This dependence of the response amplitude on the frequency of stimulation is called the optimum and the pessimum of the frequency of stimulation. For example, a-motoneuron can send impulses to a muscle with a frequency of 20 pulses, 40 pulses, 50 pulses. Depending on the frequency, the magnitude of the contractile response will be different. This is one of the ways to regulate the strength of muscle contractions lou ferrigno hulk. In reality, all skeletal muscle contractions occur in response to periodic stimulation and are tetanic. Comparison of a single muscle contraction with tetanic shows that the stress that develops during tetanus is higher than the maximum stress recorded during a single; the difference often reaches a ratio of 4 1.
For skeletal muscle is characterized by another kind of activity – contracture. After the cessation of tetanic irritation, the muscle fibers relax at first incompletely – their initial length is restored only after a certain time lou ferrigno jr swat. This phenomenon is called post-tetanic (residual) contracture. The nature of contractures consists in the residual prolonged depolarization of the muscle membrane. In the whole organism, contracture occurs under conditions of pathology and manifests itself in a long-term contracted muscle contraction.
Modes of skeletal muscle contraction. Skeletal muscle is characterized by two main modes of contraction, isometric and isotonic. The isometric mode is manifested when tension increases in the muscle during its activity (force is generated), but due to the fact that both ends of the muscles are fixed, for example, the muscle tries to lift an unbearable weight, it does not shorten. The isotonic mode is manifested when the muscle initially develops tension that can do the job, for example, lift the load. Then the muscle is shortened – it changes its length, keeping the tension equal to the weight of the lifted load. Since isotonic contraction is not exclusively isotonic (elements of isometric contraction occur at the very beginning of muscle contraction), as isometric is not completely isometric either (displacement elements do exist), it is proposed to use the term auxotonic contraction, i.e. mixed the nature.
In real practice, both modes can move from one to another lou ferrigno height. So, when the muscular wall of a hollow organ begins to contract, and the organ contains fluid and the exit from it is closed by the sphincter (heart, bladder), then an isometric situation arises, the pressure inside the hollow organ grows and its size cannot change, since the fluid does not shrink . At a certain amount of pressure, the sphincter opens and the muscle goes into the isotonic mode of contraction for the time of the expulsion of fluid from the organ cavity, when the tension remains constant.
The mechanism of muscle contraction. In order to understand the essence of the process of muscle fiber contraction, it is necessary to recall that each muscle fiber consists of myofibrils, which include longitudinally repeated blocks – sarcomeres, separated from each other by so-called Z-plates (a network of interlaced protein molecules). On top of the myofibrils are covered with a muscle fiber membrane – plasma membrane, which thickness is about 10 nm. The functional unit of the striated muscle myofibrils is the sarcomere lou ferrigno jr. In both directions from the Z-plate stretch thin filaments (filaments), consisting of the protein actin. They are intertwined with thick threads of myosin protein. Myosin filaments form the most optically dense part of the sarcomere, the A-disk lou ferrigno age. The light area in the middle of the A-disk is called zone N. Here there are only myosin threads and there is no actin. In the middle of the H-zone is the M-line, in which there are enzymes (for example, creatinase), which play an important role in energy metabolism. In the areas of mutual overlap, each myosin filament is surrounded by six actin threads, and each actin thread is surrounded by three myosin threads. Small protrusions, called transverse bridges, were found on the myosin filaments. They are located along the myosin filaments in the form of a 2-filament helix. During contraction, these bridges interlock with actin filaments lou ferrigno vs arnold schwarzenegger. The actin filament is formed by two spiral strands of globular actin molecules (G-actin), twisted one around the other. In the longitudinal grooves of the actin helix are thread-like molecules of the tropomyosin protein brent ferrigno. It is a rod-shaped fibrillar protein, the sticks connect end-to-end and form two strands twisted in a spiral around actin. A complex of globular protein molecules under the general name troponin is attached to each tropomyosin molecule.
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The myosin molecule consists of two parts of a long rod-shaped area, which is divided into the neck and tail, and the globular area, represented by two identical heads. The diameter of this part of the molecule is 4 nm, length – 20 nm. If the myosin molecule is treated with proteolytic enzyme trypsin, it will break up into two parts light meromyosin and heavy meromyosin. The latter, mainly formed by the head and neck of the molecule, has enzymatic and actin-binding activity. Myosin filaments are formed by combining individual molecules. The tails of the molecules are oriented to the middle, and the heads are directed in opposite directions and form protrusions on the filament carla ferrigno. Myosin molecules are located in the myosin filament so that the heads are evenly distributed along the entire length (except for a small area in the middle).
When observed in a microscope, it was found that the width of A-disks does not change with a reduction, while I-disks (isotropic) and H areas become narrower, that is, the width of mutual overlap between actin and myosin filaments changes. Proceeding from this, A.F. Huxley in 1954 proposed the theory of the sliding of threads to explain the mechanism of muscle contraction (Fig. 1).
According to the theory, the shortening of the sarcomere occurs due to the active sliding of thin actin threads of relatively thick myosin within the sarcomere. The length of the threads themselves is not changed lou ferrigno jr.. During contraction, each myosin head or cross bridge can connect the myosin thread to the actin one. The tilts of the heads create a joint effort, and a “stroke” occurs, pushing the actin filament to the middle of the sarcomere. The bipolar organization of myosin molecules allows actin filaments to slide in the opposite direction in both halves of the sarcomere.
