Corpus: Muscle

Muscles are the contractile organs of the human body that enable active movements. They cause a change in shape and/or position of body components (e.g., limbs, blood vessels, or glands). Muscles are made of muscle tissue, one of the 4 basic tissues of the human body.

Three different types of muscle tissue can be distinguished histologically, which are further divided into 2 classes based on their most prominent microscopic aspect.

2.1. Striated Muscle

When viewed under the microscope, striated muscle exhibits fine, characteristic transverse stripes that give it its name. Two types of striated muscles can be differentiated:

• Skeletal muscle: Also called locomotor muscle, is responsible for the body's motor function. It can usually be controlled voluntary. There are over 600 skeletal muscles in the human body.
• Cardiac muscle: It works continuously, cannot cramp, and has its own nervous system, the conduction system. The cardiac muscle can spontaneously depolarize and contains the cardiac isoform of troponin I and troponin T.

The skin musculature can be distinguished as a further type of striated muscle, although most authors classify it as skeletal muscle.

2.2. Smooth Muscle

Smooth muscle lacks cross striation, hence its designation as smooth. It is not under conscious control and is innervated and regulated by the autonomic nervous system. This includes, for example, the intestinal and vascular muscles.

Movement is achieved by exerting force on the rough and fine tissue structures to which the muscles are attached. Muscles can only actively contract — expanding energy — and passively relax. Contraction and relaxation are triggered by electrical impulses (action potentials) transmitted by nerves.

The force of a muscle can be calculated from its physiological cross-section. In case of parallel-fibered muscles, it corresponds to the anatomical cross-section.

The musculature consists mainly of 4 different protein components:

• Troponin
• Actin
• Myosin
• Tropomyosin

The strength of a muscle is directly proportional to its physiological cross-section. However, it should be noted that a relatively small muscle with a small diameter can be quite powerful, for example, in combination with an advantageous lever arrangement. Thus, the relatively small masseter muscle is considered one of the strongest muscles in humans. The strength depends on intramuscular coordination. However, the strength of the entire muscular body is mainly determined by biomechanical principles (e.g., the lever law).

Congenital and acquired muscle diseases are distinguished from myopathies. Acquired muscle diseases can have toxic, inflammatory, endocrine, metabolic, traumatic, or immunological causes, among others. Congenital muscle diseases are genetically determined.

In sports, muscles are exposed to increased stress, which results in an increased risk of injury. The most important muscle injuries include muscle strain and torn muscle fiber.

As target organs of the motor nervous system, muscles depend on intact innervation. Loss of motor nerves (paresis) leads to muscle atrophy.