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