Introduction The Muscular System Muscle Tissue Elongated cells, specialized for contraction Three types of muscle tissue: 1. Skeletal muscle striated/voluntary
2. Cardiac muscle striated/involuntary 3. Smooth muscle not striated/involuntary Striated = striped. Skeletal muscle appears striped under a microscope Skeletal Muscle Functions 1. 2.
3. 4. 5. Produces movement of the skeleton Maintains posture and body position Supports soft tissues Guards entrances and exits Maintains body temperature by generating heat
Organization of Skeletal Muscle Organ-level structure Muscle = many bundles of muscle fibers Muscle fiber = single muscle cell fiber
(elongated) Fascicle = bundle of fibers 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 Cellular Level Anatomy Sarcolemma Sarcoplasm = muscle fiber membrane
= cytoplasm of muscle cell Sarcoplasmic reticulum = stores calcium for contraction Transverse (T) tubules = network of narrow tubes that form passageways
through a muscle fiber Organization of Skeletal Muscle Myofibrils = cytoskeletal proteins; composed of myofilaments (actin and myosin) Alternating arrangement of myofilaments gives skeletal muscle a striated appearance 1. Actin = thin filament = light band
2. 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 myfobril consists of ~10,000 sarcomeres arranged end to end Arrangement of actin and myosin within the sarcomere produce a striped appearance
Sarcomere Structure Z line = marks the boundaries of each sarcomere M line = middle Sarcomere of the sarcomere Sarcoplas mic
reticulum Z line Z line M line Sarcomere Structure A band = Myosin (thick filament; dark band) I band = Actin (thin filament; light band)
Sarcomere Sarcoplas mic Z line reticulumActin Z line I band A
I band Myosin M line Sarcomere Structure Animatio n
H zone = no overlap between thick and thin filaments Sarcomere Sarcoplas mic H zone Z line reticulumActin Z line
I band A I band Myosin M line LabelA, I, M, Z, H
animatio n Actin Most abundant protein on earth Active site = location of interaction with myosin At rest, proteins (troponin and tropomyosin)
block the active site on actin, preventing actin and myosin from interacting Myosin Motor protein Muscle at Rest Actin and myosin lie side-by side Myosin heads are primed for
contraction H zone and I band are at maximum width Changes to the sarcomere during contraction I band gets smaller Z lines move closer together H zone decrease Width of A bands doesnt change
animatio n Sliding Filament Theory Model of muscle contraction Thin filament slides past the thick filament Contraction = sarcomere shortening
Involves 5 different molecules and calcium Myosin Actin Tropomyosin Troponin myosin At Rest
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 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 Ttubules Muscle Contraction 4. Calcium release: sarcoplasmic reticulum releases calcium; calcium
diffuses to sarcomeres Muscle Contraction 5. Active Site exposed: Ca2+ binds to troponin; tropomoyosin shifts; active sites for myosin are exposed Muscle Contraction 6.
Cross-bridge formation: Myosin head binds to actin Muscle Contraction 7. Powerstroke: ADP and Pi are released from myosin; myosin moves, pulling the actin with it; sarcomere shortens
Muscle Contraction 8. Release: ATP binds to myosin; myosin released from actin; ATP reverts into ADP and Pi. Myosin is ready to form another crossbridge Relaxation
9. Relaxation: Impulse stops, calcium is released from troponin; tropomyosin covers the active site Ca+ is transported back into the SR and waits for another impulse Muscle Contraction
Contraction ends when Impulses stop Calcium ion concentration returns to normal Contractio n animation
Muscle Sketches 1. Skeletal muscle (see page 113; compare with what you see on a microscope) Label nuclei, muscle fiber 2. Actin filament (see page 198) Label actin, troponin, tropomyosin, active site 3. Myosin filament (see page 198) Label head
4. Sarcomere (at rest and during contraction) Label all parts (A, I, actin, myosin, H, M, Z)
