The sternocleidomastoid muscle is also known as a large head nodule and is one of the ventral superficial neck muscles that lie between the sternum, skull base and collarbone. The main function of the bilateral muscles is the lateral flexion of the head towards the shoulder, which is made possible by unilateral contraction. Lesions of the accessory nerve affect the function of the muscle, including paralysis.

What is the sternocleidomastoid muscle?

The skeletal muscles can be divided into different muscle groups. One of them is the group of the neck muscles. Among the ventral neck muscles is the large head turner, also known as the sternocleidomastoid muscle. The muscle is colloquially referred to as a head crawler and lies between the sternum, the collarbone and the base of the skull.

He makes the superficial layer of the abdominal neck muscles out. The skeletal muscle lies on both sides of the neck. Thus, every human actually owns two muscular sternocleidomastoid muscles. The right side is called the sternocleidomastoid muscle dexter and the left side is called the sternocleidomastoid muscle sinister.

Each of these muscles has two distinct heads: a caput lateral and a caput ventral. Both of the heads run obliquely over the lateral neck area. Where the muscle is firmly attached to the clavicle, it is sometimes broadened, resulting in widening of the trapezius muscle on the arm side of the associated bone. In some cases, the two muscles are completely grown together, but both work flawlessly.

Anatomy & Construction

The sternocleidomastoid muscle is motor-innervated by the accessory nerve and also receives nerve branches from segments C1 to C3 and C4 of the cervical plexus. The blood supply is ensured by the ramus sternocleidomastoideus.

The caput lateral of the muscle originates at the upper edge and anterior surface of the medial collarbone and extends from there in meaty aponeurotic fibers approximately vertically towards the top. The medial head of the muscle originates from the anterior surface of the manubrium sterni on the sternum and pulls with a fiber in the 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 to the middle of the neck to a union of the muscles, which gives rise to a thick roundish muscle belly.

The sternocleidomastoid muscle has its approach on the lateral mastoid process and thus on the temporal bone. Through its course, the muscle divides the lateral neck into triangles. The lateral neck triangle is also called Trigonum colli lateral. The anterior neck triangle is the trigone colli mediale.

Function & Tasks

The big head turner has several tasks concerning the movement of the body. Via motor efferent nerves, commands for contraction reach the motor end plate of the muscle originating from the central nervous system. In a one-sided contraction of the muscle, the head tilts laterally towards the shoulder. So it comes to Lateralflexion or lateral bending of the head in the shoulder direction.

On the same axis of movement is the opposite lateral extension, which is also performed by the sternocleidomastoid muscle and corresponds to a slight lateral extension of the head. The extension corresponds approximately to a reclination, since it is aligned to the rear. In addition, the head rotates in the contraction of the muscle to the opposite side, whereby the skeletal muscle is also involved in the rotation of the head. With a fixation of the head the two head turner change their function and together become the respiratory aid musculature.

The right and left muscles of the sternocleidomastoid muscle thus change the volume of the chest cavity due to their contraction and relaxation, thus enabling increased breathing. Unlike the actual respiratory muscles, however, they are not necessarily vital. In addition to the sternocleidomastoid muscle, pectoral muscles and abdominal muscles are part of the respiratory aid muscles and, as such, support inhalation and expiration, but do not perform it automatically.

Diseases

Like all other muscles, the sternocleidomastoid muscle can be affected by paralysis. In the case of a paralysis of the large head turning, the lateral flexion of the head in the direction of the shoulder is no longer possible. A paralysis of the muscle is usually due to a lesion of the accessory nerve. Such a lesion may be due to an accident, for example.

Another cause is the lesion after compression, so damage to the nerve after jamming. Pressure damage can also be due to tumor. In addition, inflammation of the nerve can lead to its proportionate or complete failure. The same applies to malnutrition, poisoning and infections. Also in the context of a polyneuropathy nerve paralysis can occur, which manifest themselves in the form of a paralysis of the large head turner. The causes mentioned so far are in the peripheral nervous system. However, the connection of the muscle to the central nervous system may also be favored by lesions in the central nervous system itself.

Such a lesion may, for example, affect the spinal segments C1 to C3 and may be due to an accident due to spinal cord injury, tumor-related or inflammation-related. Inflammatory paralysis distinguishes between infections caused by microorganisms and autoimmune diseases such as MS. In ALS, the central motor neurons in turn degenerate piece by piece. This phenomenon results in progressive paralysis of all muscles.

The central motor neurons are crucial for any kind of movement. Therefore, their progressive degeneration leads to the progressive failure of any arbitrary, but also reflex motor skills. Overall, a paralysis of the sternocleidomastoid muscle can have many causes and should always be clarified neurologically.

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