Skin muscles are striated muscles between fascia and skin, which are rather underdeveloped in humans. The main function of the muscle form is skin movement, in humans mainly facial expression. Like all other muscles of the body, skin muscles can be affected by paralysis, such as peripheral facial nerve palsy.
What is a skin muscle?
Striated muscles owe their name to their typical pattern. In addition to skeletal muscle, this type of muscle includes cardiac muscle. One form of skeletal muscle is skin muscle. Skin muscles do not move the skeleton, but are located between the skin and fascia. Skin muscles thus have no contact with the skeleton. This distinguishes them from most other skeletal muscles, for which the skeleton serves as an attachment. The movement of the skin is one of the main tasks of the skin muscles. In humans, the skin muscles are much less developed than in most animals. The largest skin muscle is known to be the platysma, which covers almost the entire area of the anterior neck. The skin muscles are an essential part of the mimic musculature. Unlike animals, humans do not possess any skin muscles in the trunk region.
Anatomy and structure
The low development of human skin musculature is due to human anatomy. Humans do not need to be able to move their skin separately to fend off tiny creatures such as insects. They use their perfectly mobile hands and arms to chase insects away from their bodies. Since most animals have only limited ability to do this, they have highly developed skin muscles. Their skin muscles chase insects away from their bodies by twitching. In humans, the platysma and portions of the mimic musculature are among the most notable skin muscles. Like all striated muscles, skin muscles consist of homogeneous functional units. These sarcomeres carry the myofilaments actin and myosin with some overlap. Light I bands consist essentially of actin. Darker A bands contain mainly myosin bundles.
Function and tasks
Skin muscles move the skin through their contractions. Like all muscles, skin muscles are connected to the central nervous system by nerve pathways and receive constant information from this control center. Skin muscles contract predominantly involuntarily. This means that they play an essential role in reflex motor function. Sensory cells report to the central nervous system of various animals, for example, touches of insects or similar organisms on the skin. This information reaches the central nervous system in the form of bioelectric excitation via afferent nerve pathways. In the spinal cord, the excitation is switched to efferent pathways and transported to the skin muscle of the affected area. Via the motor end plate of the muscle, the action potential is transmitted to the respective muscle and stimulates the fibers to contract. The skin at the corresponding part of the body moves in response. If the stimulus at the beginning of the reflex arc has been caused by a settled insect, the insect is shaken off by the reflex skin movement. For the human organism, this type of skin movement plays only a minimal role. Instead of reflexive movements, human skin muscles rather perform arbitrary movements. For example, contraction of the platysma pulls down the lower jaw, the corners of the mouth, and the lower lip. If the lower jaw is held fixed, contraction of the platysma tightens and shortens the neck skin. Because the skin muscles of humans perform important tasks within facial expressions, they can be said to have, in part, a communicative and expressive function. Facial expressions are the most natural form of human expression. Studies on newborns have shown how deeply an understanding of mimic communication is anchored in the human organism.
Diseases
Peripheral facial nerve palsy corresponds to paralysis in the supply area of the facial nerve. This nerve supplies the platysma, among other organs. When paralysis occurs, the skin muscle slackens accordingly. The lower jaw, the corners of the mouth and the lower lip can no longer be pulled down by the affected person. Since the nerve supplies many other muscles of the mimic musculature in addition to the platysma, damage to the nerve structure on the affected side can result in complete paralysis of the facial expression.In addition to incomplete eyelid closure in the sense of the Bell’s phenomenon, drooping corners of the mouth and a deformed nasolabial fold may be indicative of facialis-related paresis. The muscles of the forehead area are also affected by peripheral facial nerve paresis. Thus, the patient’s forehead often appears smooth and unnaturally lacking in wrinkles. In most cases, complete facial nerve palsy is preceded by transection in its core areas or peripheral nerve course. Incomplete facial nerve palsy may also be caused by bacterial or autoimmunologic inflammation in the peripheral and central nervous systems. Nerve compression from tumors or trauma can also damage the facial nerve to the extent that facial nerve palsy results. Also conceivable are stroke-induced paralyses of the mimic muscles or platysma. Weaknesses in the skin muscles do not necessarily have to be due to actual paralysis, but can also be caused by degenerative muscle diseases such as myopathy. Myopathies come in different forms. While alcohol myopathy corresponds to intoxication, many other forms of the phenomenon are due to genetic bases such as mutations. In the context of myopathies, the mimic musculature is usually not affected by symptoms of weakness in isolation. The platysma can also gain pathological relevance apart from paralysis and myopathies. Like any other muscle, the tissue can become inflamed, for example. Radiating pain in the corresponding area is the result. Such inflammations are often caused by overload or incorrect strain. Muscle fiber tears at the platysma, on the other hand, are a rather rare occurrence.
