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Muscular system 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.• Ca2+ 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& Pi.
• 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 Ca2+ moves back into the sarcoplasmic reticulum.
Musclecontraction
Myosin head (H) attaches to actin filament (A), forming a crossbridge.
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