Functional Anatomy of Skeletal Muscle, Lecture notes of Physical Activity and Sport Sciences

Download Functional Anatomy of Skeletal Muscle and more Lecture notes Physical Activity and Sport Sciences in PDF only on Docsity! Module 2: Exercising Muscle Functional Anatomy of Skeletal Muscle When the heart beats, when partially digested food moves through the intestines, and when the body moves in any way, muscle is involved. These many and varied functions of the muscular system are performed by three distinct types of muscle: smooth muscle, cardiac muscle, and skeletal muscle. Three types of muscle  Smooth muscle is sometimes called involuntary muscle because it is not under direct conscious control. It is found in the walls of most blood vessels, allowing them to constrict or dilate to regulate blood flow. It is also found in the walls of most internal organs, allowing them to contract and relax, for example, to move food through the digestive tract, to expel urine, or to give birth.  Cardiac muscle is found only in the heart, composing the vast majority of the heart’s structure. While it shares some characteristics with skeletal muscle, like a smooth muscle it is not under conscious control. Cardiac muscle in essence controls itself, with some fine-tuning by the nervous and endocrine systems.  Skeletal muscles are under conscious control and are so named because most attach to and move the skeleton. Together with the bones of the skeleton, they make up the musculoskeletal system. Image 1: Three main types of muscle. Image source: teachpe.com Skeletal Muscle When we think of muscles, we visualize each muscle as a whole, that is, as a single unit. This is natural because a skeletal muscle seems to act as a single entity. But skeletal muscles are far more complex than that. Image 2: Basic structure of muscle (Kenney W.L et al., 2012) If a person were to dissect a muscle, he or she would first cut through an outer connective tissue covering known as the epimysium. It surrounds the entire muscle and functions to hold it together. Once through the epimysium, one would see small bundles of fibers wrapped in a connective tissue sheath. These bundles are called fasciculi, and the connective tissue sheath surrounding each fasciculus (also called a fascicle) is the perimysium. Finally, by cutting through the perimysium and using a microscope, one would see the individual muscle fibers, each of which is a muscle cell. Unlike most cells in the body, which have a single nucleus, muscle cells are multinucleated. A sheath of connective tissue, called the endomysium also covers each muscle fiber. It is generally thought that muscle fibers extend from one end of the muscle to the other; but under the microscope, muscle bellies (the thick middle parts of muscles) often divide into compartments or more transverse fibrous bands (inscriptions). Muscle fibers Muscle fibers range in diameter from 10 to 120 μm, so they are nearly invisible to the naked eye. The following sections describe the structure of the individual muscle fiber. A sarcomere is the basic functional unit of a myofibril and the basic contractile unit of muscle. Each myofibril is composed of numerous sarcomeres joined end to end at the Z- disks. Each sarcomere includes what is found between each pair of Z-disks, in this sequence:  An I-band (light zone)  An A-band (dark zone)  An H-zone (in the middle of the A-band)  An M-line in the middle of the H-zone  The rest of the A-band  A second I-band Thick filaments About two-thirds of all skeletal muscle protein is myosin, the principal protein of the thick filament. Each myosin filament typically is formed by about 200 myosin molecules. Each myosin molecule is composed of two protein strands twisted together. One end of each strand is folded into a globular head, called the myosin head. Each thick filament contains many such heads, which protrude from the thick filament to form cross-bridges that interact during muscle contraction with specialized active sites on the thin filaments. There is an array of fine filaments, composed of titin, that stabilizes the myosin filaments along their longitudinal axis. Titin filaments extend from the Z-disk to the M-line. Thin filaments Each thin filament, although often referred to simply as an actin filament, is actually composed of three different protein molecules—actin, tropomyosin, and troponin. Each thin filament has one end inserted into a Z-disk, with the opposite end extending toward the center of the sarcomere, lying in the space between the thick filaments. Nebulin, an anchoring protein for actin, coextends with actin and appears to play a regulatory role in mediating actin and myosin interactions (figure 1.5). Each thin filament contains active sites to which myosin heads can bind. Actin forms the backbone of the filament. Individual actin molecules are globular proteins (G-actin) and join together to