Skeletal Muscle Contraction & Fiber types 5/7/2019 SAP2b

Fig08.01

Skeletal Muscle Histology Fibers--represent a single cell Sarcolemma--cell membrane Sarcoplasm--the cytoplasm that contains nuclei, mitochondria, and myofibrils Sarcoplasmic reticulum--similar to but not identical to E.R.-- releases Ca+ for muscle contraction Myofibrils--contain two kinds of protein myosin--thick filaments Attach to actin during contractions (2) actin--thin filaments Also contains tropomyosin and troponin Histology-study of microscopic structure of tissues 5/7/2019 SAP2b

A Bands – dark striations; location of myosin filaments Histology Continued The alternating light and dark striations are named for their positions within the fiber A Bands – dark striations; location of myosin filaments I Bands- light striations; location of actin filaments Z Lines- attachment point of actin filaments at end of I Bands Sarcomere- Segment of myofibril between two Z lines Transverse Tubules- A membranous channel that extends inward from a muscle fiber membrane and passes through the fiber 5/7/2019 SAP2b

Sarcomere = structural unit of myofibril from Z line to Z line—striations of skeletal muscle in repeating patterns I bands – light - anchored to Z lines A bands – dark – made of overlapping thick & thin filaments H Zone – in center of A bands

Transmission Electron Microscope technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid 5/7/2019 SAP2b

Neuromuscular Junction Electrical nerve impulse (action potential) reaches axon terminal Neurotransmitter (ACh) released from synaptic vesicle across synaptic cleft ACh binds with ACh receptors on sarcolemma (muscle membrane) @ axon terminal synaptic vesicles join the synaptic cleft & release Ach. Acetic acid & choline released. Ach binds to receptors to open Na & K channels 5/7/2019 SAP2b

Neuromuscular Junction Summary: electrical signal -------> chemical signal ------->electrical signal   AP         ACh                  AP on motor neuron    in synaptic cleft       on sarcolemma AP= action potential @ axon terminal synaptic vesicles join the synaptic cleft & release Ach. Acetic acid & choline released. Ach binds to receptors to open Na & K channels 5/7/2019 SAP2b

Excitation-Contraction Coupling 4. Stimulation of ACh receptor triggers release of sodium ions 5. Na+ travels down the (Transverse) T-tubules of the sarcoplasmic reticulum 6. Stimulation of T-Tubules triggers release of Ca+ 7. Ca+ combines with troponin (troponin prevents myosin from interacting w/ actin normally) ACH receptors release Na ions----Na ions travel thru T tubules –triggers release of Ca ions Ca ions join with troponin

Sliding Filament Model of Contraction 8. Tropomyosin on actin shifts exposing binding sites for myosin heads 9. ATP is used-allows myosin heads to attach to actin exposed binding sites 10. The myosin “power stroke” the actin filaments towards the middle 11. Filaments slide past each other contracting the muscle 12. Release of myosin heads allows muscle to relax When Ca ions join with troponin -tropomyosin moves ---exposes binding sites for myosin heads Myosin heads attach to actin Myosin pulls actin filaments to middle for muscle contraction Muscle Contraction Process

5/7/2019 SAP2b

Fig08.06 When Ca ions join with troponin -tropomyosin moves ---exposes binding sites for myosin heads Myosin heads attach to actin Myosin pulls actin filaments to middle for muscle contraction

Table08.01

3 Events are recorded Latent: first milliseconds between the stimulus and contraction, when excitation-coupling contraction occurs Contraction: When cross-bridges (myosin & actin) are active between 10-100 milliseconds Relaxation: resting of muscle, re-entry of Ca+ into SR-returns to baseline Latent period- delay of time between stimulation & beginning of contraction

Energy for Muscle Contractions Comes from breaking high-energy bonds of ATP 1. Muscles are fueled by the breakdown of sugar in which ATP byproducts (ADP) are reconstructed into ATP molecules Creatine phosphate-stores excess energy released from mitochondria 2. Anaerobic glycolysis Use of ATP energy requires no O2- when ATP is depleted, muscle cells must use cellular respiration to obtain energy Anaerobic glycolysis provides 20-30 sec of ATP Once muscle is at rest-ATP stores are replenished (appx 45 sec to 2 mins)

All-or-None Response When a muscle fiber contracts, it contracts to its fullest extent. It cannot contract partially Slightly misleading---in normal use of muscles, force generated by muscles varies 5/7/2019 SAP2b

Graded Strength Principle Skeletal muscles contract with varying degrees of strength at different times. No always using 100% strength. Muscle’s contraction depends on: Number of fibers contracting simultaneously (muscle recruitment) Recruitment of motor neurons Load placed on muscle Slightly misleading---in normal use of muscles, force generated by muscles varies 5/7/2019 SAP2b

Types of Skeletal Muscle Fibers Slow oxidative (Low ATP activity) Muscle fibers that twitch at a very slow rate Resistant to fatigue Aerobic activities Fast oxidative (large ATP activity) Muscles contract at a fast rate & have large force but resistant to tiring High in oxidative & glycolytic enzymes Aerobic activities for prolonged intervals Slow Ox—Slow Twitch (one muscle contraction) ---Needs O2---Cross X running Builds lots of endurance---- Fast Ox –higher tension contractions-------- Power walking or sprinting---Needs O2---breathing fast---body undergoes cellular respiration so O2 can be produced 5/7/2019 SAP2b

Slow-greater blood supply & O2 supply to muscles = better endurance Many oxidative enzymes= aerobic respiration Fast-Few mitochondria-whiter in appearance many glycolysis enzymes for anaerobic respiration 5/7/2019 SAP2b

Types of Skeletal Muscle Fibers Fast glycolytic (high ATP) Fibers tire easily Muscle requires rest after small amt of exertion Low oxidative capacity Anaerobic activities-short durations Uses glucose for power and exercise----anaerobic Run fast—over exert yourself---body can’t make enough ATP Super out of breath because body is trying to reoup or regain the lost O2 5/7/2019 SAP2b

Location of Muscle Fibers White muscle Human Muscles Fast fibers Mixture of fibers (Pink) Typically pale Found in chicken breast Red muscle Slow fibers Typically dark Found in chicken legs 5/7/2019 SAP2b