The term contraction (Latin contrahere = to contract) is used to describe the process by which a muscle either shortens or increases its tension. There are different types of contractions with different functional significance.
What is the coronary contraction?
The term contraction (Latin contrahere = to contract) is used to describe the process by which a muscle either shortens or increases its tension. A muscle can produce two mechanical effects on the human skeleton. Either it stabilizes joints and areas of the body or it moves bones. For this to succeed, the force generated in the muscle must be transmitted to the bone. This task is performed by the tendons. The overall muscle consists of several subunits, such as muscle bundles, muscle fiber bundles, muscle fibers and, at the lowest level, the muscle cells, also called fibrils. In addition to cell organelles, these contain thousands of serially connected sarcomeres, the smallest functional units of a muscle. Each sarcomere can contract and thus develop force. The total force of a muscle is therefore the sum of the force generation of the sarcomeres involved. The functional center of each sarcomere is the actin-myosin complexes. Actin and myosin are proteins connected by cross-bridges. The thinner actin strands are attached to the outer boundaries of the sarcomere, while the thicker myosin molecules each lie between two actin filaments. When a nerve impulse reaches the muscle, calcium is released and the sarcomeres shorten or tighten under energy consumption. The myosin units pull the actin units toward the center of the sarcomere by a rowing motion of their heads. The effect on the whole muscle depends on how many sarcomeres are made to contract.
Function and task
Contractions cause 2 effects in the muscle. First, force is developed, and second, heat is generated. Muscle has poor mechanical efficiency. Approximately 80% of the energy expenditure in muscular work goes to heat generation, and only 20% to force generation. However, the heat produced makes an important contribution to regulating body temperature and optimizing metabolic processes. The force developed by the contraction is transmitted via the tendons to the attachments on the bone and leads either to movement in the joints involved or to increased tension. Whether a movement occurs depends on the goal pursued in the movement programs in the brain and transmitted to the muscles via nerve impulses. If the goal is the execution of movement sequences, all muscle chains that are necessary for the adequate action are automatically switched on, inhibiting influences are switched off. If a certain position is to be held, the command for the muscles is to stabilize body parts and joints. An important role in this process is played by the interaction between agonists (acting muscles) and their counterparts (antagonists). Thus, 3 possible types of contractions occur. In isometric contraction, the tension in the muscle increases, but no movement occurs because the antagonists or an external resistance do not allow it. Ideally, the agonists and their antagonists work together. This form of muscle work is important for all static loads, for example to stabilize the back or joints. Concentric contractions cause movement in the joint by shortening the active muscle and allowing the antagonists to move. This form of muscle work is the mechanically lightest and the most beneficial for stimulating muscle metabolism. Eccentric contractions occur when the muscle controls movements in which it is lengthened. It has to do a lot of mechanical work as it contracts while the number of crossbridges between actin and myosin is decreasing. All braking activities belong to this form of contraction.
Diseases and disorders
A typical functional disorder of muscle and contraction is muscle weakness (atrophy). It usually occurs because a muscle is not used enough (inactivity atrophy). Typically, this phenomenon is observed in bedridden patients or when limbs are immobilized (plaster cast). The contractile force of the muscles and the muscle cross-section decrease, and function is impaired to a greater or lesser extent depending on the severity and duration.Another trigger for inactivity is injury or other irritation, for example painful irritation of tendon insertions. In this case, the brain switches on protective programs that cause muscles to be used less. Inactivity atrophies can be regenerated if they do not persist for too long. The ability of muscles to contract depends on the nerve stimuli they receive from the brain. If these are absent, no contraction can occur. Nerve conduction may be impaired either centrally (brain or spinal cord) or peripherally (peripheral nervous system), or damaged entirely. The result is incomplete or complete paralysis. Causes can be injuries (paraplegia), herniated discs or inflammatory (MS, poliomyelitis) and metabolic diseases (polyneuropathy, amyotrophic lateral sclerosis). Diseases that impair contractility and have their cause in the muscle itself or at the transition between nerve and muscle are summarized under the term muscular dystrophy. Common to all is the symptomatology, possibly visible atrophy, increasing weakness and rapid fatigability. In addition, as the disease progresses, there is often pain during movements, as the effort for the weakened muscles becomes greater. Another typical feature of muscular dystrophies is the progressive remodeling of muscle tissue. The contractile elements are increasingly replaced by connective tissue, causing not only increasing weakness but also progressive immobility (contracture). These diseases are caused by genetic defects that cause irreparable damage to muscle cells, resulting in a severe reduction or complete blockage of protein formation in the muscle. Muscular dystrophies are rare diseases that to date have no cure.
