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Which Type of Stimuli Do Skeletal Muscles Need to Contract

Skeletal muscle is rarely completely relaxed or flaccid. Even if a muscle does not produce movement, it is contracted in small amounts to maintain its contractile proteins and produce muscle tone. The tension generated by muscle tone allows the muscles to continuously stabilize the joints and maintain posture. A contraction can last from a few milliseconds to 100 milliseconds, depending on the type of muscle. The voltage generated by a single contraction can be measured by a myogram that creates a graph that illustrates the amount of voltage generated over time. In combination with an electrical signaling diagram, the myogram shows three phases that each contraction goes through. The first period is the latency period during which the action potential propagates along the membrane and Ca2+ ions are released from the sarcoplasmic reticulum (SR). At this point, no tension or contraction is generated, but the conditions of contraction are defined. This is the phase where excitation and contraction are coupled, but contraction has not yet taken place. The contraction phase occurs after the latency period, when calcium is used to trigger the formation of transverse bridges.

This period lasts from the beginning of the contraction to the point of maximum stress. The last phase is the relaxation phase, where the tension decreases when the contraction stops. Calcium is pumped out of the sarcoplasm, back into the SR, and cycling across the bridge stops. The muscle returns to a resting state. There is a very short refractory period after the relaxation phase (check the previous material on the physiology of a neuromuscular compound) A sport like arm wrestling depends on muscle contractions. Arm wrestlers must contract the muscles of their hands and arms and contract them to resist the opposing power of their opponent. The wrestler, whose muscles can contract with more force, wins the match. • The contraction of skeletal muscle is achieved by sliding filaments of actin and myosin As already mentioned, when a skeletal muscle fiber contracts, the myosin heads attach to the actin to form transverse bridges, followed by thin filaments that slide over the thick filaments while the heads pull the actin, which leads to the shortening of the sarcomere, creating the tension of muscle contraction. Transverse bridges can only form where thin and thick filaments overlap; Thus, the length of the sarcomaer has a direct influence on the force that occurs when the sarcoma shortens. This is called the length-tension relationship. The propagation of an action potential and the depolarization of the sarcolemma include the excitation part of the excitation-contraction coupling, the connection of electrical activity and mechanical contraction.

The structures responsible for coupling this excitation to contraction are the T tubules and the sarcoplasmic reticulum (SR). The T tubules are extensions of the sarcolemma and thus carry the action potential along their surface and conduct the depolarization wave inside the cell. The T-shaped tubules form triads with the ends of two SRs called terminal tanks. RSs, and in particular terminal tanks, contain high concentrations of Ca2+ ions inside. When an action potential moves along the tubule T, nearby terminal cisterns open their voltage-dependent calcium release channels so that Ca2+ can diffuse into the sarcoplasm. The influx of Ca2+ increases the amount of calcium available to bind to troponin. Troponin, which is bound to Ca2+, undergoes a conformational change that causes tropomyosin to move onto the actin filament. When tropomyosin moves, the site of binding of myosin to actin is revealed.

This continues as long as excess Ca2+ is available in the sarcoplasm. When free Ca2+ is no longer available to bind to troponin, the contraction stops. In order to restore the Ca2+ mirror to a resting state, excess Ca2+ is actively transported into the SR. In a resting state, Ca2+ remains in the SR, keeping the level of sarcoplasmic Ca2+ low. Low levels of sarcoplasmic calcium prevent unwanted muscle contraction. The size of a motor unit determines its function. A small motor unit, consisting of a motor neuron and only a few muscle fibers, allows a very fine motor control of a muscle. For example, extraocular eye muscles have thousands of muscle fibers, each with 5-10 fibers, provided by a single motor neuron; This allows for exquisite control of eye movements, allowing both eyes to quickly focus on an object.

Small motor units are also involved in the many fine movements of the fingers and thumb of the hand for gripping, texting, etc. The process by which a signal is transmitted to a neuromuscular compound is shown in figure (PageIndex{2}). The sequence of events begins when an action potential is initiated in the cell body of a motor neuron and the action potential propagates along the neuron`s axon to the neuromuscular connection. Once the action potential reaches the end of the axonal termination, it causes the neurotransmitter acetylcholine (ACh) from synaptic vesicles in the axonal termination. ACh molecules diffuse through the synaptic cleft and bind to receptors in muscle fibers, initiating muscle contraction. Muscle contraction is initiated by the depolarization of the sarcolemma, which is caused by the entry of sodium ions through the sodium channels associated with ACh receptors. As already mentioned, the thick and thin filaments of myofibrils are arranged in units called sarcomeres. The sarcoma is the fundamental contractile unit of myofibril. Z lines separate each sarcomere.

The A stripes, which are located in the middle of each sarcomere, contain the thick filaments that can overlap with thin filaments. The A band divides further into the H zone, which does not contain thin filaments. The distinctive M line divides the H zone and is used to connect the central parts of the thick filaments. On both sides of the A strip are the I bands, which contain both the thin filaments and the Z line running in the middle of each I band. In the muscle fiber, the T-tubules are found next to the terminal cisterns of an internal membrane system derived from the endoplasmic reticulum, called the sarcoplasmic reticulum (SR), which is a reserve of calcium ions. Stimulation of the muscle fiber causes a wave of depolarization to descend into the T-tube and release the SR calcium ions into the sarcoplasm. Calcium is pumped into the SR to lower the concentration of calcium ions in the sarcoplasm to relax the muscle (disable contraction). Skeletal muscles are stimulated by nerve impulses that depolarize the muscle. However, not all muscle fibers in the muscle fiber are necessarily activated at the same time. Sometimes a subset of muscle fibers is activated, depending on the strength needed. FO fibers are sometimes called intermediate fibers because they have properties that lie between fast fibers and slow fibers.