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The mechanism of this process can be explained by electromechanical conjugation, i.e. transmission of the excitation signal from the membrane to the myofibrils. The key role is played by Ca ions.
hulk actor. In a relaxed state, i.e. at a low concentration of Ca ions, regulatory proteins, tropomyosin and troponin C, block the attachment of myosin transverse bridges to actin filaments. Excitation causes the release of Ca ions from the sarcoplasmic reticulum. This leads to the fact that the concentration of Ca when excited inside the fiber increases and Ca ions combine with the troponin molecule. As a result, the tropomyosin molecule is displaced, which makes it possible to attach the myosin transverse bridges to the actin filaments. This conformational change initiates a series of processes leading to muscle contraction. Therefore, at rest, the proteins troponin and tropomyosin prevent the connection of myosin bridges to actin lou ferrigno vs arnold schwarzenegger. Ca activation leads to inactivation of regulatory proteins, and bridging occurs.
The next question is how a muscle converts chemical energy into mechanical energy. The contractile proteins themselves, actin and myosin, do not possess adenosine triphosphatase (ATPase) activity. However, by contacting actin, the myosin head of the transverse bridge in the presence of Mg ions acquires ATPase activity and catalyzes the breakdown of adenosine triphosphoric acid (ATP). The ATP molecule binds to the active ATPase center of the myosin head and separates it from the actin filament how old is lou ferrigno. Hydrolysis is accompanied by conformational changes in the head of the myosin molecule, translating it into a high-energy state.
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Cyclic activity of cross bridges (rhythmic attachment and detachment), which provides muscle contraction, is possible as long as ATP hydrolysis continues, i.e., ATPase is activated and there is a sufficient concentration of Ca ions. Under normal conditions, muscle relaxation occurs due to the fact that Ca ions released during excitation begin to be actively absorbed by the sarcoplasmic reticulum (SPR). The level of Ca in the sarcoplasm decreases, and tropomyosin begins to prevent the accession of the transverse bridges. The muscle remains relaxed until the next time the membrane depolarizes again.
The muscle mass of the body consists of striated muscle. They are the most voluminous tissue of the human body, reaching 50% of body weight in men. Their activity is controlled by sensory areas of the cerebral cortex and subcortical centers of movement.
The implementation of movements and locomotion is the main function of striated muscles lou ferrigno net worth. In addition, they are the largest generator of biological energy because of their mass and perform the function of the “second heart”, facilitating the return of venous blood to the right heart, propulsively acting on the wall of peripheral veins.
All this further emphasizes the importance of movements for the vital activity of the organism. The striated muscles are subject to age-related involution – mustaches, the proportion of muscle tissue decreases to 27%, muscle tone decreases and the volume of motor activity drops sharply. This is associated with a decrease in the role of muscles as an energy generator and a decrease in the role played by the “second heart”, which leads to edema, stagnant.
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The coefficient of performance (efficiency) of muscular work is very high – 37 – 65%. The energy of muscle contraction is spent on changes in the spatial relationship of contractile proteins.
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Muscle cell The most significant protein fractions of the muscle cell are
– proteins of the myogen group – mainly glycolysis enzymes and myoalbumin;
– myoglobin – red chromoprotein, hemoglobin “sibling”, which determines the red color of muscle tissue; it contains in its composition a free, easily oxidized iron atom, binds and transports oxygen within the cytoplasm of the cell;
– globulins – enzymes and spare proteins that can be converted into myofibrillar contractile proteins during training;
– myofibrillary proteins myosin and actin, enzymes, etc .;
– nuclear proteins – nucleoproteins.
The most significant of the energy fractions are water-soluble nitrogenous compounds, such as ATP (0.25 – 0.4% of the cell’s dry residue) and creatinine phosphate (CrF) – 0.4 – 1.0% of the cell’s dry residue.
The most important nitrogen-free energy fractions include glycogen, which can be up to 2% of the dry residue of the cell. It can be in a free and bound state, and physical training significantly increases the amount of free glycogen, which significantly increases the contractility of myofibrils lou ferrigno now. In addition, fats, cholesterol and mineral salts play a role in the mechanism of muscle contraction.
Of the other inclusions in the cytoplasm of the cell, attention is drawn to the mitochondria (mitochondrial cristae). Molecular biologists believe that they are aerobic ATP synthesis and protein synthesis. The electrochemical potential of hydrogen ions is very high in the membranes of mitochondrial cristae, which makes it possible to consider them as an additional source of intracellular energy, ensuring the efficiency of electrolyte transfer through the cell membrane.
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Located on the cell membrane of the myocyte oppositely charged metal ions (potassium, calcium, magnesium, sodium, silicon, etc.) determine the work of “ionic membrane pumps.”
Muscle fibers Muscle cells are structurally organized into muscle fibers. Two types of muscle fibers are well known (we mentioned above). These are white muscle fibers, also called “fast”, and red muscle fibers – “slow”.
White fibers are the first to respond to the team’s activities from the central nervous system (CNS), playing the role of a kind of starter. Anaerobic processes of ATP resynthesis are predominantly developed in them, the main way of energy supply is anaerobic glycolysis, which flows into them at a higher rate. Also, white fibers are characterized by a relatively low number of mitochondria and myoglobin, a high activity of myosin triphosphatase, a high buffer capacity, and a well-developed AB.