| Muscle Microstructure and Macrostructure - Skeletal muscle contains: o Muscle tissue o Connective tissue o Nerves o Blood vessels o Epimysium (Fibrous connective tissue) § It covers the body >430 skeletal muscles § Continuous with the tendons at the ends of the muscle o Periosteum (The tendon is attached to bone) § A specialized connective tissue covering all bones - Muscle cells (Muscle fibers) o Cylingdrical cells 50 to 100 mm in diameter o Many nuclei situated on the periphery of the cell o Striated appearance under low magnification o Epimysium (outer layer) § Fasciculi (the muscle fibers are grouped in bundles – 150 fibers) o Perimysium (Surrounding each fasciculus, or group of fibers) o Endomysium (Surrounding individual fibers) § Sarcolemma (continuous with fiber’s membrane) ® All the connective tissue is continuous with the tendon, so tension developed in one muscle cell can develop tension in the tendon. - Motor Unit o A motor neuron and the muscle fibers it innervates o All the muscle fibers of a motor unit contract together when they are stimulated by the motor neuron o Neuromuscular junction (motor end plate) o The junction between a motor neuron (nerve cell) and muscle fibers it innervates o Each muscle cell has only one neuromuscular junction, although a single motor neuron innervates many muscle fibers (sometimes several hundred) - Sarcoplasm (Cytoplasm of a muscle fiber) o Contains contractile component (consist of protein filaments) o Stored glycogen and fat particles, enzymes, and specialized structures such as mitochondria and the sarcoplasmic reticulum o Myofibrils (each about 1 mm in diameter) § Contains apparatus that contracts the muscle cell § Myofilament · Myosin (thick – 450 in a typical myofibril) o Cross-bridge (globular heads) protrude away from the myosin filament at regular intervals · Actin (thin – 900 in a typical myofibril) o Consist of two strands arranged in a double helix ® Sarcomere · Myosin and actin filaments are organized longitudinally in the smallest contractile unit of skeletal muscle o A-band (dark band) corresponds with the alignment of the myosin filaments o I-band (light band) corresponds with areas in two adjacent sarcomeres that contain only actin filaments. o Sarcoplasmic reticulum § An intricate system of tubules is parallel to and surrounding each myofibril § It terminates as vesicles in the vicinity of the Z-lines. § Calcium ions are stored in the vesicles. § It is through the regulation of calcium that muscular contraction is controlled. o T-tubules § Short for transverse tubules § They run perpendicular to the sarcoplasmic reticulum and terminate in the vicinity of the Z-line and between two vesicles. o Triad § This pattern of a T-tubule spaced between and perpendicular to the sarcoplasmic reticular vesicles is called a triad. o Calcium is released throughout the muscle, producing a coordinated contraction. Sliding-Filament Theory of Muscular Contraction - The sliding-filament theory o It states that the actin filaments at each end of the sarcomere slide inward on myosin filaments, pulling the Z-lines toward the center of the sarcomere and thus shortening the muscle fiber; as actin filaments slide over myosin filaments, the H-zone and I-band shrink. o Troponin § A protein situated at regular intervals along the actin filament that has a high affinity for calcium ions. o Tropomyosin § This causes a shift to occur in another protein molecule. § It runs along the length of the actin filament in the groove of the double helix. o The energy for cross-bridge flexion comes from the hydrolysis (breakdown) of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate, a reaction catalyzed by the enzyme myosin ATPase. Another molecule of ATP must replave the ADP on the myosin cross-bridge head for the head to detach from the actin active site and recock. This allows the contraction process to be continued (if calcium is available to bind to the troponin molecule) or relaxation to occur (if calcium is not available). (It may be noted that calcium plays a role in regulating a large number of events in skeletal muscle besides contraction. These include glycolytic and oxidative energy metabolism, as well as protein synthesis and degradation.) Resting Phase o Under normal resting conditions little calcium is present in the myofibril (most of it is stored in the sarcoplasmic reticulum), so very few of the myosin cross-bridges are bound to actin. o No tension is developed in the muscle, so the muscle is said to be at rest. Types of Muscle Action - Concentric actions occur when the total tension developed in all cross-bridges of a muscle is sufficient to overcome any resistance to shortening. - Isometric actions occur when the tension in the cross-bridges equals the resistance to shortening and the muscle length remains