In isotonic contractions, where the tension in the muscle remains relatively constant, a load is shifted as the length of the muscle changes. With concentric contraction, the muscle shortens to move a load. An example of this is the contraction of the biceps brachii muscle when a weight from the hand is brought upwards to the body. Eccentric contraction occurs when muscle tension decreases and a muscle lengthens. This type of contraction is observed when the same hand weight is slowly and controlled lowered by the biceps brachii. Once the elastic components of the series are tightened, the muscle begins to create external tension and move a resistant object or load. This is called the contraction phase of contractions. In the gastrocnemic preparation of frog, the charge is the sensor of the recording device; In the body, it is usually a bone. Imagine in a similar way, you lift a weight with an elastic band from a table. When a skeletal muscle has been inactive for a long period of time and then stimulated to contract, all things being the same, the initial contractions produce about half the strength of subsequent contractions. Muscle tension increases in a graduated way that resembles a staircase to some. This increase in tension is called stairs, a condition in which muscle contractions become more effective.
It is also known as the “staircase effect” (Figure 10.4.5). Most of the body`s actions are the result of a combination of isotonic and isometric contractions that work together to achieve a wide range of results. These muscular activities are under the control of the nervous system. A crucial aspect of controlling the nervous system of skeletal muscles is the role of motor units. Neural control initiates the formation of actin-myosin cross bridges, which leads to the shortening of the sarcoma involved in muscle contraction. These contractions extend from the muscle fiber through the connective tissue to pull on the bone, resulting in skeletal movement. The pull exerted by a muscle is called tension, and the amount of force generated by this tension can vary. This allows the same muscles to move very light objects and very heavy objects. For individual muscle fibers, the amount of tension generated depends on the cross-section of the muscle fiber and the frequency of neuronal stimulation. A sufficiently weak electrical stimulus to a muscle does not cause contraction.
By gradually increasing the tension and stimulating the muscle again, we can determine the threshold or the minimum tension necessary to create an action potential in the muscle fiber and create a contraction. The action potential triggers the release of a Ca2+ pulse into the cytoplasm and activates the sliding filament mechanism. At the threshold or at a higher level, a stimulus thus causes a rapid cycle of contraction and relaxation called contractions (Fig. 11.13). During complete tetanus, the concentration of Ca++ ions in the sarcoplasm allows virtually all sarcomeres to form transverse bridges and shorten, allowing a contraction to continue uninterrupted (until the muscle gets tired and can no longer generate tension). The number of transverse bridges formed between actin and myosin determines the amount of tension generated by a muscle. The length of a sarcomere is optimal when the area of overlap between thin and thick filaments is at its maximum. Muscles that are too stretched or compressed do not produce maximum strength.
A motor unit is formed by a motor neuron and all muscle fibers innervated by the same motor neuron. A single contraction is called a contraction. A muscle contraction has a latency period, a contraction phase and a relaxation phase. A graduated muscle response allows a variation in muscle tension. Summation occurs when successive stimuli are added together to create a stronger muscle contraction. Tetanus is the fusion of contractions to create a continuous contraction. Increasing the number of motor neurons involved increases the number of activated motor units in a muscle, which is called recruitment. Muscle tone is the constant contraction of low level that allows posture and stability. A graduated muscle response works as follows: if the fibers are stimulated while a previous contraction is still occurring, the second contraction is stronger. This response is called wave summation because the excitation-contraction coupling effects of successive motor neuron signaling are added or added together (Figure 10.4.4a).
At the molecular level, summation occurs because the second stimulus triggers the release of more Ca++ ions, which become available to activate more cross bridges while the muscle contracts again from the first stimulus. Summation leads to a stronger contraction of the motor unit. If necessary, the maximum number of motor units in a muscle can be recruited at the same time, creating the maximum contraction force for that muscle, but it may not take very long due to the energy required to maintain the contraction. To prevent complete muscle fatigue, not all motor units are usually active at the same time, but some motor units rest while others are active, allowing for longer muscle contractions. The nervous system therefore uses recruitment as a mechanism to effectively use skeletal muscle. A single action potential of a motor neuron creates a single contraction in the muscle fibers innervated by the motor neuron. This isolated contraction is called contraction. A contraction can last between a few milliseconds and 100 milliseconds, depending on the type of muscle fiber. The voltage generated by a single contraction can be measured by a myogram, an instrument that measures the voltage generated over time (Figure 10.4.3). Large motor units deal with simple or “coarse” movements, such as . B.dem moving parts of the body against gravity.
Large motor units of the thigh or back muscles, in which a single motor neuron provides thousands of muscle fibers in a muscle, are representative of this type of activity. The absence of the weak contractions that lead to muscle tone is called hypotension or atrophy and can result from damage to parts of the central nervous system (CNS) such as the cerebellum or the loss of innervations on skelet muscle, as in poliomyelitis. Hypotonic muscles have a flaccid appearance and have functional impairments, such as . B weak reflexes. Conversely, excessive muscle tone is called hypertension, accompanied by hyperreflexia (excessive reflex reactions), often the result of damage to the upper motor neurons of the CNS. Hypertension can occur with muscle rigidity (as seen in Parkinson`s disease) or spasticity, a phasic change in muscle tone, in which a limb “folds” from passive stretching (as in some strokes). The invention of modern dry cells dates back to studies of frog muscle by the Italian anatomist Luigi Galvani (1737-98). He hung isolated frog legs on a copper hook and noticed that they contracted when hit by an iron scalpel. He attributed this to “animal electricity” in the legs. The physicist Alessandro Volta (1745-1827) further studied Galvani`s discovery. He concluded that when two different metals (such as copper hook and iron scalpel) are separated by an electrolyte solution (tissue fluids of a frog), a chemical reaction occurs that creates an electric current. .
