The muscles that are connected to the skeleton and are related to the movement of bones are called skeletal muscles. The skeletal muscles are consciously managed and for that reason, are called voluntary muscles. Skeletal muscles are likewise called striped or striated muscles due to the fact that they show alternate light and dark bands, e.g., triceps muscles and biceps.
Typically, each end of the whole muscle is attached to bone by a package of collagen, non-elastic fibers, called tendons.
Skeletal Muscle Fiber
Each muscle includes muscle bundles, which are more made up of muscle fibers or cells. Each muscle fiber is a long round cell with several oval nuclei set up just underneath its sarcolemma. Skeletal muscle fibers are huge cells. Their size is 10 – 100 mm.
The sarcoplasm of the muscle fiber is similar to the cytoplasm of other cells but it includes usually a big quantity of kept glycogen and distinct oxygen-binding protein myoglobin, a red pigment that stores oxygen.
When viewed in high magnification, each muscle fiber is seen to consist of a lot of myofibrils 1-2 mm in size that run in parallel style and extends the entire length of the cell. Bundles of these fibrils are confined by the muscle cell membrane or sarcolemma. The myofibrils include smaller contractile units called sarcomere. In each sarcomere, a series of dark and light bands appear along the length of each myofibril.
Each dark band is called A band, since it is anisotropic, i.e., it can polarize visible light. The light band called I band is isotropic or non-polarizing. It gives the cell as a whole its striped look. Each A band has a lighter stripe in its mid area called H – zone (H stands for “hele” means bright). The H-zone is bisected by a dark line called M – line. The I bands have actually midline called Z – line (Z for zwishen means in between).
A sarcomere is the area of a myofibril in between 2 successive Z – lines and is the smallest contractile unit of muscle fiber. The myofibrils contain myofilaments.
Infrastructure of Myofilament
Myofilament is comprised of thick and thin filaments. The central thick filaments extend the entire length of the A-band. The thin filaments extend throughout the I-band and partially into the A band. The thick filament which is about 16 nm in diameter is composed of myosin. Each myosin particle has a tail ending in 2 globular heads.
Myosin tail consists of two long polypeptide chains coiled together. The heads are sometimes called cross-bridges due to the fact that they connect the thick and the thin myofilaments together during contraction.
Thin filaments are 7 – 8 nm thick and are composed chiefly of actin protein molecules. The actin molecules are organized in 2 chains which twists around each other like a twisted double strand of pearls. Twisting around the actin chains are 2 strands of another protein, tropomyosin. The other significant protein in the thin filament is troponin. It is in fact three polypeptide complex, one binds to actin, another binds to tropomyosin while the third binds calcium ions. Each myosin filament is surrounded by six actin filaments on each end.
Sliding Filament Model
When muscle fiber contracts, the thin and thick filaments undergo shifting. The I-band reduces in length and Z-line gets more closed.
Huxley and A. F. Huxley and their colleagues suggested a hypothesis in 1954 to explain all occasions in contraction, this is called the “Sliding filament model” of contraction. According to this theory, the thin filaments slide past the thick one so that actin and myosin filaments overlap to a higher degree. Hence the Z-line is brought close together, I-band reduces, the H zone vanishes.
In this process of contraction, the cross-bridges of thick filament become attached to binding sites on the actin filament. The cross-bridges then contract to pull the actin filament towards the center of the sarcomere.
How the bridges are controlled
When the muscle is at rest, the tropomyosin is gotten rid of in such a way that it covers the sites on the actin chain where the head of the myosin becomes attached. When the muscle is required to contract, calcium ions bind with the troponin particle and cause them to move slightly.
This has the effect of displacing the tropomyosin and exposing the binding sites for the myosin head. Once the myosin head has actually become connected to the actin filament, ATP is hydrolyzed and the bridge goes to its cycle.
This ATP is supplied by many mitochondria present in each muscle cell. From the above account, it is cleared that ATP is needed to break the link between the myosin and the actin.
After death, the quantity of ATP in the body falls. Under these scenarios, the bridges cannot be broken and so they remain firmly bound. This leads to the body ending up being stiff, a condition referred to as rigor mortis.
Frequently Asked Questions
Q1: What surrounds the myosin filament?
Ans: Myosin filament is surrounded by 6 actin filaments.
Q2: How sarcoplasmic reticulum is different from the endoplasmic reticulum?
Ans: Sarcoplasmic reticulum has a specialized repeating pattern and is devoid of ribosomes.
Q3: What are the general characters of skeletal muscles?
Ans: The regular striped long muscles with more than one nucleus in each cell. They are under somatic control and have slow to rapid movement.
Q4: What happens when muscle contracts?
Ans: During muscle contraction, the thin and thick filament undergo shifting. The I band reduces in length and Z – lines get closer. The A-band remains unaffected.