Muscle fibers form the basic cellular and working unit of all skeletal muscles in humans. They can range in length from less than 1 mm to 50 cm with a thickness of about 0.01 to 0.2 mm. Several muscle fibers become muscle fiber bundles, which – also combined into several – form the muscle in its entirety. The multinucleated muscle fibers can respond to electrical nerve stimuli with contraction or relaxation. There are several different types of muscle fibers that differ in responsiveness, fatigue behavior, and energy metabolism.
What are muscle fibers?
Muscle fibers are the striated muscle cells that make up skeletal muscle. Other common names are muscle fiber cells or myocytes. Multinucleated muscle fibers reach a length of a few mm to 50 cm and have a diameter of 0.01 to 0.2 mm. Several parallel aligned muscle fibers are combined into muscle fiber bundles and enclosed by a membrane. The actual contractile function is performed by myofibrils, of which several hundred are present in each muscle fiber. Contraction of the muscle is achieved by the interlocking of actin and myosin filaments arranged parallel to each other, without them shortening themselves. In order to fulfill different tasks of the muscles, such as rapid strength with minimal
reaction time or endurance capacity, different types of muscle fibers can be distinguished, which differ in their reaction time and in their energy metabolism. Fast-reacting muscle fibers that fatigue quickly operate in the anaerobic range, while muscle fibers characterized by longer reaction times operate predominantly in the aerobic continuous mode. The distribution of the different types of muscle fibers within a muscle is largely genetically determined and almost certainly cannot be altered by strength and endurance training.
Anatomy and structure
Muscle fibers are the cellular building blocks of striated skeletal muscle. They are syncytial, a fusion of many single cells whose cytoplasm and nuclei are retained and become part of the new large cell. There can be up to 40 nuclei per mm in a muscle fiber. Each individual muscle fiber contains several hundred myofibrils composed of sarcomeres only about 2 µm long. The sarcomeres resemble tiny “compartments” containing parallel aligned actin myofilaments and, each laterally offset, myosin motor proteins. They are so regularly arranged one behind the other that the typical transverse striation can be seen in polarized light. In a muscle fiber 10 cm long, cica 40,000 sarcomeres are lined up in a row. On receipt of an appropriate action potential, the actin and myosin filaments virtually slide into each other, causing the muscle cell to shorten. Each myofibril, together with its associated organelles, is enveloped by a membrane, the sarcolemma. To increase mechanical strength, myofibrils also contain connective tissue fibers that are attached to the basement membrane. A very important function is performed by so-called muscle spindles or proprioceptors, which are intercalated between the muscle fibers and inform the central nervous system (CNS) about the momentary contraction state of the muscle via afferent nerve fibers.
Function and tasks
Skeletal muscles can only achieve their main functions, such as stabilizing the body, moving individual limbs, and delivering heat to the body, through the interaction of their individual muscle fibers. To ensure that all muscle fibers shorten almost simultaneously during a muscle contraction, all muscle fibers must receive the action potential for contraction (almost) simultaneously, because otherwise there would be punctual muscle tension and relaxation. The sarcolemma is responsible for transmitting the contraction command to all the muscle cells in a given muscle, and its many invaginations in the myofibrils provide the anatomical conditions for this. The extremely fast-acting FT (fast twitch) fibers, which appear pale because of their low content of myoglobin and mitochondria, are also called white muscle fibers. They develop high strength potential but fatigue quickly. The body needs this type of muscle fiber for escape or attack reactions and for high jumping or punching power.In contrast to this are the so-called slower ST fibers (slow twitch), which are also called red muscle fibers because of their high content of myoglobin and mitochondria. They develop less power, but work in the aerobic zone and fatigue much more slowly. In the event of hypothermia of the body, skeletal muscle cells can be induced by the autonomic nervous system to shiver (muscle tremors), which cannot be controlled voluntarily, ultimately converting glucose to heat and allowing body temperature to rise again.
Diseases and ailments
Diseases and disorders associated with muscle fibers can occur either as a result of direct disease and inflammation at the muscle fibers, or can be caused by lesions at the innervating nerves or at their higher-level nerve nodes. In the former case, it is a variety of possible myofibrillar myopathies, and in the latter case, it is a neuromuscular disease. Direct mechanical damage to muscle fibers can result from a muscle fiber tear if the muscle is subjected to excessive stress at specific points. Usually, several muscle fibers or even entire muscle fiber bundles are affected. Myofibrillar myopathies manifest as progressive muscle weakness and wasting, which can be attributed to one or more genetic defects. While muscle tremor in the form of shivering can be considered a normal process, muscle tremor (tremor) can also be triggered by a wide variety of neurological diseases. A distinction is made between a resting, action, movement or intention tremor. The different types of tremor provide initial clues to the nature of possible lesions present in the brain. A serious impairment of the muscle fibers can be caused by a disease of the motor neurons. Either the first (primary) motoneurons, whose axons originate in the motor cortex, or the second motoneurons, which originate in the spinal cord, are then affected. Amyotrophic lateral sclerosis (ALS) belongs to the group of motor neuron diseases. It announces itself by muscle weakness or muscle stiffness and takes a variably progressive course.
