The sternocleidomastoid muscle is also known as the great head turner and is one of the ventral superficial neck muscles located between the sternum, base of the skull, and clavicle. The main function of the bilateral muscles is lateral flexion of the head toward the shoulder, which is made possible by unilateral contraction. Lesions of the accessorius nerve impair the function of the muscle to the point of paralysis.
What is the sternocleidomastoid muscle?
Skeletal muscles can be divided into different muscle groups. One of them is the group of neck muscles. Among the ventrally located neck muscles is the large head turner, also known as the sternocleidomastoid muscle. The muscle is colloquially known as the head-nicker and is located between the sternum, clavicle and base of the skull. It makes up the superficial layer of the ventral cervical muscles. The skeletal muscle is located on both sides of the neck. Thus, each person actually has two musculi sternocleidomastoidei. The one on the right side is called the sternocleidomastoid dexter muscle and the one on the left side is called the sternocleidomastoid sinister muscle. Each of these muscles has two distinct heads: a caput laterale and a caput ventrale. Both of the heads each run obliquely across the lateral surface of the neck. Where the muscle is attached to the clavicle, it is sometimes widened, resulting in a widening of the trapezius muscle on the arm side of the associated bone. In isolated cases, the two muscles are completely fused, but both function properly.
Anatomy and structure
The sternocleidomastoid muscle receives motor innervation from the accessorius nerve and also receives nerve branches from segments C1 to C3 and C4 of the cervical plexus. Blood supply is provided by the sternocleidomastoid ramus. The caput laterale of the muscle originates at the superior edge and anterior surface of the medial clavicle and extends from there in fleshy aponeurotic fibers approximately vertically upward. The caput medial of the muscle originates each from the anterior surface of the manubrium sterni at the sternum and pulls with a fibers in cranial, lateral and dorsal direction. Between the respective origins of the two muscle heads there is a triangular gap on each side. Only in the further course it comes on middle of the neck to a union of the muscles, which lets a thick roundish muscle belly develop. The sternocleidomastoid muscle has its insertion at the lateral mastoid process and thus at the temporal bone. Through its course, the muscle divides the lateral neck region into triangles. The lateral neck triangle is also called the trigonum colli laterale. The anterior triangle of the neck is the trigonum colli mediale.
Function and tasks
The large head turner has several roles in terms of body movement. Via motor efferent nerves, commands to contract reach the motor endplate of the muscle, originating in the central nervous system. When the muscle contracts unilaterally, the head tilts laterally towards the shoulder. This results in lateral flexion or lateral bending of the head in the direction of the shoulder. On the same axis of motion lies the opposite lateral extension, which is also performed by the sternocleidomastoid muscle and corresponds to a slight lateral extension of the head. The extension approximates reclination because it is directed posteriorly. In addition, when the muscle contracts, the head rotates to the opposite side, which means that the skeletal muscle also participates in the rotation of the head. When the head is fixed, the two head turners change their function and together become the respiratory accessory muscles. Thus, when the head is fixed, the right and left sternocleidomastoid muscles each change the volume of the thoracic cavity through their contraction and relaxation, allowing for increased breathing. Unlike the actual respiratory muscles, however, they are not necessarily vital. In addition to the musculi sternocleidomastoidei, pectoral muscles and abdominal muscles are among the respiratory accessory muscles and, as such, support inspiration and expiration, but do not perform them automatically.
Diseases
Like all other muscles, the sternocleidomastoid muscle can be affected by paralysis. If the large head turner is paralyzed, lateral flexion of the head toward the shoulder is no longer possible.Paralysis of the muscle is usually due to a lesion of the accessorius nerve. Such a lesion can be caused by an accident, for example. Another cause is a lesion following compression, i.e. damage to the nerve following entrapment. Pressure damage can also be tumor-related. In addition, inflammation of the nerve can lead to its partial or complete failure. The same applies to malnutrition, poisoning and infections. Nerve paralysis can also occur in the context of polyneuropathy, which manifests itself in the form of paralysis of the large head turner. The causes mentioned up to this point are in the peripheral nervous system. However, the connection of the muscle to the central nervous system can also be favored by lesions in the central nervous system itself. Such a lesion can, for example, affect the spinal cord segments C1 to C3 and be accident-related, spinal cord infarct-related, tumor-related, or inflammation-related. In the case of inflammation-related paralyses, a distinction must be made between infections caused by microorganisms and autoimmune diseases such as MS. In ALS, on the other hand, the central motor neurons degenerate bit by bit. This phenomenon results in a progressive paralysis of all muscles. The central motor neurons are crucial for any kind of movement. Therefore, their progressive degeneration leads to the progressive loss of any voluntary, but also reflex motor function. Overall, paralysis of the sternocleidomastoid muscle can accordingly have many causes and should always be clarified neurologically.
