Sternocleidomastoid muscle

Of the Sternocleidomastoid muscle is also as big head turner known and is one of the ventrally superficial neck muscles that lie between the sternum, base of the skull and collarbone. The main task 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 impair the function of the muscle and even lead to paralysis.

What is the sternocleidomastoid muscle?

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

It makes up the superficial layer of the belly-valued neck muscles. The skeletal muscle lies on either side of the neck. So every person actually has two sternocleidomastoid muscles. The right side is called the sternocleidomastoid muscle and the left side is called the sternocleidomastoideus sinister muscle.

Each of these muscles has two different heads: a lateral head and a ventral head. Both of the heads run diagonally across the side of the neck. Where the muscle is attached to the collarbone, it is sometimes widened, causing the trapezius muscle to widen on the arm side of the associated bone. In individual cases the two muscles are completely fused together, but both function perfectly.

Anatomy & structure

The sternocleidomastoid muscle is 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 lateral caput of the muscle arises from the upper edge and anterior surface of the medial clavicle and from there runs in fleshy aponeurotic fibers approximately vertically upwards. The medial head of the muscle arises from the anterior surface of the sternum manubrium on the sternum and pulls with one of its fibers in the cranial, lateral and dorsal directions. Between the respective origins of the two muscle heads there is a triangular gap on each side. Only in the further course does the muscles join in the middle of the neck, creating a thick, rounded muscle belly.

The sternocleidomastoid muscle attaches to the lateral mastoid process and thus to the temporal bone. Through its course, the muscle divides the lateral neck area into triangles. The lateral neck triangle is also called the Trigonum colli laterale. The front triangle of the neck is the trigonum colli mediale.

Function & tasks

The great head turner has several tasks in terms of moving the body. Via motor efferent nerves, commands for contraction reach the motor end plate of the muscle, which originate from the central nervous system. With a one-sided contraction of the muscle, the head tilts sideways towards the shoulder. So there is lateral flexion or lateral bending of the head in the shoulder direction.

The opposite lateral extension, which is also carried out by the sternocleidomastoid muscle and corresponds to a slight lateral extension of the head, lies on the same axis of movement. The extension corresponds approximately to a reclination, as it is directed backwards. In addition, when the muscle contracts, the head turns to the opposite side, which means that the skeletal muscle is also involved in the rotation of the head. When the head is fixed, the two head turner change their function and together become the auxiliary breathing muscles.

When the head is fixed, the right and left sternocleidomastoid muscles change the volume of the chest cavity through their contraction and relaxation and thus enable increased breathing. Unlike the actual respiratory muscles, however, they are not absolutely essential. In addition to the sternocleidomastoid muscles, the chest muscles and abdominal muscles are part of the auxiliary breathing muscles and, as such, support inspiration and expiration, but do not perform them automatically.

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Diseases

Like all other muscles, the sternocleidomastoid muscle can be affected by paralysis. If the head is paralyzed, it is no longer possible to flex the head laterally towards the shoulder. Paralysis of the muscle is usually due to a lesion of the accessory nerve. Such a lesion can be caused by an accident, for example.

Another cause is the lesion after compression, i.e. damage to the nerve after it has been blocked. Pressure damage can also be caused by the tumor. In addition, inflammation of the nerve can lead to its partial or complete failure. The same goes for 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 turn. The causes mentioned so far are in the peripheral nervous system. The connection of the muscle to the central nervous system can, however, also be favored by lesions in the central nervous system itself.

Such a lesion can affect the spinal cord segments C1 to C3, for example, and can be caused by an accident, a spinal cord infarction, a tumor or an inflammation. In the case of inflammation-related paralysis, a distinction must be made between infections caused by microorganisms and autoimmune diseases such as MS. In ALS, the central motor neurons degenerate bit by bit. This phenomenon leads to a progressive paralysis of all muscles.

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