form strands of actin molecules. Two strands then twist into a helical pattern, much like two strands of pearls twisted together. Tropomyosin is a tube-shaped protein that twists around the actin strands. Troponin is a more complex protein that is attached at regular intervals to both the actin strands and the tropomyosin. Tropomyosin and troponin work together in an intricate manner along with calcium ions to maintain relaxation or initiate contraction of the myofibril. Muscle Fiber Contraction An alpha motor neuron is a nerve cell that connects with and innervates many muscle fibers. A single a-motor neuron and all the muscle fibers it directly signals are collectively termed a motor unit. The synapse or gap between the a-motor neuron and a muscle fiber is referred to as a neuromuscular junction. This is where communication between the nervous and muscular systems occurs. Image 1: A motor unit includes one alpha motor neuron and all of the muscle fibers it innervates. (Kenney W.L et al., 2012) Excitation-Contraction Coupling The complex sequence of events that triggers a muscle fiber to contract is termed excitation-contraction coupling because it begins with the excitation of a motor nerve and results in the contraction of the muscle fibers. The process is initiated by a nerve impulse, or action potential, from the brain or spinal cord to an a-motor neuron. The action potential arrives at the alpha motor neuron’s dendrites, specialized receptors on the neuron’s cell body. From here, the action potential travels down the axon to the axon terminals, which are located very close to the plasmalemma. Role of Calcium in the Muscle Fiber In addition to depolarizing the fiber membrane, the action potential travels over the fiber’s network of tubules (T-tubules) to the interior of the cell. The arrival of an electrical charge causes the adjacent SR to release large quantities of stored calcium ions (Ca2+) into the sarcoplasm. In the resting state, tropomyosin molecules cover the myosin-binding sites on the actin molecules, preventing the binding of the myosin heads. Once calcium ions are released from the SR, they bind to the troponin on the actin molecules. Troponin, with its strong affinity for calcium ions, is believed to then initiate the contraction process by moving the tropomyosin molecules off the myosin-binding sites on the actin molecules. Because tropomyosin normally covers the myosin-binding sites, it blocks the attraction between the myosin cross-bridges and actin molecules. However, once the tropomyosin has been lifted off the binding sites by troponin and calcium, the myosin heads can attach to the binding sites on the actin molecules. Sliding Filament Theory When muscle contracts, muscle fibers shorten. How do they shorten? The explanation for this phenomenon is termed the sliding filament theory. When the myosin cross-bridges are activated, they bind with actin, resulting in a conformational change in the cross-bridge, which causes the myosin head to tilt and to drag the thin filament toward the center of the sarcomere. This tilting of the head is referred to as the power stroke. The pulling of the thin filament past the thick filament shortens the sarcomere and generates force. When the fibers are not contracting, the myosin head remains in contact with the actin molecule, but the molecular bonding at the site is weakened or blocked by tropomyosin. Energy for Muscle Contraction Muscle contraction is an active process, meaning that it requires energy. In addition to the binding site for actin, a myosin head contains a binding site for the molecule adenosine triphosphate (ATP). The myosin molecule must bind with ATP for muscle contraction to occur because ATP supplies the needed energy. Muscle Relaxation Muscle contraction continues as long as calcium is available in the sarcoplasm. At the end of a muscle contraction, calcium is pumped back into the SR, where it is stored until a new action potential arrives at the muscle fiber membrane. Calcium is returned to the SR by an active calcium-pumping system. This is another energy-demanding process that also relies on ATP. Thus, energy is required for both the contraction and relaxation phases. When the calcium is pumped back into the SR, troponin and tropomyosin return to the resting conformation. This blocks the linking of the myosin cross-bridges and actin molecules and stops the use of ATP. As a result, the thick and thin filaments return to their original relaxed state. Muscle Fiber Types Not all muscle fibers are alike. A single skeletal muscle contains fibers having different speeds of shortening and ability to generate maximal force: type I (also called slow or slow-twitch) fibers and type II (also called fast or fast-twitch) fibers. Type I fibers take approximately 110 ms to reach peak tension when stimulated. Type II fibers, on the other hand, can reach peak tension in about 50 ms.