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KINESIOL 2C03 Midterm: Midterm 1

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Audrey Hicks

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Neuromuscular Exercise Physiology Muscle fasciculus  muscle fibre  myofibril  sarcomeres  myofilaments (actin and myosin) Muscle: skeletal muscle makes up 40-45% of adult male and 23-25% of adult female body weight  Encased by epimysium (tough)  Divided into fasciculi  Divided into compartments called inscriptions (6-pack) Fasciculus  Encased and divided by perimysium Muscle Fibres  1000s of fibres per muscle (biceps = 200,000)  Fibres are 10-100 um thick (largest as thick as human hair)  Length: a few mm to several cm long (about 1 um space between fibers)  Polygonal in shape to allow greater “packing” density (instead of cylindrical)  Surrounded by endomysium  Multinucleated (fusion of myoblasts)  Sarcolemma: cell membrane around each muscle cell  Divided into myofibrils Myofibrils  Make up ~80-85% of contents of fibre  1-2 um in diameter  A few hundred to a few thousand myofibrils per fibre  Run whole length of muscle fibre  Divide into myofilaments (sarcomeres in series) Myofilaments (make up sarcomere)  Actin: “thin” filaments, pearl strand, 6nm wide, 1 um long o Troponin every 7 globular actin o Tropomyosin  Myosin: “thick” filaments, cross bridge heads, 16 nm wide, 1.5 um long  M-line: middle  Z-line: connect actin, divide sarcomeres  A band: length of myosin  I (light) band: w/o myosin  H zone: no cross bridges (middle)  6 actin per 1 myosin  Sarcomeres can vary in length depending on degree of filament overlap  Sarcomeres can vary in number depending on muscle length Cytoskeleton and Auxillary Proteins  Auxillary proteins form the cytoskeleton  Cytoskeleton keeps sarcomeres properly aligned  Connect lines and disks See notes for Longitudinal vs. Cross-section structure Working Range of Sarcomere  Long: 3.5 um, looks stretched out  “Optimal”: 2 um  Short: no more “compression”, 1.5 um  Working range: 3.5-1.5 um = 2 um  See working range of myofibril/muscle fibre in notes The Longest Fibre at Optimal length (Sartorius!)  ~12 cm = 120 mm = 120,000 um  Optimal is 2 um  # sarcomeres = 120,000/2 = 60,000 Myosin  Myosin molecule = head + tail (rod)  All tails point towards midpoint  Heads bend towards tails (midpoint)  Myosin heads pull actin filaments with them to middle  Myosin CB = myosin head  Myosin molecule has a “double” head  Myosin is made of heavy and light chains o Two heavy chains in one myosin molecule = 2 heads  Not enough heads for each globular actin  BUT every actin has a myosin binding site  ‘Bare zone’ – segment of myosin myosin filament without CBs (H-zone) (.2-.3 um) Crossbridge Power Stroke  Always want to cause sarcomere shortening  Success determined by external load imposed on muscle  Asynchronous - not like a rowing boat o If all detached at same time, z-line would spring back Energy for CB Power Stroke  ATP is broken down into ADP and inorganic phosphate (P) and inergy o Fuelled by myosin ATPase  ATP is responsible for energy for the power stroke and detachment of myosin from actin 1. ATP binds to myosin head causing detachment from actin 2. ATP splits and CB springs upright (ADP and Pi) 3. Pi is released and CB binds to actin 4. Powerstroke and ADP release – energy pulls Z-line Crossbridge Power Stroke  CB cycle time ~50ms  CB attachment time ~3 ms  ~50% of CBs attached at one time o Assume maximal isometric contraction Regulatory Proteins in CB cycling  Thin filaments have troponin and tropomyosin  Troponin o Troponin-C: bind with Ca2+, Troponin T: bind with other regulatory proteins, Troponin-I: inhibit actin-myosin binding at rest  Tropomyosin o Long thread like protein at rest covers actin-myosin binding sites (each covers 7 actin globules) Role of Ca2+ in Crossbridge Cycling  Released Ca2+ binds to troponin  Troponin pulls tropomyosin aside, exposing actin binding sites  Myosin head binds to actin – CB cycle What if Ca2+ is removed?  