The scalenus medius muscle is the longest scalenus muscle and is classified as a neck muscle and respiratory accessory muscle. The skeletal muscle is also called the middle rib elevator and, when contracted bilaterally, enlarges the thorax to facilitate forced inspiration. With the scalenus anterior muscle, the muscle forms the scalenus gap, which gains pathologic relevance in scalenus syndrome.
What is the scalenus medius muscle?
The cervical or ventral neck muscles consist of various skeletal muscles that contribute to the anterolateral muscle mass of the neck. Neck muscles are sometimes called the neck muscles, which are basically more like back muscles. One of the skeletal muscles of the neck is the scalenus medius muscle. The Latin adjective “scalenus” means something like “uneven-sided” or “crooked” and thus already refers to the morphology of the neck muscle. The scalenus medius muscle is better known as the medial rib cage muscle. To be distinguished from the middle rib holder is the scalenus anterior muscle, which is also classified as part of the neck musculature and, together with the scalenus medius muscle, forms the so-called scalenus gap. In total, there are three mucoli scaleni. The third scalenus muscle is the scalenus posterior muscle. All three musculi scaleni are referred to as hypaxial skeletal muscles and are located in the thoracic region. Each half of the body is equipped with a middle rib elevator.
Anatomy and structure
The origins of the scalenus medius muscle correspond to the transverse processes of cervical vertebrae. More specifically, they are cervical vertebrae three through seven. Its insertion takes the medial rib holder in most cases at the first or second rib. The muscle runs from here dorsal to the subclavian artery and occasionally attaches to the outer surface of the ribs. The scalenus medius muscle is the longest scalenus muscle in human anatomy. Between the medial rib holder and the shorter scalenus anterior muscle is a space also known as the posterior scalenus gap. At this point, the subclavian artery passes through alongside the brachial plexus to enter the axilla. Innervation of the scalenus medius muscle is provided by the anterior branches of various spinal nerves. More specifically, the spinal nerves from spinal cord segments C4 to C7 are involved in the innervation of the cervical muscle.
Function and Tasks
The scalenus medius muscle makes major contributions to the motor function of the neck. The muscle moves the neck to the side during unilateral contractions. Thus, the medial rib elevator tilts the cervical spine during unilateral contraction. On the other hand, if the skeletal muscles contract on both sides of the body, it pulls the neck down. The contractions of the muscle not only affect the neck motor function, but also show influence on the general trunk motor function. Especially the bilateral contraction of the skeletal muscle changes something in the morphology of the trunk and thorax. During bilateral contraction, the scalenus medius muscle raises the upper ribs. This connection has helped the muscle to be called the “middle rib lifter.” The raised ribs automatically change the thorax. Primarily, the bony thorax increases in volume due to muscle contraction. Like the other two musculi scaleni, the musculus scalenus medius thus belongs to the auxiliary respiratory musculature, which performs important tasks during inspiration. For example, the scalenus anterior muscle raises the first rib when contracted bilaterally and the cervical spine is fixed, which also causes the thorax to expand. The scalenus posterior muscle assists in expanding the bony thorax when contracted bilaterally, and the scalenus medius muscle expands the bony thorax when inhaled by contracting bilaterally. Consequently, like all other components of the inspiratory respiratory support musculature, the scalenus medius muscle supports breathing during increased or forced inspiration. The respiratory support musculature should not be confused with the respiratory musculature proper, which consists of the diaphragm and intercostal musculature.
Diseases
The scalenus medius muscle may acquire pathologic significance in the context of various compression syndromes. Sometimes the best known phenomenon in this context is the scalenus syndrome.Compression syndrome is sometimes referred to in the literature as cervical rib syndrome or Naffziger syndrome. This nerve compression syndrome is from the group of thoracic outlet syndromes. In the phenomenon, the brachial plexus becomes jammed in the scalenus gap between the medius and anterior scalenus muscles. Different deficits in the neurological area can be the result. Since the brachial plexus innervates the shoulder and chest muscles motorically and is also involved in the sensitive motor innervation of the arms and hands, scalenus syndrome patients often suffer from load-dependent pain in the shoulder and arm area. In individual cases, the sensory innervation of the hand may be disturbed by nerve compression. Hypesthesias and paresthesias are the result. In some cases, the sensory disturbances are associated with circulatory disturbances. The latter is especially true if the subclavian artery is also affected by the compression. In addition to numbness and feelings of heaviness, paralysis of the arm or chest muscles may occur. In extreme cases, paralysis-related atrophy of the musculature may occur during the course of the disease, particularly affecting the small muscles of the hand. The region between the scalenus anterior and medius muscles is a bottleneck for the brachial plexus, especially when patients have additional cervical ribs. Such extra ribs are one of the most common causes of scalenus syndrome. Hypertrophic muscles may also be causative. Hypertrophies of muscles result in enlargement due to an increase in cell volume while the number of cells remains the same. This phenomenon usually develops in the context of muscles from functional stress or hormonal stimulation.