Muscular system

SKELETAL MUSCLE Skeletal muscle is made up of hundreds of muscle fibers –Fibers consists of threadlike myofibrils –Myofibrils composed of smaller myofilaments –Striations reflect the overlapping of muscle filaments

Skeletal Muscles Structure Muscle are composed of bundles of muscle fibers, which in turn are made of bundles of myofibrils. Muscle fiber: Sarcolemma: the plasma membrane with inward extensions form T tubules. Sarcoplasm: refers to the cytoplasm. Sarcoplasmic reticulum: the ER in muscle. Myofilaments actin and myosin, which are organized into contractile units called Sarcomeres: basic units connected end-to-end by Z- line to form myofibrils.

The Myofilaments The thick filaments and the thin filaments. These two filaments are arranged within the sarcomere in an overlapping manner. Thin filaments are composed of the protein actin, the helical backbone of thin filament. Each actin protein contains an active site which interacts with the myosin head. Two other proteins are present in the thin filaments, tropomyosin and troponin. Thick filaments are composed of a myosin. The head extends out from the filament forming cross bridges which interact with the thin filaments

Sarcomere –Contractile unit –Actin (thin) filaments –Myosin (thick) filaments

Steps in muscle contraction –Acetylcholine released by a motor neuron combines with receptors on the surface of a muscle fiber –Calcium ions released from the sarcoplasmic reticulum

–Calcium ions bind to troponin in the actin filaments causing the troponin to change shape –Troponin pushes tropomyosin away from the active sites on the actin filaments –ATP binds to myosin

–ATP is split, putting the myosin head in a high-energy state –Energized myosin heads bind to the exposed active sites on the actin filaments –The actin filament is pulled toward the center of the sarcomere

–Myosin head binds a new ATP –Myosin head detaches from the actin –Myosin reattaches to new active sites so that the filaments are pulled past one another –Muscle continues to shorten

STIMULATION Contraction of skeletal muscle is initiated when an action potential traveling down a motor neuron reaches the neuromuscular junction. Motor neuron releases acetylcholine into synaptic cleft, which binds with receptors on muscle fiber. Depolarizes (change in electric charge) the sarcolemma of the muscle fiber. This action potential travels down the inward- projecting T tubules that reach deep into the muscle fiber. Depolarization of T tubules opens calcium channels in the sarcoplasmic reticulum.

Causing the to release of stored calcium ions. Ca 2+ then diffuse into the myofibrils and bind to troponin complex, which change its shape. Pushing tropomyosin away from the active sites on the actin filament. Expose myosin-binding sites, which are capable of interacting with myosin heads, forming cross bridges after ATP breakdown to ADP& P i. A new ATP binds to myosin heads, breaking the cross bridges and myosin detach from actin. Tropomyosin then covers active sites on the actin molecules and relaxation occurs. After contraction, ACH inactivated, the Ca 2+ moves back into the sarcoplasmic reticulum.

Muscle contraction

Myosin head (H) attaches to actin filament (A), forming a crossbridge.

Providing energy for muscle contraction –ATP hydrolysis provides the energy to “cock” the myosin –Creatine phosphate is used for intermediate energy storage –Glycogen is the fuel stored in muscle fibers

Antagonistic action of skeletal muscles –Agonist muscle contracts –Antagonist muscle relaxes –Groups of muscles work together –Series of separate stimuli timed close together produces a smooth, sustained contraction

Muscle action