Smooth muscles can be divided into two subgroups: one unit and several units. Individual smooth muscle cells can be found in the intestines and blood vessels. Since these cells are connected to each other by lacunar junctions, they can contract as functional syncytium. Individual smooth muscle cells contract myogenically, which can be modulated by the autonomic nervous system. The muscle fiber creates tension through the action of actin and the cross-bridge cycle of myosin. Under tension, the muscle can lengthen, shorten or remain the same. Although the term contraction implies a shortening, compared to the muscles, these are muscle fibers that create tension with the help of motor neurons. Different types of muscle contractions occur and they are defined by changes in muscle length during contraction. University of California, San Diego: “Muscle Physiology: Types of Contractions.” In isotonic contraction, the tension in the muscle remains constant despite a change in muscle length. [1] [3] [4] [5] This occurs when the contraction force of a muscle corresponds to the overall load on the muscle. The strength of skeletal muscle contractions can be roughly divided into contractions, summation and tetanus. A contraction is a unique cycle of contraction and relaxation generated by an action potential in the muscle fiber itself.
[26] The time between a motor nerve stimulus and the subsequent contraction of the innervated muscle is called the latency period, which typically lasts about 10 ms and is caused by the time it takes for the nerve action potential to propagate, the chemical transmission time to the neuromuscular transition, and then the subsequent stages of excitation-contraction coupling. [27] Unlike isotonic contractions, isometric contractions generate force without changing the length of the muscle. This is typical of the muscles of the hands and forearm: the muscles do not change length and the joints do not move, so the strength is sufficient for taking. An example is when the muscles of the hand and forearm grasp an object; The joints of the hand do not move, but the muscles generate enough force to prevent the object from falling. The end of the transverse bridge cycle (and leaving the muscle in the latch state) occurs when myosin`s light-chain phosphatase removes phosphate groups from myosin heads. Phosphorylation of 20 kDa myosin light chains is well correlated with the speed of shortening of smooth muscles. Meanwhile, there is a rapid increase in energy consumption, as measured by oxygen consumption. A few minutes after the start of induction, calcium levels decrease significantly, phosphorylation of light chains 20 kDa-myosin decreases, and energy use decreases; However, the strength of the tonic smooth muscles is preserved.
During muscle contraction, rapidly circular transverse bridges form between activated actin and phosphorylated myosin, which generate strength. Power maintenance is thought to result from dephosphorylated “locking bridges” that slowly become cyclical and maintain power. A number of kinases such as rho kinase, DAPK3 and protein kinase C are thought to participate in the prolonged phase of contraction, and the flow of Ca2+ may be significant. Concentric contractions. This type of contraction occurs when your muscle is actively shortened. Your muscle tenses when you activate it to lift something heavier than normal, which creates tension. If another muscle action potential needs to be created before the complete relaxation of a muscle contraction, the next contraction simply adds to the previous contraction, thus creating a summation. Summation can be obtained in two ways:[28] Frequency summation and multiple fiber summation. In frequency summation, the force exerted by skeletal muscle is controlled by varying the frequency with which action potentials are sent to muscle fibers. The action potentials do not arrive synchronously on the muscles, and during a contraction, a fraction of the fibers in the muscle fire at a certain point in time.
In a typical case, when people strain their muscles as hard as possible, about a third of the fibers in each of those muscles fire immediately[citation needed], although this ratio can be influenced by various physiological and psychological factors (including Golgi`s tendon organs and Renshaw cells). This “low” level of contraction is a protective mechanism to prevent avulsion of the tendon – the force generated by a contraction of 95% of all fibers is enough to damage the body. When the central nervous system sends a weak signal in multifiber summation to contract a muscle, the smaller motor units, which are more excitable than the larger ones, are stimulated first. As the signal strength increases, more power units are excited in addition to the larger drive units, with the larger drive units having up to 50 times more contractile force than the smaller ones. As more and larger motor units are activated, the muscle contraction force becomes stronger and stronger. A concept known as the size principle allows for a gradation of muscle strength during a small contraction in small steps, which then gradually increase when larger amounts of strength are needed. Concentric and eccentric muscle contractions. These two types of contraction often go hand in hand. A concentric muscle contraction will help you lift something heavy.
We often talk about positive work. concentric: (Of a movement), in the direction of the contraction of a muscle. (Z.B. Extension of the forearm on the elbow joint in contraction of the triceps and other extensor muscles of the elbow. In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts. [8] This happens when the force generated by the muscle exceeds the load that counteracts its contraction. The sliding filament theory describes a process used by muscles for contraction. It is a cycle of repetitive events that cause a thin filament to slide over a thick filament, creating tension in the muscle. It was developed independently in 1954 by Andrew Huxley and Rolf Niedergerke, as well as Hugh Huxley and Jean Hanson.[21] [22] [23] Physiologically, this contraction on the sarcoma is not uniform; The central position of thick filaments becomes unstable and can move during contraction. However, the action of elastic proteins such as titin is believed to maintain a uniform tension on the sarcomere, pulling the thick filament into a central position. [24] Eccentric contraction leads to the lengthening of a muscle. Such contractions slow down the muscle joints (which act as “brakes” for concentric contractions) and can change the position of the load force.
These contractions can be both voluntary and involuntary. During an eccentric contraction, the muscle elongates under tension due to an opposite force greater than the force generated by the muscle. Instead of working to pull a joint towards muscle contraction, the muscle acts to slow down the joint at the end of a movement or control the repositioning of a load. With the exception of reflexes, all skeletal muscle contractions occur as a result of conscious exertion that comes from the brain. The brain sends electrochemical signals via the nervous system to the motor neuron, which innervates several muscle fibers. [17] In some reflexes, the contraction signal may come from the spinal cord through a feedback loop with gray matter. Other actions such as locomotion, breathing and chewing have a reflex aspect: contractions can be triggered both consciously and unconsciously. Isotonic: From or with muscle contraction against resistance, in which the length of the muscle changes.
The antonym is isometric. Isotonic movements are either concentric (muscle shortening) or eccentric (muscle extensions). Muscle contraction is the tightening, shortening or lengthening of the muscles when you perform an activity. This can happen when you hold or pick up something, or when you stretch or exercise with weights. Muscle contraction is often followed by muscle relaxation when the contracted muscles return to their normal state. An isometric contraction of a muscle creates tension without changing its length. [1] [3] [4] [5] An example can be found when the muscles of the hand and forearm grasp an object; The joints of the hand do not move, but the muscles generate enough force to prevent the object from falling. .