Introduction The Muscular System

Striated = striped. Skeletal muscle appears striped under a microscope Muscle Tissue Elongated cells, specialized for contraction Three types of muscle tissue: Skeletal muscle – striated/voluntary Cardiac muscle – striated/involuntary Smooth muscle – not striated/involuntary Striated = striped. Skeletal muscle appears striped under a microscope

Skeletal Muscle Functions Produces movement of the skeleton Maintains posture and body position Supports soft tissues Guards entrances and exits Maintains body temperature by generating heat Skeletal muscles pull on tendons attached to bones and thereby result in movement. 2. Continuous muscle contractions maintain posture. Without this constant action, you could not sit upright without collapsing.

Organization of Skeletal Muscle Organ-level structure “Muscle” = many bundles of muscle fibers Muscle fiber = single muscle cell (elongated) Fascicle = bundle of fibers fiber fascicle muscle

Organization of Skeletal Muscle Organ-level structure Connective Tissue layers: Endomysium (around a fiber) Perimysium (around a fascicle) Epimysium (outer layer) Connective tissue layers converge to become the tendon

Cellular Level Anatomy Sarcolemma = muscle fiber membrane Sarcoplasm = cytoplasm of muscle cell Sarcoplasmic reticulum = stores calcium for contraction Transverse (T) tubules = network of narrow tubes that form passageways through a muscle fiber Each muscle fiber contains hundreds to thousands of myofibrils. Myofibrils are responsible for muscle contraction. Because they are attached to the sarcolemma at each end of the cell, their contraction shortens the entire cell. Scattered among the myofibrils are mitochondria and granules of glycogen, a source of glucose. The breakdown of glucose and the activity of the mitochondria provide the ATP needed to power muscluar contractions

Organization of Skeletal Muscle Myofibrils = cytoskeletal proteins; composed of myofilaments (actin and myosin) Alternating arrangement of myofilaments gives skeletal muscle a striated appearance Actin = thin filament = light band Myosin = thick filament = dark band

Organization of Skeletal Muscle Nuclei (many) sarcolemma mitochondria sarcoplasm Myofibril; myofilaments

Organization of Skeletal Muscle Hierarchy of organization: Muscle fascicle  fiber  myofibrils  myofilaments (actin/myosin)

The Sarcomere Functional unit of skeletal muscle Each myofibril consists of ~10,000 sarcomeres arranged end to end Arrangement of actin and myosin within the sarcomere produce a striped appearance Each myofibril consists of 10,000 sarcomeres arranged end to end. The sarcomere is the smallest functional unit of the muscle fiber. Interactions between the thick and thin filaments of sarcomeres are responsible for muscle contraction.

Sarcoplasmic reticulum Sarcomere Structure Z line = marks the boundaries of each sarcomere M line = middle of the sarcomere Sarcomere Sarcoplasmic reticulum Z line Z line M line

Sarcoplasmic reticulum Sarcomere Structure A band = Myosin (thick filament; dark band) I band = Actin (thin filament; light band) Sarcomere Sarcoplasmic reticulum Myosin Actin Z line Z line Neither type of filament spans the entire length of an individual sarcomere. The thick filaments lie in the center of the sarcomere, thin filaments at either end of the sarcomere. Differences in densities between actin and myosin account for the banded appearance of the sarcomere. M line I band A band I band

Sarcoplasmic reticulum Sarcomere Structure Animation H zone = no overlap between thick and thin filaments Sarcomere Sarcoplasmic reticulum Myosin Actin Z line H zone Z line M line I band A band I band

Label…A, I, M, Z, H animation

Muscle at Rest Actin and myosin lie side-by side Myosin heads are “primed” for contraction H zone and I band are at maximum length

Changes to the sarcomere during contraction I band gets smaller Z lines move closer together H zone decreases Length of A bands doesn’t change Length of filaments doesn’t change animation

Sliding Filament Theory Model of muscle contraction in which actin “slides” past myosin to cause sarcomere shortening Involves 5 different molecules and calcium Myosin Actin Tropomyosin Troponin ATP myosin

Role of Nervous System Brain/spinal cord sends an impulse to the muscle Impulse = electrical signal Neuromuscular junction = nerve + muscle fiber Acteylcholine = neurotransmitter, activates the muscle Impulse travels through T-tubules triggering release of calcium from the sarcoplasmic reticulum

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At Rest

Muscle Contraction 1. The brain or spinal cord sends an impulse to the muscle; acetylcholine released at the neuromuscular junction

Muscle Contraction 2. Impulse triggered in the sarcolemma of the muscle fiber; impulse travels along T-tubules

Muscle Contraction 3. Calcium release: sarcoplasmic reticulum releases calcium; calcium diffuses through muscle fiber

Muscle Contraction 4. Active Site exposed: Ca2+ binds to troponin; tropomoyosin shifts; active sites for myosin are exposed

Muscle Contraction 5. Cross-bridge formation: Myosin head binds to actin

Muscle Contraction 6. Powerstroke: ADP and Pi are released from myosin; myosin moves, pulling the actin with it; sarcomere shortens

Muscle Contraction 7. Release: ATP binds to myosin; myosin released from actin; ATP reverts into ADP and Pi. Myosin is ready to form another crossbridge

Relaxation 8. Relaxation: Impulse stops, calcium is released from troponin; tropomyosin covers the active site Ca+ is transported back into the SR

Contraction animation Muscle Contraction Contraction ends when nerve impulses stop AND calcium ion concentration returns to normal Contraction animation

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Muscle Sketches Skeletal muscle (see page 113) Label nuclei, muscle fiber Actin filament (see page 198) Label actin, troponin, tropomyosin, active site Myosin filament (see page 198) Label head Sarcomere (at rest and during contraction) Label all parts (A, I, actin, myosin, H, M, Z)