relatively constant. - Eccentric actions occur when the tension developed in the cross-bridges is less than the resistance, and the muscle lengthens despite contact between the myosin cross-bridge heads and the actin filaments. Relaxation - Relaxation occurs when the stimulation of the motor nerve stops. - Calcium is pumped back into the sarcoplasmic reticulum, which prevents the link between the actin and myosin molecules. - Relaxation is brought about by the return of the actin and myosin filaments to their unbound state. Force Production Physiological Factors - A greater amount of calcium in a myofibril results in more myosin cross-bridge heads binding with actin filaments and thus more tension in that muscle. - The amount of calcium released from the sarcoplasmic reticulum vesicles is related to how frequently the innervating motor neuron stimulates the muscle. - Force production is controlled in two main ways; o Frequency of stimulation of motor units o Number of motor units activated. - It is possible that some training-induced increases in strength may occur because of increases in neural activation of motor units. - It is possible during training-mediated decreases in antagonist activity also affect strength. - Variation in nerve conduction and in myosin cross-bridge cycling velocities between “fast” and “slow” muscle fibers also affects the number of cross-bridge cycling velocities affects the number of cross-bridge heads bound to actin filaments. This means that maximal force production in a muscle does not occur instantaneously. Consequently, maximal force production may not occur early in the range of motion, especially during fast movements. - Since high force development may be important in strength development, development of strength may be retarded under training conditions that do not allow high-tension development in the early part of the range of motion. High tension is developed in muscle even before movement occurs when lifting weights because the weights must be supported isometrically. This is called preloading. Since neural activation controls calcium release from the sarcoplasmic reticulum, any factor that affects the neural activation of the muscle will influence the application of force within that muscle. For example, motivation, excitatory reflex activity, and inhibitory reflex activity all influence the activity of the motor neuron. These factors will be discussed in detail in chapter 2. Velocity of Shortening - Force production is inversely related to velocity of shortening during concentric actions; in other words, during faster movements, less force production is possible, and when lifting heavier loads, slower movements will occur. This is probably due to a smaller number of cross-bridge contacts on actin filaments at any instant as the velocity of shortening increases. - The relationship is different for eccentric actions. As the velocity of eccentric actions increases, maximal force production also increases. The force capabilities are typically 120% to 160 % greater in eccentric actions than in concentric actions. This means that when overloading eccentrically, very heavy resistances may be needed, but when training for explosive concentric movements, relatively light resistance may be more suitable. Cross-Sectional Area - The maximum force capability of a muscle is related to the cross-sectional area of the muscle. Angle of Pennation - Not all muscle has sarcomeres aligned along the long axis of the muscle. Some muscle is pinnate, and the angle of pennation can affect the number of sarcomeres per cross-sectional area and thus the maximal force capabilities. Sarcomere and Muscle Length - The amount of force that a muscle can exert is related to its length. Peak force production is usually seen at resting or slightly greater than resting length. This is known as the length-tension relationship of muscle. Torque - The integration of the length-tension relationship and differences in mechanical advantage in the joint lever system contribute to variations in the maximal torque that can be produced through the range of motion of any joint. - An understanding of the relationship between torque and joint position is important for development of proper technique in all sport activities. Prestretching - Prestretching a muscle just prior to a concentric action can enhance force production during the subsequent contraction. The increase in force production is called stretch-shortening potentiation or, more commonly, the stretch-shortening cycle. This enhancement is probably caused by the combined effects of the use of elastic energy in the muscle (Primarily from stretching the myosin cross-bridges) and stretch-reflex potentiation (activation of the myoatic stretch reflex caused by a rapid stretch) of muscle. |
| Muscle Physiology |