MUSCLE CONTRACTION AND RELAXATION

Muscle Contraction:
A process leading to shortening and/or development of tension in muscle tissue.

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Muscle contraction occurs by a sliding filament mechanism whereby actin filaments slide inward among the myosin filaments.

Myofibrils:
Any of the elongated contractile threads found in striated muscle cells.

Muscles are made up of a hierarchy of smaller and smaller units.
-Skeletal muscles are made of fibers.
-Each fiber is a single cell with many nuclei.
-The fibers consist of smiler myofibrils. ☺️

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Each myofibril is made up of thin filaments (actin+tropomyosin+troponin) and thick filaments (myosin).

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The myofibril is divided into think and thin filaments called a sarcomere.

SARCOMERE

• Z-line: the borders of the sarcomere
• I-band: area near Z-lines where there are only thin filaments.
• I-band: region that corresponds to the length of the thick filaments.
• H-zone: consist of thick filaments.

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Muscle contractions occur when myosin breaks down an ATP molecule to change its shape to high energy confirmation.

-The myosin holds onto the ADP+P.
-The myosin will into an actin filament.
-The myosin then releases the ADP+P and relaxes into its low energy state.
-Since it is still attached to the actin, this results in a force on the actin.
-Another ATP molecule binds to the myosin changing its low energy confirmation.
-Repeat.

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This process is regulated by Calcium ion (Ca+2) levels in the muscle cell cytoplasm.
-A signal is sent to the muscle cell by a neuron through the use of Acetylcholine.
-This signal causes the sarcoplasmic reticulum to release Ca+2 ions.

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-Ca+ ions bind to troponin molecules which causes the shape of troponin molecules to change. This exposes he tropomyosin and allows the myosin to attach to it.

-Phosphate binds with ADP to create ATP
-Troponin and tropomyosin regulate actin by blocking sites on actin required for complex formation with myosin.

-The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in the sarcoplasm.

-The active pumping of calcium ions into the sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This causes the removal of calcium ions from the troponin.
-This the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases.

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