiet breathing in upper chest breathing patterns. Prolonged coughing can overuse these muscles as well, and they may be especially problematic to asthma sufferers. Pain can come from myofascial trigger points in the scalenes or from thoracic outlet entrapment syndromes associated with the muscles.7
Origins, Insertions, and Actions
Origins: The Anterior Scalene (front scalene) originates on the anterior tubercles of the transverse processes of the third or fourth to the sixth cervical vertebrae.
The Scalenus Medius (middle scalene) originates on the posterior tubercles of the transverse processes of the first or second to seventh cervical vertebrae.
The Scalenus Posterior (rear scalene) attaches by two or three tendons from the posterior tubercles of the transverse processes of the the fifth or sixth to the seventh cervical vertebra (the last two or three).
Insertions: The scalenus anterior inserts onto the scalene tubercle and cranial crest of the firt rib, in front of the subclavian groove. The middle scalene inserts onto the cranial surface of the first rib, between the scalene tubercle and the subclavian groove. The posterior scalene inserts onto the outer surface of the second rib.
Actions: As above, the scalenes function as fixers and elevators of the first and second ribs during inspiration. The anterior and medial scalenes elevate the first rib and the posterior scalene elevates the second rib.
It is generally accepted that, acting unilaterally, they flex the head to the same side and acting bilaterally the flex the head forward (cervical flexion). Their roles as rotators of the neck given differently by different texts. Some report that all three scalenes rotate the head to the same side and some report that they all rotate it to the opposite side. Some report different functions for each scalene. According to Buford, et al., a multiple single-subject study on anesthetized macaques and human cadaver follow up revealed all three muscles as contralateral rotators of the cervical spine (rotating the head to the opposite side).4 The scalenes also help to laterally stabilize the neck, which is especially suited to the scalenus posterior.7
Sources of Scalene Trouble and Trigger Points
As stated above, breathing habits can be a cause of the scalenes being overworked. Here is a list of possible causes of scalene trouble which can lead to trigger points in the muscles or the neurovascular entrapment syndrome: