Muscle Fibers

• Muscle fiber is the muscle cell• Each skeletal muscle contains hundreds to

thousands of these fiber cells

Epimysium

• The connective tissue covering that covers the entire muscle

• Outer most covering of the muscle

Perimysium

• Surrounds bundles of 10 to 100 muscle fibers/cells called fascicles

• Second/middle covering of muscle tissue.

Fascicles

• A bundle of 10 to 100 muscle fibers/cells

Endomysium

• Third or Innermost covering• Covers the muscle fiber/cell• Just outside the sarcolemma

Tendon

• A cord of dense regular connective tissue composed of collagen fibers.

• The tissues of the epimysium, perimysium, endomysium, and sarcolemma come together to form the tendon

• The tendon is connected to the periosteum of the bone

• Tendons connect muscle to bone

Sarcolemma

• The plasma membrane (cell membrane) of the muscle cell

• Defines the individual muscle fiber from its surroundings

• Folds inward to form the transverse tubules• Involved in active and passive transport

Transverse Tubules (T-Tubules)

• Located on the exterior bundles of myofibrils of the muscle fiber/cell

• Located in the middle of the terminal cistern. • Tubes tunnel inward through the myofibrils

into the interior of the muscle cell• Store calcium ions (Ca2+)

Sarcoplasm

• Similar to the cytoplasm of other cells.• Contains:– Sarcoplasmic reticulum– Many mitochondria– Myoglobin

• Main function is to provide an aqueous environment for calcium ion (Ca2+) exchange between the sarcoplasmic reticulum and the thin filament.

Sarcoplasmic Reticulum

• Similar to the endoplasmic reticulum of other eukaryotic cells

• Network of fluid-filled membrane-enclosed tubules extending from the terminal cisterns

• Main function is to store calcium ions (Ca2+) required for muscle contraction

Myoglobin

• Reddish pigment similar to hemoglobin in blood.• Stores oxygen until it is needed by mitochondria

to generate ATP• Myoglobin binds oxygen when oxygen is plentiful

in the sarcoplasm and releases oxygen when it is scarce in the sarcoplasm.

• Myoglobin is part of aerobic cellular respiration

Myofibrils

• Myofibrils contain the following and all their components.o Thin filamentso Thick filamentso Sarcomereso Z-Discso A-Bando H-Zoneo I-Band

Thin Filament

• Main structural helix of the thin filament is composed of protein called actin.– Other proteins involved in the structure of thin filament• Tropomyosin – Block the myosin binding site in relaxed muscle• Troponin – Hold tropomyosin in place on thin filament

• Contains the myosin –binding site

Thick Filaments

• Composed of protein called myosin• Structured like golf clubs twisted together• Golf club handles are the myosin tails• The heads of the golf clubs are the myosin

heads• Thick filaments are secured at their centers by

the M-Line

Sarcomeres

• The basic functional unit of myofibrils and striated muscle fibers. (Skeletal and Cardiac)

• A sarcomeres is the area between two Z-discs

Z-Discs

• Zigzagging zones• Space between two Z-discs is a sarcomere• Thin Filament is attached to the Z-discs

A-Band

• Darker striation of muscle• Extends the entire length of the thick filament• Thin and thick filaments overlap at the ends of

the A-band• The H-zone is in the middle of the A-band

H-Zone

• The center of the A-band• Section in relaxed muscle where there is no

thin filaments• When a muscle contracts the H-zone

disappears because the thin filaments overlap in the H-Zone

I-Band

• Extends from the end of a thick filament in one sarcomere across the Z-disc and ends at the next thick filament in the adjacent sarcomere.

• In a relaxed muscle, there are no thick filaments in the I-band.

• In a contracted muscle, the I-band disappears.• Thick filaments move into the I-band during

contraction

Myosin

• Protein making up the thick filament• Structured like golf clubs twisted together• Golf club handles are the myosin tails• The heads of the golf clubs are the myosin

heads

Myosin Heads

• Cluster of myosin protein forming heads at the ends of the thick filament

• When the myosin head is attached to the myosin-binding site on the thin filament a crossbridge is formed

• Power Stroke - When the myosin head moves the thin filament toward the M-line in contraction.

Actin

• Main structural protein in the thin filament– Actin combine with the following to form the thin

filament• Tropomyosin – Block the myosin binding site in relaxed

muscle• Troponin – Hold tropomyosin in place on thin filament

Tropomyosin

• Protein that forms rod-shaped, helical-stripes down the thin filament.

• In a relaxed muscle, the tropomyosin covers the myosin-binding sites, this blocks the myosin heads from attaching to the thin filament.

• When the calcium ion Ca2+ is present in the sarcoplasm, troponin causes tropomyosin to move, which allows the myosin heads to attach to the thin filament and muscles to contract.

Troponin

• Smaller, ball shaped protein that is interspaced down the thin filament.

• Calcium ion (Ca2+) will attach to the troponin.• This causes tropomyosin to move, which

allows the myosin heads to attach to the thin filament and muscles to contract.

Sliding-Filament Mechanism

• Myosin heads of the thick filaments pull on the thin filaments

• Thin filaments slide toward the center of the sarcomere.

• As the thin filaments slide, the I-bands and H-zones become narrower.

• I-bands and H-zones disappear altogether when maximum contraction is reached

• The sliding-filament mechanism is how muscles contract.