Muscle tissues

skeletal muscle

attached to bone; voluntary; striated muscle

cells are long and multinucleated

smooth muscle

do not exhibit cross-striation; involuntary; cover wall of internal organs

cells are narrow and spindle-shaped, single and centrally located nucleus

cardiac muscle

wall of heart; involuntary; striated muscle

cells are branched, intercalated discs

fusiform muscles

Cylindrically shaped in the center and tapers at the ends

Ex: biceps, triceps

unipennate muscle

Fibers that run only one side of the muscle

Ex: palmar interosseous

bipennate muscle

Two rows of oblique muscle fibers run both sides of tendon

Ex: dorsal interosseous

digastric muscle

double belly; parallel muscle fibers

Ex: omohyoid

Quadrate muscles

multi-belly muscle with tendinous intersection

Ex: rectus abdominis

levels of organization of skeletal muscle

muscle>fascicle>myofibers (muscle fiber)>myofibrils>myofilaments>actin (thin filaments) & myosin (thick filaments)

Epimysium

surrounds entire muscle

Perimysium

surrounds fascicles

Endomysium

Surrounds individual muscle fibers

muscle fiber (myofiber)

bundle of myofibrils; nuclei + mitochondria + T-tubules + sarcoplasmic reticulum + terminal cisternae

Triad of a muscle fiber

T-tubule + 2 terminal cisternae of SR

cross bridge cycle

repeated interactions between myosin and actin filaments -> muscle contraction

thin filaments

double-stranded helix actin

tropomyosin and troponin complexes: masks myosin-binding site

thick filaments

myosin

Sacromere

contractile unit of a muscle fiber

muscle contraction steps

1) Motor neuron releases acetylcholine into the neuromuscular junction and causes the depolarization of the sarcolemma.

2) Depolarization spreads down the sarcolemma to the T-tubules, triggering the release of Ca2+ ions.

3) Ca2+ binds to troponin, causing a shift in tropomyosin and exposure of the myosin-binding site on the actin filament.

4) Shortening of the sarcomere occurs as the myosin head binds to the exposed sites of actin, forming a cross-bridge and pulling the actin filament along the thick filament, resulting in contraction.

5) Muscle relaxes when acetylcholine is degraded by acetylcholine esterase and the allowing Ca2+ is brought back into the SR. ATP binds to myosin head, allowing it to relax from actin.

red fibers

slow-twitch; many myoglobin and capillaries -> oxidative phosphorylation to produce ATP; slow contraction but fatigue resistant

muscles to maintain erect posture, leg muscles of marathon runners

white fiber

fast-twitch; abundant glycogen -> anaerobic glycolysis to produce ATP; fast contraction but fatigue-prone

leg muscles of short-distance sprinters

extreme muscle work -> oxygen deficiency -> ischemic pain -> lactic acid -> muscle fever

intermediate fibers

fast-twitch; both myoglobin & glycogen -> oxidative and glycolytic activity; intermediate contraction speed and fatigue

leg muscles of middle-distance runners, swimmers and hockey players

isotonic contraction

muscle shortens against a constant load to produce movement

isometric contraction

muscle tension increases as the muscle maintains a fixed overall length

concentric action

insertion approach origin

concentric inverse: origin approach insertion

eccentric action

origin and insertion move away from each other