Muscles: Myosin and Myosin Filaments Essay

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NOTE!! These files are provided for the sole purpose of assisting BSci 110 students to study for exams in the class. Some of the material in these files may be copyrighted, and it is not OK for you to share these files with anyone who is not a student in this class or to use them for any purpose other than to study for the exams of our class. Thanks, Carl Johnson

Muscles

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Types of Muscles in Vertebrates

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All three types of muscle use actin and myosin, but they are controlled and organized differently

Purves 47.1

Smooth Muscle

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Smooth muscle cells are the simplest muscle cells: single nucleus per cell spindle-shaped often in smooth sheets not "striated" because the actin and myosin are not arranged regularly within the cell as is true for cardiac and skeletal muscle cells Smooth muscle: moves food through the digestive tract (involuntary contractions) controls the flow of blood (surrounds blood vessels) empties the urinary bladder

Cardiac Muscle

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Cardiac muscles are branched into a meshwork. They appear striated because of the regular arrangement of their actin and myosin filaments. Cardiac muscle cells are in electrical contact with one another by gap junctions, and depolarizations begun at one point in the heart rapidly spread through the muscle mass. Although heart activity is modified by the autonomic nervous system, the heart will beat without nervous input because of special pacemaker muscle cells that have a self-generated heartbeat.

Skeletal Muscle

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All voluntary movements are controlled by skeletal muscle. Skeletal muscle is also called "striated muscle" because of its striped appearance (due to the regular arrangement of their actin and myosin filaments). Skeletal muscle cells are called muscle fibers. They are large and have many nuclei because they are a fusion of many individual cells.

Skeletal Muscle
Skeletal muscle moves the body by contraction (muscle can't actively extend). Movement around joints is accomplished by antagonistic muscle pairs—one contracting, the other relaxing. Connective tissue: Ligaments hold bones together at a joint. Tendons attach the muscles to bones.

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(bends the joint) (straightens the joint)

tendon

(knee cap)

ligaments

Purves 47.16

Skeletal Muscle

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C&R 50.34

Skeletal Muscle

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Each muscle fiber (= single cell) is packed with bundles of myofibrils, each made up of thin actin units surrounding thick myosin units.

single muscle fiber (a single cell)

Purves 47.3

Sarcomeres of Skeletal Muscle
Myofibrils consist of repeating units called sarcomeres. Each sarcomere is bounded by Z lines, which anchor the thin actin filaments. At the center is the A band, housing all the myosin filaments.

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Purves 47.3
The M band contains proteins that support the myosin filaments. The H zone and I band are areas where actin and myosin do not overlap and therefore appear less dense.

Sarcomeres of Skeletal Muscle
The bundles of myosin filaments are held in register by the protein titin. Titin runs the full length of the sarcomere from Z line to Z line, and each titin molecule runs through the myosin bundle. Electron microscope photograph

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Purves 47.3

Sarcomeres of Skeletal Muscle
Cross-section of a sarcomere at different places: Actin = thin filaments Myosin = thick filaments

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Contraction of Sarcomeres

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Actin and myosin slide past each other as the muscle contracts. Hugh Huxley and Andrew Huxley called this mechanism the "sliding filament" theory of muscle contraction.

Contraction of Sarcomeres
When a muscle contracts, the sarcomere shortens, the H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments were sliding into the region occupied by the myosin filaments.

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C&R 50.27

Contraction of Sarcomeres myofibril 14

relaxed

width of sarcomere myofibril

width of sarcomere
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