Muscle Contraction & The Sliding Filament Theory, Lecture notes of Physiology

Download Muscle Contraction & The Sliding Filament Theory and more Lecture notes Physiology in PDF only on Docsity! Figure 1: Muscle Structure 1.1 (above) Breakdown of skeletal muscle into its smallest parts 1.2 (below) Structure of the Sarcomere Muscle Contraction & The Sliding Filament Theory The Sliding Filament Theory explains the basis of skeletal muscle contraction and specifically deals with the movement of myosin heads along an actin fiber in the sarcomere. The first step to understanding this theory stems from an understanding of the functional parts of the muscle and its terminology. Once the components of the system are known, the mechanical process of muscle contraction follows naturally. Accordingly, this document outlines the parts of the muscle before providing a simplified explanation of the mechanism behind muscle contraction. Functional Parts of the Muscle The process behind the sliding filament theory depends heavily on the functional parts of the muscle. The muscle contains bundles of muscle fibers, with each individual muscle fiber termed a myofibril (Figure 1.1). Each myofibril represents the fusion of many cells into one large cell containing multiple nuclei. In addition, the myofibril includes the actin and myosin filaments, which together form the basis of muscle movement. The arrangement of these filaments delineates the boundaries of the muscle’s contractile unit called the sarcomere. The Sliding Filament Theory generally explains the movement of each sarcomere; accordingly, the structure of the sarcomere serves as the most important component of this mechanism. The majority of the myosin in the sarcomere is found in the thick filaments shown in Figure 1.2, while actin myofilaments compose the thin filaments. The thin filaments array themselves mainly within the I bands, while the thick filaments populate the A bands. The Z line (occasionally called “Z disk”) lays in the middle of the I band, and the M line divides the A band into two visually distinct sections. The distance from Z line to Z line determines the length of the sarcomere. Other proteins associate with the sarcomere, with titin often considered the most important of these proteins. Titin attaches the thick filaments to the Z line and extends across the sarcomere to the M line. In doing so, it acts like a “molecular ruler” and determines how far the sarcomere stretches. Another protein, nebulin, anchors the thin filaments to the Z disk. Figure 1.1 Figure 1.2 Major Steps of the Sliding Filament Theory Before any muscle contraction can take place, the muscle must receive a signal from the nervous system. Until this signal arrives, a protein complex (the troponin-tropomyosin complex) blocks the myosin head from binding to the thin filaments. Three subunits, called I, T, and C, form troponin. Troponin I prevents the actual binding of the myosin head, while Troponin T connects troponin to tropomyosin. Meanwhile, Troponin C awaits the signal from the nervous system, which releases calcium ions (Ca2+) into the sarcoplasmic reticulum surrounding the myofibril. Once released, the Ca2+ binds to Troponin C, and a conformational change in troponin allows for muscle contraction to occur. Now, the mechanism of the Sliding Filament Theory can be explained. Muscle contraction involves four major steps (shown in Figure 2): 1) ATP binds to the myosin head. The cross bridge between the two fibers breaks as the myosin head dissociates from the actin myofilament. 2) ATP is hydrolyzed. ATP breaks into ADP and Pi causing a conformational change of the myosin head into its high-energy conformation. 3) Myosin head attaches to actin myofilament. The formation of a new cross bridge leads to the release of Pi 4) Pi release triggers the “power stroke.” A conformational change in the myosin head moves the actin and myosin filaments relative to each other. 1 2 3 4 Figure 2: Sliding Filament Theory – The four major steps that result in the ultimate movement of myosin down the actin myofilament.