Tropomyosin slides back over actin binding sites  CBs can’t bind to actin binding sites  CB cycling stops (relaxation) Neuromuscular Transmission (Ca2+ release and removal) (pg. 20 HB and 280 text)  Neuromuscular junction = Nerve terminal (sole foot) + motor end plate (muscle)  Sarcolemma is folded to increase density of channels  Synaptic vesicles have acetylcholine (Ach)  Membrane potential of muscle fibre is negative at rest (-70mV)  1 AP = twitch 1. Nerve AP arrives at presynaptic terminal 2. AP causes voltage gated Ca2+ gates to open in presynaptic membrane 3. Calcium causes the release of ACh from the synaptic vesicles 4. ACh inyo synaptic cleft 5. ACh binds to Na+ channels of postsynaptic membrane 6. Na2+ into motor end plate and depolarizes (inside becomes positive) 7. If threshold it met, AP occurs (Na+ channels close at peak) 8. ACh is broken down by acetylcholinesterase into acetic acid and choline to be recycled 9. K+ rushes out (repolarization) Muscle Action Potential  Muscle action potentials flows into and through Transverse-tubules in sarcolemma to myofibrils and myofilaments (towards centre)  “Triad” – sarcoplasmic reticulum + t-tubule + SR o There are 2 triads per sarcomere in human skeletal muscle  Sarcolemma: excitable membrane surrounding the muscle fibre; conducts the MAP initiated at the NMJ to T-tubules  Transverse Tubules: inward extensions of sarcolemma which conduct MAP into the interior of the muscle fibre  Sarcoplasmic reticulum: tubular network surrounding each myofibril o Junction between SR and t-tubule is called a “triad” o Ca2+ ~10,000 x greater o Ca2+ storage (at rest)  Low permeability to Ca2+ (can’t leave SR)  Ca2 pump sequesters Ca2+ from sarcoplasm (pumps back in) o Ca2+ release (in response to MAP)  Ca2+ channels open  Ca2+ efflux from SR  Ca2+ binds to troponin o Ca2+ re-uptake (after MAP)  Ca2+ channels close  SR pumps Ca2+ back into SR  Requires ATP  CB cycling stops Excitation-Contraction Coupling  MAP along sarcolemma  MAP down through T-tubules  MAP excites voltage sensing protein in T-tubule  Voltage sensing protein excites SR Ca2++ release channels to open  Ca+ release from SR  Ca+ binds to troponin  Tropomyosin pulled aside exposing actin binding sites for myosin  Myosin actin binding  CB cycling Twitch, Summation and Tetanus  Only one MAP despite two directions (stone in water effect)  Twitch = contration (Ca++ release) and relaxation (Ca++ re-uptake) o Isometric in this case o Contractile response to single nerve impulse/MAP (1) o Rare in normal function o Evoked artificially in research to study muscle function o Explain “summation” concept of contraction  Contraction rate is faster than relaxation  Voltage can be released until there is a plateau (and muscle fibres are engaged)  Summation – when MAPs are close enough and 2 action potential finishes before 1 finishes  Increased frequency = increased summation = increased force (up to a point)  Tetanus: contractile response to a “train” of MAPs causing summation o At a given frequency o Unfused and fused (no relaxation or force variation) depends on frequency of MAPs  Twitch/Tetanus Ratio = a/b = ~10% normal o Tetanus/Twitch ratio  Summation causes tetanus to be stronger than a twitch Mechanisms of Summation 1. Taking up the Series-Elastic Component (SEC)  Consists of tendons, connective tissue, cytoskeleton and crossbridges  Twitch: rising phase only lasts ~50-100 ms i. Insufficient time to take up SEC ii. External force does not equal potential force  Tetanus: can last seconds  sufficient time to take up SEC i. External force = potential force  Stiffer tendons = more force than elastic  Potential CB force is based on active # of CBs 2. Greater Ca2+ release from SR  Increase frequency of MAPs (increase frequency of stimulation)  Increase release of Ca2+ from SR  Increase # of active CBs  Increase force of contraction  Increased frequency = increased force  See charts in notes (pg 33-35) Contraction Types  Contraction: generate force  Action: movement  Concentric: shortening – force is greater than external load  Isometric: fixed length – force is equal to external load  Eccentric: lengthening – force is less than external load o Movement is controlled o Don’t need as many muscle fibres (recruitment of motor neurons is controlled)  Isotonic: constant force  Isovelocity: constant velocity  Plyometric: eccentric  Often the same action ca
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