Physiology Exam 3 Skeletal Muscle Review of skeletal muscle Tendon muscle to bone Ligament bone to bone knee Skeletal muscles are composed of individual muscle fibers that contract when stimulated by a somatic motor neuron Each motor neuron branches to innervate a number of muscle fibers Activation of varying numbers of motor neurons results in gradations in the strength of contraction of the whole muscle Skeletal muscles are usually attached to bone on each end by tough connective tissue tendons The muscle tension when a muscle contracts causes movement of the bones at a joint where one of the attached bones moves more than the other The more movable bony attachment of the muscle is the insertion and is pulled toward its less movable attachment known as its origin When flexor muscles contract they decrease the angle of a joint Contraction of extensor muscles increases the angle of their attached bones at the joint The prime mover of any skeletal movement is the agonist muscle Flexors and extensors that act on the same joint to produce opposite actions are antagonist muscles Structure of Skeletal muscles 3 Types of connective tissue Epimysium an outer sheath of fibrous connective tissue that extends into the body of the muscle subdividing it into fascicles Perimysium connective tissue sheath surrounding each fascicle A muscle fascicle is composed of many muscle fibers myofibers The myofibers are surrounded by a plasma membrane sarcolemma enveloped by a thin connective tissue layer called an endomysium All the connective tissue is continuous which is why muscle fibers do not normally pull out of the tendons when they contract Facts of skeletal muscle fibers Muscle fibers are unusually elongated in shape but have the same organelles that are present in other cells Unlike most other cells in the body skeletal muscle fibers are multinucleate contain multiple nuclei because each muscle fiber is formed from the union of several embryonic myoblast cells Muscle fibers are not capable of cell division born with a certain number of them and muscle fibers get bigger when you lift weight Distinctive feature of skeletal muscle fibers is their striated appearance The striations are produced by alternating dark and light bands that appear to span the width of the fiber The dark bands are called A bands and the light bands are called I bands Thin dark lines can be seen in the middle of the I bands these are Z lines A band is anisotropic and I band is isotropic Skeletal Muscle fibers Each muscle fiber or muscle cell is composed of many myofibrils 1 micrometer in diameter densely packed in parallel rows from one end of the muscle fiber to the other When a muscle fiber is seen with an electron microscope its striations do not extend all the way across its width Rather the dark A bands and light I bands that produce the striations are seen within each myofibril Because the dark and light bands of different myofibrils are stacked in register aligned vertically from one side of the muscle fiber to the other and the individual myofibrils are not visible with a light microscope the entire muscle fiber appears to be striated Myofibrils are divided into units called sarcomeres and inside the sarcomeres are smaller filaments called myofilaments The A bands are seen to contain thick filaments 110 angstroms thick stacked in register and give A band its dark appearance The lighter I band contains thin filaments 50 to 60 angstroms thick The thick filaments are composed of the protein myosin and the thin filaments are composed of the protein actin Skeletal Muscle Fibers Each thin filament extends partway into the A bands on each side Because the thick and thin filaments overlap at the edges of each A band the edges of the A bands are darker in appearance than the central region These central lighter regions of the A bands are called the H bands The central H bands contain only thick filaments that are not overlapped by thin filaments Center of A band is the M line M lines serve to anchor the thick filaments helping them stay together during a contraction The arrangement of thick and thin filaments between a pair of Z lines forms a repeating pattern that serves as the basic subunit of striated muscle contraction These subunits from Z to Z are known as sarcomeres Filaments of titin the largest protein in the human body and runs right down the middle of thick myofilament The springlike portion of titin within the I bands is highly folded when the muscle is short but unfolds and develops passive tension when the sarcomere is stretched Titin contributes to the elastic recoil of muscles that helps them to return to their resting lengths when they relax Sliding Filament Theory of Contraction When a muscle contracts it decreases in length as a result of the shortening of its individual fibers Shortening of the muscle fibers is produced by shortening of their myofibrils which occurs as a result of the shortening of the sarcomeres The thin filaments composing the I band however do not shorten The thick and thin filaments remain the same length during muscle contraction Shortening of the sarcomeres is produced not by shortening of the filaments but rather by the sliding of thin filaments over and between the thick filaments In the process of contraction the thin filaments on either side of each A band slide deeper and deeper toward the center producing increasing amounts of overlap with the thick filaments The I bands containing only thin filaments and H bands containing only thick filaments get shorter during contraction Cross bridges Sliding of the filaments is produced by the action of numerous cross bridges that extend out from the thick myofilaments toward the thin myofilaments These cross bridges are part of the myosin proteins that extend from the axis of the thick filaments to form arms that terminate in globular heads A myosin protein has two globular heads that serve as cross bridges The orientation of the myosin heads on one side of a sarcomere is opposite to that of the other side so that when myosin heads form cross bridges by attaching to actin on each side of the sarcomere they can pull the actin from each side toward the center Isolated muscles are easily stretched because myosin heads are not attached to actin when the muscle is at rest Each globular myosin head of a cross bridge contains an ATP binding site closely associated with an actin binding site The globular heads function as myosin ATPase