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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 inwardextensions form T tubules.

• Sarcoplasm: refers to the cytoplasm.

• Sarcoplasmic reticulum: the ER in muscle.

• Myofilaments actin and myosin, which areorganized 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 thesarcomere 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 whichinteracts with the myosin head.

Two other proteins are present in the thin

filaments, tropomyosin and troponin.

Thick filaments are composed of a myosin. Thehead 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 theactin 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 thecenter 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 motorneuron reaches the neuromuscular junction.

• Motor neuron releases acetylcholine into synapticcleft, which binds with receptors on muscle fiber.

• Depolarizes (change in electric charge) thesarcolemma of the muscle fiber.

• This action potential travels down the inward-

projecting T tubules that reach deep into themuscle 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 totroponin complex, which change its shape.

• Pushing tropomyosin away from the active siteson the actin filament.

• Expose myosin-binding sites, which are capable

of interacting with myosin heads, forming crossbridges after ATP breakdown to ADP& P

i.

• A new ATP binds to myosin heads, breaking thecross bridges and myosin detach from actin.

• Tropomyosin then covers active sites on the actinmolecules and relaxation occurs.

• After contraction, ACH inactivated, the Ca2+

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



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