Inhibition of the Actin Filament by the Troponin Tropomyosin Complex. A pure actin filament without the presence of the troponin-tropomyosin complex (but in the presence of magnesium ions and ATP) binds instantly and strongly with the heads of the myosin molecules. Then, if the troponin-tropomyosin complex is added to the actin filament, the binding between myosin and actin does not take place. Therefore, it is believed that the active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin tropomyosin complex. Consequently, the sites cannot attach to the heads of the myosin filaments to cause con traction. Before contraction can take place, the inhibitory effect of the troponin-tropomyosin complex must itself be inhibited.

Activation of the Actin Filament by Calcium Ions. In the presence of large amounts of calcium ions, the inhibitory effect of the troponin-tropomyosin on the actin filaments is itself inhibited. The mechanism of this inhibition is not known, but one suggestion is as follows: When calcium ions combine with troponin C, each molecule of which can bind strongly with up to four calcium ions, the troponin complex supposedly undergoes a con formational change that in some way tugs on the tropomyosin molecule and moves it deeper into the groove between the two actin strands. This action “uncovers” the active sites of the actin, thus allowing these active sites to attract the myosin cross-bridge heads and cause contraction to proceed. Although this mechanism is hypothetical, it does emphasize that the normal relation between the troponin-tropomyosin complex and actin is altered by calcium ions, producing a new condition that leads to contraction.

Interaction of the “Activated” Actin Filament and the Myosin Cross-Bridges—The “Walk-Along” Theory of Contraction. As soon as the actin filament is activated by the calcium ions, the heads of the cross-bridges from the myosin filaments become attracted to the active sites of the actin filament, and this, in some way, causes con traction to occur. Although the precise manner in which this interaction between the cross-bridges and the actin causes contraction is still partly theoretical, one hypothesis for which considerable evidence exists is the “walk along” (or “ratchet”) theory of contraction.

Figure 1 demonstrates this postulated walk-along mechanism for contraction. The figure shows the heads of two cross-bridges attaching to and disengaging from active sites of an actin filament. When a head attaches to an active site, this attachment simultaneously causes pro found changes in the intramolecular forces between the head and arm of its cross-bridge. The new alignment of forces causes the head to tilt toward the arm and to drag the actin filament along with it. This tilt of the head is called the power stroke. Immediately after tilting, the head then automatically breaks away from the active site. Next, the head returns to its extended direction. In this position, it combines with a new active site farther down along the actin filament; the head then tilts again to cause a new power stroke, and the actin filament moves another step. Thus, the heads of the cross-bridges bend back and forth and step by step walk along the actin filament, pulling the ends of two successive actin filaments toward the center of the myosin filament.

Fig1. The “walk-along” mechanism for contraction of the  muscle.

Each one of the cross-bridges is believed to operate independently of all others, each attaching and pulling in a continuous repeated cycle. Therefore, the greater the number of cross-bridges in contact with the actin filament at any given time, the greater the force of contraction.

ATP as the Energy Source for Contraction—Chemical Events in the Motion of the Myosin Heads. When a muscle contracts, work is performed and energy is required. Large amounts of ATP are cleaved to form ADP during the contraction process, and the greater the amount of work performed by the muscle, the greater the amount of ATP that is cleaved; this phenomenon is called the Fenn effect. The following sequence of events is believed to be the means by which this effect occurs:

 1. Before contraction begins, the heads of the cross bridges bind with ATP. The ATPase activity of the myosin head immediately cleaves the ATP but leaves the cleavage products, ADP plus phosphate ion, bound to the head. In this state, the conformation of the head is such that it extends perpendicularly toward the actin filament but is not yet attached to the actin.

 2. When the troponin-tropomyosin complex binds with calcium ions, active sites on the actin filament are uncovered and the myosin heads then bind with these sites, as shown in Figure1.

 3. The bond between the head of the cross-bridge and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge and pro viding the power stroke for pulling the actin filament. The energy that activates the power stroke is the energy already stored, like a “cocked” spring, by the conformational change that occurred in the head when the ATP molecule was cleaved earlier.

 4. Once the head of the cross-bridge tilts, release of the ADP and phosphate ion that were previously attached to the head is allowed. At the site of release of the ADP, a new molecule of ATP binds. This binding of new ATP causes detachment of the head from the actin.

5. After the head has detached from the actin, the new molecule of ATP is cleaved to begin the next cycle, leading to a new power stroke. That is, the energy again “cocks” the head back to its perpendicular condition, ready to begin the new power stroke cycle.

6. When the cocked head (with its stored energy derived from the cleaved ATP) binds with a new active site on the actin filament, it becomes uncocked and once again provides a new power stroke.

Thus, the process proceeds again and again until the actin filaments pull the Z membrane up against the ends of the myosin filaments or until the load on the muscle becomes too great for further pulling to occur.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم حفظ النظام ينهي استعداداته الخاصة بحفل تخرج طالبات الجامعات العاشر

العتبة العباسية المقدسة تنشر أكاليل الزهور ومظاهر الفرح بمناسبة ذكرى ولادة أمير المؤمنين (عليه السلام)

مجمع الشيخ الكليني يواصل تحضيراته لاستقبال المشاركات في حفل تخرج طالبات الجامعات العاشر

وصول المشاركات في فعاليات مهرجان روح النبوة الثقافي النسوي العالمي الثامن إلى كربلاء

المزيد

الآخبار الصحية

ماذا يحدث لجسمك عند تناول الوجبات السريعة يوميا؟

لصحة عظام المرأة.. الشاي أفضل أم القهوة؟

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أطعمة صحية تساعد على التعافي من "الإنفلونزا الخارقة"

المزيد

الآخبار التكنلوجية

دراسة تكشف قدرة الحيتان على سماع الأصوات منخفضة وعالية التردد من مسافة بعيدة

تجربة جديدة: الأسفلت المدعوم بالطحالب يقاوم الحفر ويقلل الانبعاثات الكربونية

تسريبات تكشف ملامح أول هاتف قابل للطي من "أبل"

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المزيد

مواضيع ذات صلة

Secretion of Acetylcholine by The Nerve Terminals

Mechanics of Skeletal Muscle Contraction

Energetics of Muscle Contraction

Molecular characteristics of the contractile filaments

General Mechanism of Muscle Contraction

Physiological Anatomy of Skeletal Muscle

المفاصل Joints

العضلة القلبية (Cardiac muscle)

العضلة الناعمة متعددة الوحدات (multi-unit smooth muscle)

العضلة الناعمة المفردة (single-unit smooth muscle)

العضلة الناعمة (smooth muscle)

آلية التقلص والانبساط العضلي