bmeg 350 muscle (slide 1)

Muscle Fiber

long, multinucleated cell specialized for contraction

Sarcomere

 smallest functional unit of muscle; boundaries: Z-line to Z

Actin (thin filament)

provides binding sites for myosin; regulated by troponin (binds Ca2+), tropomyosin (blocks BS)

Myosin (thick filament)

motor protein with ATPase activity, forms cross-bridges with actin

Muscle fiber types

type 1 (slow twitch; aerobic (oxidative) metabolism, type 2 (intermediate fast-twitch; aerobic (oxidative) and anaerobic (glycolytic) metabolism, type 2X (fast twitch; anaerobic metabolism)

Phosphagen system

8-10s, 100m sprinter; synthesize ATP more rapid than cell respiration

Anaerobic system

1-2min, 400m swimmer; more sustained pool of ATP, produce lactic acid

Aerobic system

unlimited; marathon; constant oxygen; large yield ATP

Oxygen debt

extra consumption of oxygen repays oxygen debt after completion of hard exercise (reconvert LA accumulated back to glucose; reconvert adenosine monophosphate and ADP → ATP, reconvert creatine and phosphate → phosphocreatine; establish normal oxygen w/ hemoglobin/myoglobin; raise oxygen in lungs to normal)

Skeletal muscle

bone; long, cylindrical; multiple, peripheral nucleus; striations; voluntary; move whole body

Cardiac muscle

heart; branches; usually single, central nucleus; intercolated disks; striations; autorhythmic; involiuntary; contract heart to deliver blood

Smooth muscle

walls/hollow organs, blood vessels, glands; spingle-shaped; single, central nucleus; cell-to-cell attach; autorhythmic; involuntary; compress organs/ducts/tubes etc

Sliding filament theory

muscle shortens b/c actin slides over myosin; filaments do not shorten; sarcomere shortens; at rest, tropomyosin covers myosin binding sites on actin, preventing contraction. When muscle contracts, calcium binds to troponin → shape change that moves tropomyosin to reveal binding site and allow contraction

Cross-Bridge Cycle

1. ATP binds → myosin detaches 2. ATP hydrolyzed → myosin cocked 3. Ca2+ exposes BS, 4. Cross-bridge forms 5. Power stroke 6. ADP released → rigor until ATP binds 

Excitation-Contraction Coupling (convert elec signal → contraction): 1. Muscle AP 2. AP down T-tubules 3. DHPR activates RyR 4. Ca released from SR 5. Ca binds troponin 6. Contraction begin

Relaxation

Ca pumped back into SR by SERCA; needs ATP

Motor units

1 motor neuron + muscle fibers it innervates; small units → precision; large → F

Henneman’s size principle

MU recruited in size order to move increasingly heavier loads

Motor end-plate & innervation

NMJ, axon terminal releases ACh; MP → ACh-receptor in MF sarcolemma; when ACh released, diffuse across synaptic cleft and bind to receptor

Relaxation of MF

Ca++ ions pumped back into sarcoplasmic reticulum, causes tropomyosin to reshielf binding sits on actin strands; muscle stop contracting when no ATP

Ideal length sarcomere

 thick and thin filaments overlap 80-120%

Force regulation

 increase F by recruiting more MU, inc firing freq, optimal muscle length

Force summation

adding together individual twitch to increase intensity of overall muscle contraction; either increase number of MU contracting simulateneously (MF summation) or increasing freq of contraction (freq summation) → tetanization

Twitch

single AP from motor neuron → single contraction to MF of its MU

Latent period

AP propagated along sarcolemma and Ca++ ions are released from SR’ excitation and contraction are coupled

Contraction phase

 Ca++ ions have bound to troponin; tropomyosin has shifted away from myosin-binding sites on actin, cross-bridxges formed, sarcomeres shortening to peak tension

Relaxation phase

when tension decreases and contraction stops, Ca++ ions are pumped out of the sarcoplasm into SR, and cross-bridge cycling stops, returning the muscle fibers to their resting state

Concentric (isotonic)

bicep brachii during upward phase; tension and shortens; pulls forearm upwards → flexion of elbow

Eccentric (isotonic)

contraction bicep during downward; tension and lengthens; slows lowering of forearm and controls extension of elbow

Isometric

holding weight still; tension and stays same length; slows flexion and extension of the elbow

Stretch reflex

muscle spindle signals muscle to contract (so spindle goes back to original shape), prevents overstretch; detect muscle length

Autogenic inhibition

golgi tendon organ stimulated → muscle relax by stopping contraction; detects tension